Search

CD45 is a cell surface phosphatase that shapes the T cell receptor signaling threshold but does not have a known ligand. A family of adenovirus proteins, including E3/49K, exploits CD45 to evade immunity by binding to the extracellular domain of CD45, resulting in the suppression of T cell signaling. We determined the cryo-EM structure of this complex and found that the E3/49K protein is composed of three immunoglobulin domains assembled as "beads on a string" that compel CD45 into a closely abutted dimer by cross-linking the CD45 D3 domain, leading to steric inhibition of its intracellular phosphatase activity. Inspired by the E3/49K mechanism, we engineered CD45 surrogate ligands that can fine-tune T cell activation by dimerizing CD45 into different orientations and proximities. The adenovirus E3/49K protein has taught us that, despite a lack of a known ligand, CD45 activity can be modulated by extracellular dimerizing ligands that perturb its phosphatase activity and alter T cell responses.

Cytokine release syndrome (CRS), commonly known as cytokine storm, is an acute systemic inflammatory response that is a significant global health threat. Interleukin-6 (IL-6) and interleukin-1 (IL-1) are key pro-inflammatory cytokines involved in CRS and are hence critical therapeutic targets. Current antagonists, such as tocilizumab and anakinra, target IL-6R/IL-1R but have limitations due to their long half-life and systemic anti-inflammatory effects, making them less suitable for acute or localized treatments. Here we present the de novo design of small protein antagonists that prevent IL-1 and IL-6 from interacting with their receptors to activate signaling. The designed proteins bind to the IL-6R, GP130 (an IL-6 co-receptor), and IL-1R1 receptor subunits with binding affinities in the picomolar to low-nanomolar range. X-ray crystallography studies reveal that the structures of these antagonists closely match their computational design models. In a human cardiac organoid disease model, the IL-1R antagonists demonstrated protective effects against inflammation and cardiac damage induced by IL-1β. These minibinders show promise for administration via subcutaneous injection or intranasal/inhaled routes to mitigate acute cytokine storm effects., (© 2024. The Author(s).)

In natural proteins, structured loops have central roles in molecular recognition, signal transduction and enzyme catalysis. However, because of the intrinsic flexibility and irregularity of loop regions, organizing multiple structured loops at protein functional sites has been very difficult to achieve by de novo protein design. Here we describe a solution to this problem that designs tandem repeat proteins with structured loops (9-14 residues) buttressed by extensive hydrogen bonding interactions. Experimental characterization shows that the designs are monodisperse, highly soluble, folded and thermally stable. Crystal structures are in close agreement with the design models, with the loops structured and buttressed as designed. We demonstrate the functionality afforded by loop buttressing by designing and characterizing binders for extended peptides in which the loops form one side of an extended binding pocket. The ability to design multiple structured loops should contribute generally to efforts to design new protein functions., (© 2024. The Author(s).)

The loading of processed peptides on to major histocompatibility complex II (MHC-II) molecules for recognition by T cells is vital to cell-mediated adaptive immunity. As part of this process, MHC-II associates with the invariant chain (Ii) during biosynthesis in the endoplasmic reticulum to prevent premature peptide loading and to serve as a scaffold for subsequent proteolytic processing into MHC-II-CLIP. Cryo-electron microscopy structures of full-length Human Leukocyte Antigen-DR (HLA-DR) and HLA-DQ complexes associated with Ii, resolved at 3.0 to 3.1 Å, elucidate the trimeric assembly of the HLA/Ii complex and define atomic-level interactions between HLA, Ii transmembrane domains, loop domains, and class II-associated invariant chain peptides (CLIP). Together with previous structures of MHC-II peptide loading intermediates DO and DM, our findings complete the structural path governing class II antigen presentation., Competing Interests: Competing interests statement:The authors declare no competing interest.

Thrombopoietin (THPO or TPO) is an essential cytokine for hematopoietic stem cell (HSC) maintenance and megakaryocyte differentiation. Here, we report the 3.4 Å resolution cryoelectron microscopy structure of the extracellular TPO-TPO receptor (TpoR or MPL) signaling complex, revealing the basis for homodimeric MPL activation and providing a structural rationalization for genetic loss-of-function thrombocytopenia mutations. The structure guided the engineering of TPO variants (TPO mod ) with a spectrum of signaling activities, from neutral antagonists to partial- and super-agonists. Partial agonist TPO mod decoupled JAK/STAT from ERK/AKT/CREB activation, driving a bias for megakaryopoiesis and platelet production without causing significant HSC expansion in mice and showing superior maintenance of human HSCs in vitro. These data demonstrate the functional uncoupling of the two primary roles of TPO, highlighting the potential utility of TPO mod in hematology research and clinical HSC transplantation., Competing Interests: Declaration of interests K.C.G., N.T., and I.S.H. are inventors on a United States provisional patent (63/519,687, filed 15 August 2023) related to the mutant TPO molecules described in the manuscript., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

The interleukin-4 (IL-4) cytokine plays a critical role in modulating immune homeostasis. Although there is great interest in harnessing this cytokine as a therapeutic in natural or engineered formats, the clinical potential of native IL-4 is limited by its instability and pleiotropic actions. Here, we design IL-4 cytokine mimetics (denoted Neo-4) based on a de novo engineered IL-2 mimetic scaffold and demonstrate that these cytokines can recapitulate physiological functions of IL-4 in cellular and animal models. In contrast with natural IL-4, Neo-4 is hyperstable and signals exclusively through the type I IL-4 receptor complex, providing previously inaccessible insights into differential IL-4 signaling through type I versus type II receptors. Because of their hyperstability, our computationally designed mimetics can directly incorporate into sophisticated biomaterials that require heat processing, such as three-dimensional-printed scaffolds. Neo-4 should be broadly useful for interrogating IL-4 biology, and the design workflow will inform targeted cytokine therapeutic development., (© 2023. The Author(s), under exclusive licence to Springer Nature America, Inc.)

