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Endolysins produced by bacteriophages play essential roles in the release of phage progeny by degrading the peptidoglycan layers of the bacterial cell wall. Bacteriophage-encoded endolysins have emerged as a new class of antibacterial agents to combat surging antibiotic resistance. The crystal structure of mtEC340M, an engineered endolysin EC340 from the PBEC131 phage that infects Escherichia coli, was determined. The crystal structure of mtEC340M at 2.4 Å resolution consists of eight α-helices and two loops. The three active residues of mtEC340M were predicted by structural comparison with peptidoglycan-degrading lysozyme.

Interactions involving Epstein–Barr virus (EBV) LMP2A and Nedd4 family E3 ubiquitin–protein ligases promote the ubiquitination of LMP2A-associated proteins, which results in the perturbation of normal B-cell signaling. Here, we solved the solution structure of the WW2 domain of hAIP4 and investigated the binding mode involving the N-terminal domain of LMP2A and the WW2 domain. The WW2 domain presented a conserved WW domain scaffold with a three-stranded anti-parallel β-sheet and bound two PY motifs via different binding mechanisms. Our NMR titration and ITC data demonstrated that the PY motifs of LMP2A can recognize and interact weakly with the XP groove of the WW2 domain (residues located around the third β-strand), and then residues between two PY motifs optimize the binding by interacting with the loop 1 region of the WW2 domain. In particular, the residue Val15 in the hairpin loop region between β1 and β2 of the WW2 domain exhibited unique changes depending on the terminal residues of the PY motif. This result suggested that the hairpin loop is responsible for additional interactions outside the XP groove, and this hypothesis was confirmed in a deuterium exchange experiment. These weak but wide interactions can stabilize the complex formed between the PY and WW domains.

Cellular metabolites act as important signaling cues, but are subject to complex unknown chemistry. Kynurenine is a tryptophan metabolite that plays a crucial role in cancer and the immune system. Despite its atypical, non-ligand-like, highly polar structure, kynurenine activates the aryl hydrocarbon receptor (AHR), a PER, ARNT, SIM (PAS) family transcription factor that responds to diverse environmental and cellular ligands. The activity of kynurenine is increased 100–1000-fold by incubation or long-term storage and relies on the hydrophobic ligand-binding pocket of AHR, with identical structural signatures for AHR induction before and after activation. We purified trace-active derivatives of kynurenine and identified two novel, closely related condensation products, named trace-extended aromatic condensation products (TEACOPs), which are active at low picomolar levels. The synthesized compound for one of the predicted structures matched the purified compound in both chemical structure and AHR pharmacology. Our study provides evidence that kynurenine acts as an AHR pro-ligand, which requires novel chemical conversions to act as a receptor agonist.

The aryl hydrocarbon receptor (AHR) belongs to the PAS (PER-ARNT-SIM) family transcription factors and mediates broad responses to numerous environmental pollutants and cellular metabolites, modulating diverse biological processes from adaptive metabolism, acute toxicity, to normal physiology of vascular and immune systems. The AHR forms a transcriptionally active heterodimer with ARNT (AHR nuclear translocator), which recognizes the dioxin response element (DRE) in the promoter of downstream genes. We determined the crystal structure of the mammalian AHR-ARNT heterodimer in complex with the DRE, in which ARNT curls around AHR into a highly intertwined asymmetric architecture, with extensive heterodimerization interfaces and AHR interdomain interactions. Specific recognition of the DRE is determined locally by the DNA-binding residues, which discriminates it from the closely related hypoxia response element (HRE), and is globally affected by the dimerization interfaces and interdomain interactions. Changes at the interdomain interactions caused either AHR constitutive nuclear localization or failure to translocate to nucleus, underlying an allosteric structural pathway for mediating ligand-induced exposure of nuclear localization signal. These observations, together with the global higher flexibility of the AHR PAS-A and its loosely packed structural elements, suggest a dynamic structural hierarchy for complex scenarios of AHR activation induced by its diverse ligands.

Esterases comprise a group of enzymes that catalyze the cleavage and synthesis of ester bonds. They are important in biotechnological applications owing to their enantioselectivity, regioselectivity, broad substrate specificity, and the fact that they do not require cofactors. In a previous study, we isolated the esterase Est25 from a metagenomic library. Est25 showed catalytic activity toward the (R,S)-ketoprofen ethyl ester but had low enantioselectivity toward the (S)-ketoprofen ethyl ester. Because (S)-ketoprofen has stronger anti-inflammatory effects and fewer side effects than (R)-ketoprofen, enantioselectivity of this esterase is important. In this study, we generated Est25 mutants with improved enantioselectivity toward the (S)-ketoprofen ethyl ester; improved enantioselectivity of mutants was established by analysis of their crystal structures. The enantioselectivity of mutants was influenced by substitution of Phe72 and Leu255. Substituting these residues changed the size of the binding pocket and the entrance hole that leads to the active site. The enantioselectivity of Est25 (E = 1.1 ± 0.0) was improved in the mutants F72G (E = 1.9 ± 0.2), L255W (E = 16.1 ± 1.1), and F72G/L255W (E = 60.1 ± 0.5). Finally, characterization of Est25 mutants was performed by determining the optimum reaction conditions, thermostability, effect of additives, and substrate specificity after substituting Phe72 and Leu255.