Fine-tuning of protein-protein interactions occurs naturally through coevolution, but this process is difficult to recapitulate in the laboratory. We describe a platform for synthetic protein-protein coevolution that can isolate matched pairs of interacting muteins from complex libraries. This large dataset of coevolved complexes drove a systems-level analysis of molecular recognition between Z domain-affibody pairs spanning a wide range of structures, affinities, cross-reactivities, and orthogonalities, and captured a broad spectrum of coevolutionary networks. Furthermore, we harnessed pretrained protein language models to expand, in silico, the amino acid diversity of our coevolution screen, predicting remodeled interfaces beyond the reach of the experimental library. The integration of these approaches provides a means of simulating protein coevolution and generating protein complexes with diverse molecular recognition properties for biotechnology and synthetic biology.

Monoclonal antibodies (Abs) that recognize major histocompatability complex (MHC)-presented tumor antigens in a manner similar to T cell receptors (TCRs) have great potential as cancer immunotherapeutics. However, isolation of 'TCR-mimic' (TCRm) Abs is laborious because Abs have not evolved the structurally nuanced peptide-MHC restriction of αβ-TCRs. Here, we present a strategy for rapid isolation of highly peptide-specific and 'MHC-restricted' Abs by re-engineering preselected Abs that engage peptide-MHC in a manner structurally similar to that of conventional αβ-TCRs. We created structure-based libraries focused on the peptide-interacting residues of TCRm Ab complementarity-determining region (CDR) loops, and rapidly generated MHC-restricted Abs to both mouse and human tumor antigens that specifically killed target cells when formatted as IgG, bispecific T cell engager (BiTE) and chimeric antigen receptor-T (CAR-T). Crystallographic analysis of one selected pMHC-restricted Ab revealed highly peptide-specific recognition, validating the engineering strategy. This approach can yield tumor antigen-specific antibodies in several weeks, potentially enabling rapid clinical translation., (© 2023. The Author(s).)

Interleukin-21 (IL-21) plays a critical role in generating immunological memory by promoting the germinal center reaction, yet clinical use of IL-21 remains challenging because of its pleiotropy and association with autoimmune disease. To better understand the structural basis of IL-21 signaling, we determine the structure of the IL-21-IL-21R-γc ternary signaling complex by X-ray crystallography and a structure of a dimer of trimeric complexes using cryo-electron microscopy. Guided by the structure, we design analogs of IL-21 by introducing substitutions to the IL-21-γc interface. These IL-21 analogs act as partial agonists that modulate downstream activation of pS6, pSTAT3, and pSTAT1. These analogs exhibit differential activity on T and B cell subsets and modulate antibody production in human tonsil organoids. These results clarify the structural basis of IL-21 signaling and offer a potential strategy for tunable manipulation of humoral immunity., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Wnt morphogens are critical for embryonic development and tissue regeneration. Canonical Wnts form ternary receptor complexes composed of tissue-specific Frizzled (Fzd) receptors together with the shared LRP5/6 coreceptors to initiate β-catenin signaling. The cryo-EM structure of a ternary initiation complex of an affinity-matured XWnt8-Frizzled8-LRP6 complex elucidates the basis of coreceptor discrimination by canonical Wnts by means of their N termini and linker domains that engage the LRP6 E1E2 domain funnels. Chimeric Wnts bearing modular linker "grafts" were able to transfer LRP6 domain specificity between different Wnts and enable non-canonical Wnt5a to signal through the canonical pathway. Synthetic peptides comprising the linker domain serve as Wnt-specific antagonists. The structure of the ternary complex provides a topological blueprint for the orientation and proximity of Frizzled and LRP6 within the Wnt cell surface signalosome.

Human leucocyte antigen B*27 (HLA-B*27) is strongly associated with inflammatory diseases of the spine and pelvis (for example, ankylosing spondylitis (AS)) and the eye (that is, acute anterior uveitis (AAU)) 1 . How HLA-B*27 facilitates disease remains unknown, but one possible mechanism could involve presentation of pathogenic peptides to CD8 + T cells. Here we isolated orphan T cell receptors (TCRs) expressing a disease-associated public β-chain variable region-complementary-determining region 3β (BV9-CDR3β) motif 2-4 from blood and synovial fluid T cells from individuals with AS and from the eye in individuals with AAU. These TCRs showed consistent α-chain variable region (AV21) chain pairing and were clonally expanded in the joint and eye. We used HLA-B*27:05 yeast display peptide libraries to identify shared self-peptides and microbial peptides that activated the AS- and AAU-derived TCRs. Structural analysis revealed that TCR cross-reactivity for peptide-MHC was rooted in a shared binding motif present in both self-antigens and microbial antigens that engages the BV9-CDR3β TCRs. These findings support the hypothesis that microbial antigens and self-antigens could play a pathogenic role in HLA-B*27-associated disease., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The IL-17 family of cytokines and receptors have central roles in host defence against infection and development of inflammatory diseases 1 . The compositions and structures of functional IL-17 family ligand-receptor signalling assemblies remain unclear. IL-17E (also known as IL-25) is a key regulator of type 2 immune responses and driver of inflammatory diseases, such as allergic asthma, and requires both IL-17 receptor A (IL-17RA) and IL-17RB to elicit functional responses 2 . Here we studied IL-25-IL-17RB binary and IL-25-IL-17RB-IL-17RA ternary complexes using a combination of cryo-electron microscopy, single-molecule imaging and cell-based signalling approaches. The IL-25-IL-17RB-IL-17RA ternary signalling assembly is a C2-symmetric complex in which the IL-25-IL-17RB homodimer is flanked by two 'wing-like' IL-17RA co-receptors through a 'tip-to-tip' geometry that is the key receptor-receptor interaction required for initiation of signal transduction. IL-25 interacts solely with IL-17RB to allosterically promote the formation of the IL-17RB-IL-17RA tip-to-tip interface. The resulting large separation between the receptors at the membrane-proximal level may reflect proximity constraints imposed by the intracellular domains for signalling. Cryo-electron microscopy structures of IL-17A-IL-17RA and IL-17A-IL-17RA-IL-17RC complexes reveal that this tip-to-tip architecture is a key organizing principle of the IL-17 receptor family. Furthermore, these studies reveal dual actions for IL-17RA sharing among IL-17 cytokine complexes, by either directly engaging IL-17 cytokines or alternatively functioning as a co-receptor., (© 2022. The Author(s).)

Interleukin 27 (IL-27) is a heterodimeric cytokine that functions to constrain T cell-mediated inflammation and plays an important role in immune homeostasis. Binding of IL-27 to cell surface receptors, IL-27Rα and gp130, results in activation of receptor-associated Janus Kinases and nuclear translocation of Signal Transducer and Activator of Transcription 1 (STAT1) and STAT3 transcription factors. Despite the emerging therapeutic importance of this cytokine axis in cancer and autoimmunity, a molecular blueprint of the IL-27 receptor signaling complex, and its relation to other gp130/IL-12 family cytokines, is currently unclear. We used cryogenic-electron microscopy to determine the quaternary structure of IL-27, composed of p28 and Epstein-Barr Virus-Induced 3 (Ebi3) subunits, bound to receptors, IL-27Rα and gp130. The resulting 3.47 Å resolution structure revealed a three-site assembly mechanism nucleated by the central p28 subunit of the cytokine. The overall topology and molecular details of this binding are reminiscent of IL-6 but distinct from related heterodimeric cytokines IL-12 and IL-23. These results indicate distinct receptor assembly mechanisms used by heterodimeric cytokines with important consequences for targeted agonism and antagonism of IL-27 signaling., Competing Interests: NC, CG, KJ, NT No competing interests declared, KG is the founder of Synthekine, (© 2022, Caveney, Glassman et al.)

The design of proteins that bind to a specific site on the surface of a target protein using no information other than the three-dimensional structure of the target remains a challenge 1-5 . Here we describe a general solution to this problem that starts with a broad exploration of the vast space of possible binding modes to a selected region of a protein surface, and then intensifies the search in the vicinity of the most promising binding modes. We demonstrate the broad applicability of this approach through the de novo design of binding proteins to 12 diverse protein targets with different shapes and surface properties. Biophysical characterization shows that the binders, which are all smaller than 65 amino acids, are hyperstable and, following experimental optimization, bind their targets with nanomolar to picomolar affinities. We succeeded in solving crystal structures of five of the binder-target complexes, and all five closely match the corresponding computational design models. Experimental data on nearly half a million computational designs and hundreds of thousands of point mutants provide detailed feedback on the strengths and limitations of the method and of our current understanding of protein-protein interactions, and should guide improvements of both. Our approach enables the targeted design of binders to sites of interest on a wide variety of proteins for therapeutic and diagnostic applications., (© 2022. This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.)

Cytokines are powerful immune modulators that initiate signaling through receptor dimerization, but natural cytokines have structural limitations as therapeutics. We present a strategy to discover cytokine surrogate agonists by using modular ligands that exploit induced proximity and receptor dimer geometry as pharmacological metrics amenable to high-throughput screening. Using VHH and scFv to human interleukin-2/15, type-I interferon, and interleukin-10 receptors, we generated combinatorial matrices of single-chain bispecific ligands that exhibited diverse spectrums of functional activities, including potent inhibition of SARS-CoV-2 by surrogate interferons. Crystal structures of IL-2R:VHH complexes revealed that variation in receptor dimer geometries resulted in functionally diverse signaling outputs. This modular platform enabled engineering of surrogate ligands that compelled assembly of an IL-2R/IL-10R heterodimer, which does not naturally exist, that signaled through pSTAT5 on T and natural killer (NK) cells. This "cytokine med-chem" approach, rooted in principles of induced proximity, is generalizable for discovery of diversified agonists for many ligand-receptor systems., Competing Interests: Declaration of interests K.C.G., M.Y., J.R., and Q.L. are co-inventors on a provisional patent 63/306,882 based upon the technology described in this manuscript. K.C.G. is the founder of Synthekine Therapeutics., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Adoptive cell therapy using engineered T cell receptors (TCRs) is a promising approach for targeting cancer antigens, but tumor-reactive TCRs are often weakly responsive to their target ligands, peptide-major histocompatibility complexes (pMHCs). Affinity-matured TCRs can enhance the efficacy of TCR-T cell therapy but can also cross-react with off-target antigens, resulting in organ immunopathology. We developed an alternative strategy to isolate TCR mutants that exhibited high activation signals coupled with low-affinity pMHC binding through the acquisition of catch bonds. Engineered analogs of a tumor antigen MAGE-A3-specific TCR maintained physiological affinities while exhibiting enhanced target killing potency and undetectable cross-reactivity, compared with a high-affinity clinically tested TCR that exhibited lethal cross-reactivity with a cardiac antigen. Catch bond engineering is a biophysically based strategy to tune high-sensitivity TCRs for T cell therapy with reduced potential for adverse cross-reactivity.

Cytokines signal through cell surface receptor dimers to initiate activation of intracellular Janus kinases (JAKs). We report the 3.6-angstrom-resolution cryo-electron microscopy structure of full-length JAK1 complexed with a cytokine receptor intracellular domain Box1 and Box2 regions captured as an activated homodimer bearing the valine→phenylalanine (VF) mutation prevalent in myeloproliferative neoplasms. The seven domains of JAK1 form an extended structural unit, the dimerization of which is mediated by close-packing of the pseudokinase (PK) domains from the monomeric subunits. The oncogenic VF mutation lies within the core of the JAK1 PK interdimer interface, enhancing packing complementarity to facilitate ligand-independent activation. The carboxy-terminal tyrosine kinase domains are poised for transactivation and to phosphorylate the receptor STAT (signal transducer and activator of transcription)-recruiting motifs projecting from the overhanging FERM (four-point-one, ezrin, radixin, moesin)-SH2 (Src homology 2)-domains. Mapping of constitutively active JAK mutants supports a two-step allosteric activation mechanism and reveals opportunities for selective therapeutic targeting of oncogenic JAK signaling.

Affinity maturation of protein–protein interactions is an important approach in the development of therapeutic proteins such as cytokines. Typical experimental strategies involve targeting the cytokine-receptor interface with combinatorial libraries and then selecting for higher-affinity variants. Mutations to the binding scaffold are usually not considered main drivers for improved affinity. Here we demonstrate that computational design can provide affinity-enhanced variants of interleukin-2 (IL-2) “out of the box” without any requirement for interface engineering. Using a strategy of global IL-2 structural stabilization targeting metastable regions of the three-dimensional structure, rather than the receptor binding interfaces, we computationally designed thermostable IL-2 variants with up to 40-fold higher affinity for IL-2Rβ without any library-based optimization. These IL-2 analogs exhibited CD25-independent activities on T and natural killer (NK) cells both in vitro and in vivo, mimicking the properties of the IL-2 superkine “super-2” that was engineered through yeast surface display [A. M. Levin et al., Nature, 484, 529–533 (2012)]. Structure-guided stabilization of cytokines is a powerful approach to affinity maturation with applications to many cytokine and protein–protein interactions.