In toxin-antitoxin systems, many antitoxin proteins that neutralize their cognate toxin proteins also bind to DNA to repress transcription, and the DNA-binding affinity of the antitoxin is affected by its toxin. We solved crystal structures of the antitoxin HigA (apo-SfHigA) and its complex with the toxin HigB (SfHigBA) from Shigella flexneri. The apo-SfHigA shows a distinctive V-shaped homodimeric conformation with sequestered N-domains having a novel fold. SfHigBA appears as a heterotetramer formed by N-terminal dimerization of SfHigB-bound SfHigA molecules. The conformational change in SfHigA upon SfHigB binding is mediated by rigid-body movements of its C-domains, which accompanied an overall conformational change from wide V-shaped to narrow V-shaped dimer. Consequently, the two putative DNA-binding helices (α7 in each subunit) are repositioned to a conformation more compatible with canonical homodimeric DNA-binding proteins containing HTH motifs. Collectively, this study demonstrates a conformational change in an antitoxin protein, which occurs upon toxin binding and is responsible for regulating antitoxin DNA binding.

The inositol 1,4,5-triphosphate receptor (IP3R), an IP3-gated Ca2+ release channel on the endoplasmic reticulum (ER) membrane, plays a critical role in maintaining cytosolic Ca2+ homeostasis in cells. Particularly, ATP increases IP3R activity by binding to ATPA, a putative glycine-rich Walker A-type motif (GXGXXG) specific to type 1 IP3R (IP3R1). Here, we established an efficient process to produce the ATPA containing domain of IP3R1 (IP3R1-ATPA) using a chaperone co-expression system in Escherichia coli. The recombinant protein was well expressed as a soluble form and showed a high thermostability. Circular dichroism results indicated a mainly α-helical conformation of the purified protein. Additionally, model structures of IP3R1-ATPA were calculated and validated using different modeling algorithms. The structural models of IP3R1-ATPA not only supported the observed high thermostability, but also suggested a potential ATP binding site.

Shigella flexneriis a Gram-negative, anaerobic bacterium in the genusShigellathat can cause diarrhoea in humans. SF173, a hypothetical protein fromS. flexneri5a strain M90T, has been cloned, overexpressed, purified and crystallized as a part of laboratory-scale structural genomics project. The SF173 protein was crystallized using the sitting-drop vapour-diffusion method in the presence of 0.8 Msuccinic acid pH 7.0 at 293 K. Preliminary X-ray diffraction analysis revealed that the crystal diffracted to 1.47 Å resolution and belonged to space groupI432, with unit-cell parametersa=b=c= 110.245 Å.

Fragment engineering of monoclonal antibodies (mAbs) has emerged as an excellent paradigm to develop highly efficient therapeutic and/or diagnostic agents. Engineered mAb fragments can be economically produced in bacterial systems using recombinant DNA technologies. In this work, we established recombinant production in Escherichia coli for monovalent antigen-binding fragment (Fab) adopted from a clinically used anticancer mAB drug cetuximab targeting epidermal growth factor receptor (EGFR). Recombinant DNA constructs were designed to express both polypeptide chains comprising Fab in a single vector and to secrete them to bacterial periplasmic space for efficient folding. Particularly, a C-terminal engineering to confer an interchain disulfide bond appeared to be able to enhance its heterodimeric integrity and EGFR-binding activity. Conformational relevance of the purified final product was validated by mass spectrometry and crystal structure at 1.9 A resolution. Finally, our recombinant cetuximab-Fab was found to have strong binding affinity to EGFR overexpressed in human squamous carcinoma model (A431) cells. Its binding ability was comparable to that of cetuximab. Its EGFR-binding affinity was estimated at approximately 0.7 nM of Kd in vitro, which was quite stronger than the binding affinity of natural ligand EGF. Hence, the results validate that our construction could serve as an efficient platform to produce a recombinant cetuximab-Fab with a retained antigen-binding functionality.

Heat shock protein (Hsp)70 is a molecular chaperone that maintains protein homoeostasis during cellular stress through two opposing mechanisms: protein refolding and degradation. However, the mechanisms by which Hsp70 balances these opposing functions under stress conditions remain unknown. Here, we demonstrate that Hsp70 preferentially facilitates protein refolding after stress, gradually switching to protein degradation via a mechanism dependent on ARD1-mediated Hsp70 acetylation. During the early stress response, Hsp70 is immediately acetylated by ARD1 at K77, and the acetylated Hsp70 binds to the co-chaperone Hop to allow protein refolding. Thereafter, Hsp70 is deacetylated and binds to the ubiquitin ligase protein CHIP to complete protein degradation during later stages. This switch is required for the maintenance of protein homoeostasis and ultimately rescues cells from stress-induced cell death in vitro and in vivo. Therefore, ARD1-mediated Hsp70 acetylation is a regulatory mechanism that temporally balances protein refolding/degradation in response to stress., The chaperone Hsp70 has a dual role, promoting both protein refolding and protein degradation. Seo and Park et al. show that Hsp70 acetylation enhances protein refolding after stress, and that subsequent deacetylation progressively promotes ubiquitin ligase binding and protein degradation.

We report the crystal structure and bioinformatic analysis of SF173, a functionally uncharacterized protein from the human enteropathogenic bacteria Shigella flexneri. The structure shows a tightly interlinked dimer formed by adimeric core comprising α2 and α3 helices from both subunits and swapping the N-terminal α1 helix of each monomer. Structural inspection and genomic analysis results suggest that the SF173 might play its putative function by binding to SF172, the partially overlapped upstream product in the operon. As YaeO (an SF172 orthologue) has been identified to be an inhibitor of the bacterial transcription terminator Rho protein, SF173 is suggested to be involved in the regulation of Rho-dependent transcription termination, by inhibiting the Rho protein binding to SF172/YaeO.