Multiple sclerosis (MS) is a heterogenous autoimmune disease in which autoreactive lymphocytes attack the myelin sheath of the central nervous system. B lymphocytes in the cerebrospinal fluid (CSF) of patients with MS contribute to inflammation and secrete oligoclonal immunoglobulins 1,2 . Epstein-Barr virus (EBV) infection has been epidemiologically linked to MS, but its pathological role remains unclear 3 . Here we demonstrate high-affinity molecular mimicry between the EBV transcription factor EBV nuclear antigen 1 (EBNA1) and the central nervous system protein glial cell adhesion molecule (GlialCAM) and provide structural and in vivo functional evidence for its relevance. A cross-reactive CSF-derived antibody was initially identified by single-cell sequencing of the paired-chain B cell repertoire of MS blood and CSF, followed by protein microarray-based testing of recombinantly expressed CSF-derived antibodies against MS-associated viruses. Sequence analysis, affinity measurements and the crystal structure of the EBNA1-peptide epitope in complex with the autoreactive Fab fragment enabled tracking of the development of the naive EBNA1-restricted antibody to a mature EBNA1-GlialCAM cross-reactive antibody. Molecular mimicry is facilitated by a post-translational modification of GlialCAM. EBNA1 immunization exacerbates disease in a mouse model of MS, and anti-EBNA1 and anti-GlialCAM antibodies are prevalent in patients with MS. Our results provide a mechanistic link for the association between MS and EBV and could guide the development of new MS therapies., (© 2022. This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.)

Human cytomegalovirus (HCMV) encodes G protein-coupled receptors (GPCRs) US28 and US27 , which facilitate viral pathogenesis through engagement of host G proteins. Here we report cryo-electron microscopy structures of US28 and US27 forming nonproductive and productive complexes with Gi and Gq, respectively, exhibiting unusual features with functional implications. The "orphan" GPCR US27 lacks a ligand-binding pocket and has captured a guanosine diphosphate-bound inactive Gi through a tenuous interaction. The docking modes of CX3CL1-US28 and US27 to Gi favor localization to endosome-like curved membranes, where US28 and US27 can function as nonproductive Gi sinks to attenuate host chemokine-dependent Gi signaling. The CX3CL1-US28-Gq/11 complex likely represents a trapped intermediate during productive signaling, providing a view of a transition state in GPCR-G protein coupling for signaling. Our collective results shed new insight into unique G protein-mediated HCMV GPCR structural mechanisms, compared to mammalian GPCR counterparts, for subversion of host immunity.

RTN4-binding proteins were widely studied as "NoGo" receptors, but their physiological interactors and roles remain elusive. Similarly, BAI adhesion-GPCRs were associated with numerous activities, but their ligands and functions remain unclear. Using unbiased approaches, we observed an unexpected convergence: RTN4 receptors are high-affinity ligands for BAI adhesion-GPCRs. A single thrombospondin type 1-repeat (TSR) domain of BAIs binds to the leucine-rich repeat domain of all three RTN4-receptor isoforms with nanomolar affinity. In the 1.65 Å crystal structure of the BAI1/RTN4-receptor complex, C-mannosylation of tryptophan and O-fucosylation of threonine in the BAI TSR-domains creates a RTN4-receptor/BAI interface shaped by unusual glycoconjugates that enables high-affinity interactions. In human neurons, RTN4 receptors regulate dendritic arborization, axonal elongation, and synapse formation by differential binding to glial versus neuronal BAIs, thereby controlling neural network activity. Thus, BAI binding to RTN4/NoGo receptors represents a receptor-ligand axis that, enabled by rare post-translational modifications, controls development of synaptic circuits., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Interleukin-22 (IL-22) acts on epithelial cells to promote tissue protection and regeneration, but can also elicit pro-inflammatory effects, contributing to disease pathology. Here, we engineered a high-affinity IL-22 super-agonist that enabled the structure determination of the IL-22-IL-22Rα-IL-10Rβ ternary complex to a resolution of 2.6 Å. Using structure-based design, we systematically destabilized the IL-22-IL-10Rβ binding interface to create partial agonist analogs that decoupled downstream STAT1 and STAT3 signaling. The extent of STAT bias elicited by a single ligand varied across tissues, ranging from full STAT3-biased agonism to STAT1/3 antagonism, correlating with IL-10Rβ expression levels. In vivo, this tissue-selective signaling drove tissue protection in the pancreas and gastrointestinal tract without inducing local or systemic inflammation, thereby uncoupling these opposing effects of IL-22 signaling. Our findings provide insight into the mechanisms underlying the cytokine pleiotropy and illustrate how differential receptor expression levels and STAT response thresholds can be synthetically exploited to endow pleiotropic cytokines with enhanced functional specificity., Competing Interests: Declaration of interests R.A.S., L.T.H., and K.C.G. are co-inventors on a provisional patent based on discoveries described in this manuscript. A.M.H. holds intellectual property related to IL-22 and has received research funding related to IL-22 from Genentech and Evive Biotech. K.C.G. is the founder of Synthekine., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Interleukin-12 (IL-12) and IL-23 are heterodimeric cytokines that are produced by antigen-presenting cells to regulate the activation and differentiation of lymphocytes, and they share IL-12Rβ1 as a receptor signaling subunit. We present a crystal structure of the quaternary IL-23 (IL-23p19/p40)/IL-23R/IL-12Rβ1 complex, together with cryoelectron microscopy (cryo-EM) maps of the complete IL-12 (IL-12p35/p40)/IL-12Rβ2/IL-12Rβ1 and IL-23 receptor (IL-23R) complexes, which reveal "non-canonical" topologies where IL-12Rβ1 directly engages the common p40 subunit. We targeted the shared IL-12Rβ1/p40 interface to design a panel of IL-12 partial agonists that preserved interferon gamma (IFNγ) induction by CD8 + T cells but impaired cytokine production from natural killer (NK) cells in vitro. These cell-biased properties were recapitulated in vivo, where IL-12 partial agonists elicited anti-tumor immunity to MC-38 murine adenocarcinoma absent the NK-cell-mediated toxicity seen with wild-type IL-12. Thus, the structural mechanism of receptor sharing used by IL-12 family cytokines provides a protein interface blueprint for tuning this cytokine axis for therapeutics., Competing Interests: Declaration of interests K.C.G. and C.R.G. are co-inventors on a provisional patent based on discoveries described in this manuscript. K.C.G. is the founder of Synthekine., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Frizzleds (Fzd) are the primary receptors for Wnt morphogens, which are essential regulators of stem cell biology, yet the structural basis of Wnt signaling through Fzd remains poorly understood. Here we report the structure of an unliganded human Fzd5 determined by single-particle cryo-EM at 3.7 Å resolution, with the aid of an antibody chaperone acting as a fiducial marker. We also analyzed the topology of low-resolution XWnt8/Fzd5 complex particles, which revealed extreme flexibility between the Wnt/Fzd-CRD and the Fzd-TM regions. Analysis of Wnt/β-catenin signaling in response to Wnt3a versus a 'surrogate agonist' that cross-links Fzd to LRP6, revealed identical structure-activity relationships. Thus, canonical Wnt/β-catenin signaling appears to be principally reliant on ligand-induced Fzd/LRP6 heterodimerization, versus the allosteric mechanisms seen in structurally analogous class A G protein-coupled receptors, and Smoothened. These findings deepen our mechanistic understanding of Wnt signal transduction, and have implications for harnessing Wnt agonism in regenerative medicine., Competing Interests: NT, SM, DW, KJ, YM, JB, NA, AK, CG No competing interests declared, CJ, KG KCG and CYJ are founders of Surrozen Therapeutics., (© 2020, Tsutsumi et al.)

Interleukin-2 (IL-2) is a small α-helical cytokine that regulates immune cell homeostasis through its recruitment to a high-affinity heterotrimeric receptor complex (IL-2Rα/IL-2Rβ/γ c ). IL-2 has been shown to have therapeutic efficacy for immune diseases by preferentially expanding distinct T cell compartments, and several regulatory T cell (T reg )-biasing anti-IL-2 antibodies have been developed for combination therapies. The conformational plasticity of IL-2 plays an important role in its biological actions by modulating the strength of receptor and drug interactions. Through an NMR analysis of milliseconds-timescale dynamics of free mouse IL-2 (mIL-2), we identify a global transition to a sparse conformation which is regulated by an α-helical capping "switch" at the loop between the A and B helices (AB loop). Binding to either an anti-mouse IL-2 monoclonal antibody (mAb) or a small molecule inhibitor near the loop induces a measurable response at the core of the structure, while locking the switch to a single conformation through a designed point mutation leads to a global quenching of core dynamics accompanied by a pronounced effect in mAb binding. By elucidating key details of the long-range allosteric communication between the receptor binding surfaces and the core of the IL-2 structure, our results offer a direct blueprint for designing precision therapeutics targeting a continuum of conformational states., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

Ligand-induced dimerization is the predominant mechanism through which secreted proteins activate cell surface receptors to transmit essential biological signals. Cytokines are a large class of soluble proteins that dimerize transmembrane receptors into precise signaling topologies, but there is a need for alternative, engineerable ligand scaffolds that specifically recognize and stabilize these protein interactions. Recombinant antibodies can potentially serve as robust and versatile platforms for cytokine complex stabilization, and their specificity allows for tunable modulation of dimerization equilibrium. Here, we devised an evolutionary strategy to isolate monovalent antibody fragments that bridge together two different receptor subunits in a cytokine-receptor complex, precisely as the receptors are disposed in their natural signaling orientations. To do this, we screened a naive antibody library against a stabilized ligand-receptor ternary complex that acted as a "molecular cast" of the natural receptor dimer conformation. Our selections elicited "stapler" single-chain variable fragments (scFvs) of antibodies that specifically engage the interleukin-4 receptor heterodimer. The 3.1 Å resolution crystal structure of one such stapler revealed that, as intended, this scFv recognizes a composite epitope between the two receptors as they are positioned in the complex. Extending our approach, we evolved a stapler scFv that specifically binds to and stabilizes the interface between the interleukin-2 cytokine and one of its receptor subunits, leading to a 15-fold enhancement in interaction affinity. This demonstration that scFvs can be selected to recognize epitopes that span protein interfaces presents new opportunities to engineer structurally defined antibodies for a broad range of research and therapeutic applications., (© 2019 Spangler et al.)

To discriminate between closely related members of a protein family that differ at a limited number of spatially distant positions is a challenge for drug discovery. We describe a combined computational design and experimental selection approach for generating binders targeting functional sites with large, shape complementary interfaces to read out subtle sequence differences for subtype-specific antagonism. Repeat proteins are computationally docked against a functionally relevant region of the target protein surface that varies in the different subtypes, and the interface sequences are optimized for affinity and specificity first computationally and then experimentally. We used this approach to generate a series of human Frizzled (Fz) subtype-selective antagonists with extensive shape complementary interaction surfaces considerably larger than those of repeat proteins selected from random libraries. In vivo administration revealed that Wnt-dependent pericentral liver gene expression involves multiple Fz subtypes, while maintenance of the intestinal crypt stem cell compartment involves only a limited subset.

Although tunable signaling by G protein-coupled receptors can be exploited through medicinal chemistry, a comparable pharmacological approach has been lacking for the modulation of signaling through dimeric receptors, such as those for cytokines. We present a strategy to modulate cytokine receptor signaling output by use of a series of designed C2-symmetric cytokine mimetics, based on the designed ankyrin repeat protein (DARPin) scaffold, that can systematically control erythropoietin receptor (EpoR) dimerization orientation and distance between monomers. We sampled a range of EpoR geometries by varying intermonomer angle and distance, corroborated by several ligand-EpoR complex crystal structures. Across the range, we observed full, partial, and biased agonism as well as stage-selective effects on hematopoiesis. This surrogate ligand strategy opens access to pharmacological modulation of therapeutically important cytokine and growth factor receptor systems., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The cytokine interferon-γ (IFNγ) is a central coordinator of innate and adaptive immunity, but its highly pleiotropic actions have diminished its prospects for use as an immunotherapeutic agent. Here, we took a structure-based approach to decoupling IFNγ pleiotropy. We engineered an affinity-enhanced variant of the ligand-binding chain of the IFNγ receptor IFNγR1, which enabled us to determine the crystal structure of the complete hexameric (2:2:2) IFNγ-IFNγR1-IFNγR2 signalling complex at 3.25 Å resolution. The structure reveals the mechanism underlying deficits in IFNγ responsiveness in mycobacterial disease syndrome resulting from a T168N mutation in IFNγR2, which impairs assembly of the full signalling complex. The topology of the hexameric complex offers a blueprint for engineering IFNγ variants to tune IFNγ receptor signalling output. Unexpectedly, we found that several partial IFNγ agonists exhibited biased gene-expression profiles. These biased agonists retained the ability to induce upregulation of major histocompatibility complex class I antigen expression, but exhibited impaired induction of programmed death-ligand 1 expression in a wide range of human cancer cell lines, offering a route to decoupling immunostimulatory and immunosuppressive functions of IFNγ for therapeutic applications.

We describe a de novo computational approach for designing proteins that recapitulate the binding sites of natural cytokines, but are otherwise unrelated in topology or amino acid sequence. We use this strategy to design mimics of the central immune cytokine interleukin-2 (IL-2) that bind to the IL-2 receptor βγ c heterodimer (IL-2Rβγ c ) but have no binding site for IL-2Rα (also called CD25) or IL-15Rα (also known as CD215). The designs are hyper-stable, bind human and mouse IL-2Rβγ c with higher affinity than the natural cytokines, and elicit downstream cell signalling independently of IL-2Rα and IL-15Rα. Crystal structures of the optimized design neoleukin-2/15 (Neo-2/15), both alone and in complex with IL-2Rβγ c , are very similar to the designed model. Neo-2/15 has superior therapeutic activity to IL-2 in mouse models of melanoma and colon cancer, with reduced toxicity and undetectable immunogenicity. Our strategy for building hyper-stable de novo mimetics could be applied generally to signalling proteins, enabling the creation of superior therapeutic candidates.

T cell receptors (TCRs) bind to peptide-major histocompatibility complex (pMHC) with low affinity ( K d ∼ μM), which is generally assumed to facilitate cross-reactive TCR "scanning" of ligands. To understand the relationship between TCR/pMHC affinity and cross-reactivity, we sought to engineer an additional weak interaction, termed "velcro," between the TCR and pMHC to probe the specificities of TCRs at relatively low and high affinities. This additional interaction was generated through an eight-amino acid peptide library covalently linked to the N terminus of the MHC-bound peptide. Velcro was selected through an affinity-based isolation and was subsequently shown to enhance the cognate TCR/pMHC affinity in a peptide-dependent manner by ∼10-fold. This was sufficient to convert a nonstimulatory ultra-low-affinity ligand into a stimulatory ligand. An X-ray crystallographic structure revealed how velcro interacts with the TCR. To probe TCR cross-reactivity, we screened TCRs against yeast-displayed pMHC libraries with and without velcro, and found that the peptide cross-reactivity profiles of low-affinity ( K d > 100 μM) and high-affinity ( K d ∼ μM) TCR/pMHC interactions are remarkably similar. The conservation of recognition of the TCR for pMHC across affinities reveals the nature of low-affinity ligands for which there are important biological functions and has implications for understanding the specificities of affinity-matured TCRs., Competing Interests: The authors declare no conflict of interest.

TCR-signaling strength generally correlates with peptide-MHC binding affinity; however, exceptions exist. We find high-affinity, yet non-stimulatory, interactions occur with high frequency in the human T cell repertoire. Here, we studied human TCRs that are refractory to activation by pMHC ligands despite robust binding. Analysis of 3D affinity, 2D dwell time, and crystal structures of stimulatory versus non-stimulatory TCR-pMHC interactions failed to account for their different signaling outcomes. Using yeast pMHC display, we identified peptide agonists of a formerly non-responsive TCR. Single-molecule force measurements demonstrated the emergence of catch bonds in the activating TCR-pMHC interactions, correlating with exclusion of CD45 from the TCR-APC contact site. Molecular dynamics simulations of TCR-pMHC disengagement distinguished agonist from non-agonist ligands based on the acquisition of catch bonds within the TCR-pMHC interface. The isolation of catch bonds as a parameter mediating the coupling of TCR binding and signaling has important implications for TCR and antigen engineering for immunotherapy., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Interleukin-2 (IL-2) has been shown to suppress immune pathologies by preferentially expanding regulatory T cells (T regs ). However, this therapy has been limited by off-target complications due to pathogenic cell expansion. Recent efforts have been focused on developing a more selective IL-2. It is well documented that certain anti-mouse IL-2 antibodies induce conformational changes that result in selective targeting of T regs . We report the generation of a fully human anti-IL-2 antibody, F5111.2, that stabilizes IL-2 in a conformation that results in the preferential STAT5 phosphorylation of T regs in vitro and selective expansion of T regs in vivo. When complexed with human IL-2, F5111.2 induced remission of type 1 diabetes in the NOD mouse model, reduced disease severity in a model of experimental autoimmune encephalomyelitis and protected mice against xenogeneic graft-versus-host disease. These results suggest that IL-2-F5111.2 may provide an immunotherapy to treat autoimmune diseases and graft-versus-host disease.

Human cytomegalovirus has hijacked and evolved a human G-protein-coupled receptor into US28, which functions as a promiscuous chemokine 'sink' to facilitate evasion of host immune responses. To probe the molecular basis of US28's unique ligand cross-reactivity, we deep-sequenced CX3CL1 chemokine libraries selected on 'molecular casts' of the US28 active-state and find that US28 can engage thousands of distinct chemokine sequences, many of which elicit diverse signaling outcomes. The structure of a G-protein-biased CX3CL1-variant in complex with US28 revealed an entirely unique chemokine amino terminal peptide conformation and remodeled constellation of receptor-ligand interactions. Receptor signaling, however, is remarkably robust to mutational disruption of these interactions. Thus, US28 accommodates and functionally discriminates amongst highly degenerate chemokine sequences by sensing the steric bulk of the ligands, which distort both receptor extracellular loops and the walls of the ligand binding pocket to varying degrees, rather than requiring sequence-specific bonding chemistries for recognition and signaling., Competing Interests: TM, KS, KJ, NT, JB, JI, DW, GH, OL, HP, MR, KG No competing interests declared, (© 2018, Miles et al.)

Type III interferons (IFN-λs) signal through a heterodimeric receptor complex composed of the IFN-λR1 subunit, specific for IFN-λs, and interleukin-10Rβ (IL-10Rβ), which is shared by multiple cytokines in the IL-10 superfamily. Low affinity of IL-10Rβ for cytokines has impeded efforts aimed at crystallizing cytokine-receptor complexes. We used yeast surface display to engineer a higher-affinity IFN-λ variant, H11, which enabled crystallization of the ternary complex. The structure revealed that IL-10Rβ uses a network of tyrosine residues as hydrophobic anchor points to engage IL-10 family cytokines that present complementary hydrophobic binding patches, explaining its role as both a cross-reactive but cytokine-specific receptor. H11 elicited increased anti-proliferative and antiviral activities in vitro and in vivo. In contrast, engineered higher-affinity type I IFNs did not increase antiviral potency over wild-type type I IFNs. Our findings provide insight into cytokine recognition by the IL-10R family and highlight the plasticity of type III interferon signaling and its therapeutic potential., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Interleukin-2 (IL-2) is a pleiotropic cytokine that regulates immune cell homeostasis and has been used to treat a range of disorders including cancer and autoimmune disease. IL-2 signals via interleukin-2 receptor-β (IL-2Rβ):IL-2Rγ heterodimers on cells expressing high (regulatory T cells, Treg) or low (effector cells) amounts of IL-2Rα (CD25). When complexed with IL-2, certain anti-cytokine antibodies preferentially stimulate expansion of Treg (JES6-1) or effector (S4B6) cells, offering a strategy for targeted disease therapy. We found that JES6-1 sterically blocked the IL-2:IL-2Rβ and IL-2:IL-2Rγ interactions, but also allosterically lowered the IL-2:IL-2Rα affinity through a "triggered exchange" mechanism favoring IL-2Rα(hi) Treg cells, creating a positive feedback loop for IL-2Rα(hi) cell activation. Conversely, S4B6 sterically blocked the IL-2:IL-2Rα interaction, while also conformationally stabilizing the IL-2:IL-2Rβ interaction, thus stimulating all IL-2-responsive immune cells, particularly IL-2Rβ(hi) effector cells. These insights provide a molecular blueprint for engineering selectively potentiating therapeutic antibodies., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Chemokines are small proteins that function as immune modulators through activation of chemokine G protein-coupled receptors (GPCRs). Several viruses also encode chemokines and chemokine receptors to subvert the host immune response. How protein ligands activate GPCRs remains unknown. We report the crystal structure at 2.9 angstrom resolution of the human cytomegalovirus GPCR US28 in complex with the chemokine domain of human CX3CL1 (fractalkine). The globular body of CX3CL1 is perched on top of the US28 extracellular vestibule, whereas its amino terminus projects into the central core of US28. The transmembrane helices of US28 adopt an active-state-like conformation. Atomic-level simulations suggest that the agonist-independent activity of US28 may be due to an amino acid network evolved in the viral GPCR to destabilize the receptor's inactive state., (Copyright © 2015, American Association for the Advancement of Science.)

Notch receptors guide mammalian cell fate decisions by engaging the proteins Jagged and Delta-like (DLL). The 2.3 angstrom resolution crystal structure of the interacting regions of the Notch1-DLL4 complex reveals a two-site, antiparallel binding orientation assisted by Notch1 O-linked glycosylation. Notch1 epidermal growth factor-like repeats 11 and 12 interact with the DLL4 Delta/Serrate/Lag-2 (DSL) domain and module at the N-terminus of Notch ligands (MNNL) domains, respectively. Threonine and serine residues on Notch1 are functionalized with O-fucose and O-glucose, which act as surrogate amino acids by making specific, and essential, contacts to residues on DLL4. The elucidation of a direct chemical role for O-glycans in Notch1 ligand engagement demonstrates how, by relying on posttranslational modifications of their ligand binding sites, Notch proteins have linked their functional capacity to developmentally regulated biosynthetic pathways., (Copyright © 2015, American Association for the Advancement of Science.)

Topoisomerases are essential cellular enzymes that maintain the appropriate topological status of DNA and are the targets of several antibiotic and chemotherapeutic agents. High-throughput (HT) analysis is desirable to identify new topoisomerase inhibitors, but standard in vitro assays for DNA topology, such as gel electrophoresis, are time-consuming and are not amenable to HT analysis. We have exploited the observation that closed-circular DNA containing an inverted repeat can release the free energy stored in negatively supercoiled DNA by extruding the repeat as a cruciform. We inserted an inverted repeat containing a fluorophore-quencher pair into a plasmid to enable real-time monitoring of plasmid supercoiling by a bacterial topoisomerase, Escherichia coli gyrase. This substrate produces a fluorescent signal caused by the extrusion of the cruciform and separation of the labels as gyrase progressively underwinds the DNA. Subsequent relaxation by a eukaryotic topoisomerase, human topo IIα, causes reintegration of the cruciform and quenching of fluorescence. We used this approach to develop a HT screen for inhibitors of gyrase supercoiling. This work demonstrates that fluorescently labeled cruciforms are useful as general real-time indicators of changes in DNA topology that can be used to monitor the activity of DNA-dependent motor proteins.

KPC beta-lactamases hydrolyze the "last resort" beta-lactam antibiotics (carbapenems) used to treat multidrug resistant infections and are compromising efforts to combat life-threatening Gram-negative bacterial infections in hospitals worldwide. Consequently, the development of novel inhibitors is essential for restoring the effectiveness of existing antibiotics. The beta-lactamase inhibitor protein (BLIP) is a competitive inhibitor of a number of class A beta-lactamases. In this study, we characterize the previously unreported interaction between KPC-2 beta-lactamase and BLIP. Biochemical results show that BLIP is an extremely potent inhibitor of KPC enzymes, binding KPC-2 and KPC-3 with subnanomolar affinity. To understand the basis of affinity and specificity in the beta-lactamase-BLIP system, the crystallographic structure of the KPC-2-BLIP complex was determined to 1.9 A resolution. Computational alanine scanning was also conducted to identify putative hot spots in the KPC-2-BLIP interface. Interestingly, the two complexes making up the KPC-2-BLIP asymmetric unit are distinct, and in one structure, the BLIP F142 loop is absent, in contrast to homologous structures in which it occupies the active site. This finding and other sources of structural plasticity appear to contribute to BLIP's promiscuity, enabling it to respond to mutations at the beta-lactamase interface. Given the continuing emergence of antibiotic resistance, the high-resolution KPC-2-BLIP structure will facilitate its use as a template for the rational design of new inhibitors of this problematic enzyme.

Cryptophanes represent an exciting class of xenon-encapsulating molecules that can be exploited as probes for nuclear magnetic resonance imaging. The 1.70 A resolution crystal structure of a cryptophane-derivatized benezenesulfonamide complexed with human carbonic anhydrase II shows how an encapsulated xenon atom can be directed to a specific biological target. The crystal structure confirms binding measurements indicating that the cryptophane cage does not strongly interact with the surface of the protein, which may enhance the sensitivity of 129Xe NMR spectroscopic measurements in solution.

Despite the similarity in the active site pockets of carbonic anhydrase (CA) isozymes I and II, the binding affinities of benzenesulfonamide inhibitors are invariably higher with CA II as compared to CA I. To explore the structural basis of this molecular recognition phenomenon, we have designed and synthesized simple benzenesulfonamide inhibitors substituted at the para position with positively charged, negatively charged, and neutral functional groups, and we have determined the affinities and X-ray crystal structures of their enzyme complexes. The para-substituents are designed to bind in the midsection of the 15 A deep active site cleft, where interactions with enzyme residues and solvent molecules are possible. We find that a para-substituted positively charged amino group is more poorly tolerated in the active site of CA I compared with CA II. In contrast, a para-substituted negatively charged carboxylate substituent is tolerated equally well in the active sites of both CA isozymes. Notably, enzyme-inhibitor affinity increases upon neutralization of inhibitor charged groups by amidation or esterification. These results inform the design of short molecular linkers connecting the benzenesulfonamide group and a para-substituted tail group in "two-prong" CA inhibitors: an optimal linker segment will be electronically neutral, yet capable of engaging in at least some hydrogen bond interactions with protein residues and/or solvent. Microcalorimetric data reveal that inhibitor binding to CA I is enthalpically less favorable and entropically more favorable than inhibitor binding to CA II. This contrasting behavior may arise in part from differences in active site desolvation and the conformational entropy of inhibitor binding to each isozyme active site.

The atomic-resolution crystal structures of human carbonic anhydrases I and II complexed with "two-prong" inhibitors are reported. Each inhibitor contains a benzenesulfonamide prong and a cupric iminodiacetate (IDA-Cu(2+)) prong separated by linkers of different lengths and compositions. The ionized NH(-) group of each benzenesulfonamide coordinates to the active site Zn(2+) ion; the IDA-Cu(2+) prong of the tightest-binding inhibitor, BR30, binds to H64 of CAII and H200 of CAI. This work provides the first evidence verifying the structural basis of nanomolar affinity measured for two-prong inhibitors targeting the carbonic anhydrases.

The crystal structure of F65A/Y131C murine alpha-carbonic anhydrase V (CAV), covalently modified at cysteine residues with 4-chloromethylimidazole, is reported at 1.88 A resolution. This modification introduces a methylimidazole (MI) group at residue C131 in the active site with important consequences. F65A/Y131C-MI CAV exhibits an up to 3-fold enhancement of catalytic activity over that of wild-type CAV [Earnhardt, J. N., Wright, S. K., Qian, M., Tu, C., Laipis, P. J., Viola, R. E., and Silverman, D. N. (1999) Arch. Biochem. Biophys. 361, 264-270]. In this modified CAV variant, C131-MI acts as a proton shuttle, facilitating the deprotonation of a zinc-bound water molecule to regenerate the nucleophilic zinc-bound hydroxide ion. A network of three hydrogen-bonded water molecules, across which proton transfer likely proceeds, bridges the zinc-bound water molecule and the C131-MI imidazole group. The structure of F65A/Y131C-MI CAV is compared to structures of Y64H/F65A murine CAV, wild-type human alpha-carbonic anhydrase II, and the gamma-carbonic anhydrase from Methanosarcina thermophilain an effort to outline common features of catalytic proton shuttles.

Novel aryl derivatives of benzamidine were synthesized and tested for their inhibitory potency against bovine trypsin, rat skin tryptase, human recombinant granzyme A, human thrombin, and human plasma kallikrein. All compounds show competitive inhibition against these proteases with Ki values in the micromolar range. X-ray structures were determined to 1.8 A resolution for trypsin complexed with two of the para-substituted benzamidine derivatives, 1-(4-amidinophenyl)-3-(4-chlorophenyl)urea (ACPU) and 1-(4-amidinophenyl)-3-(4-phenoxyphenyl)urea (APPU). Although the inhibitors do not engage in direct and specific interactions outside the S1 pocket, they do form intimate indirect contacts with the active site of trypsin. The inhibitors are linked to the enzyme by a sulfate ion that forms an intricate network of three-centered hydrogen bonds. Comparison of these structures with other serine protease structures with noncovalently bound oxyanions reveals a pair of highly conserved oxyanion-binding sites in the active site. The positions of noncovalently bound oxyanions, such as the oxygen atoms of sulfate, are distinct from the positions of covalent oxyanions of tetrahedral intermediates. Noncovalent oxyanion positions are outside the "oxyanion hole." Kinetics data suggest that protonation stabilizes the ternary inhibitor/oxyanion/protease complex. In sum, both cations and anions can mediate Ki. Cation mediation of potency of competitive inhibitors of serine proteases was previously reported by Stroud and co-workers [Katz, B. A., Clark, J. M., Finer-Moore, J. S., Jenkins, T. E., Johnson, C. R., Ross, M. J., Luong, C., Moore, W. R., and Stroud, R. M. (1998) Nature 391, 608-612].