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Many enzymes are dynamic entities, sampling conformational states that are relevant for catalytic activity. Crystal structures of catalytic intermediates suggest, however, that not all enzymes require structural changes for activity. The single-domain enzyme xylanase fromBacillus circulans(BCX) is involved in the degradation of hemicellulose. We demonstrate that BCX in solution undergoes minimal structural changes during catalysis. NMR spectroscopy results show that the rigid protein matrix provides a frame for fast substrate binding in multiple conformations, accompanied by slow, enzyme induced substrate distortion. Therefore, we propose a model in which the rigid enzyme takes advantage of substrate flexibility to induce a conformation that facilitates catalysis.One Sentence SummaryThe rigid matrix of BCX uses substrate flexibility in Michaelis complex formation.

Strong (1)H-(1)H coupling can significantly reduce the accuracy of (1)J(CH) measured from frequency differences in coupled HSQC spectra. Although accurate (1)J(CH) values can be extracted from spectral simulation, it would be more convenient if the same accurate (1)J(CH) values can be obtained experimentally. Furthermore, simulations reach their limit for residual dipolar coupling (RDC) measurement, as many significant, but immeasurable RDCs are introduced into the spin system when a molecule is weakly aligned, thus it is impossible to have a model spin system that truly represents the real spin system. Here we report a new J modulated method, constant-time INEPT CT-HSQC (CTi-CT-HSQC), to accurately measure one-bond scalar coupling constant and RDCs without strong coupling interference. In this method, changing the spacing between the two 180° pulses during a constant time INEPT period selectively modulates heteronuclear coupling in quantitative J fashion. Since the INEPT delays for measuring one-bond carbon-proton spectra are short compared to (3)J(HH), evolution due to (strong) (1)H-(1)H coupling is marginal. The resulting curve shape is practically independent of (1)H-(1)H coupling and only correlated to the heteronuclear coupling evolution. Consequently, an accurate (1)J(CH) can be measured even in the presence of strong coupling. We tested this method on N-acetyl-glucosamine and mannose whose apparent isotropic (1)J(CH) values are significantly affected by strong coupling with other methods. Agreement to within 0.5Hz or better is found between (1)J(CH) measured by this method and previously published simulation data. We further examined the strong coupling effects on RDC measurements and observed an error up to 100% for one bond RDCs using coupled HSQC in carbohydrates. We demonstrate that RDCs can be obtained with higher accuracy by CTi-CT-HSQC, which compensates the limitation of simulation method.

Background Recognition of histone modifications by specialized protein domains is a key step in the regulation of DNA-mediated processes like gene transcription. The structural basis of these interactions is usually studied using histone peptide models, neglecting the nucleosomal context. Here, we provide the structural and thermodynamic basis for the recognition of H3K36-methylated (H3K36me) nucleosomes by the PSIP1-PWWP domain, based on extensive mutational analysis, advanced nuclear magnetic resonance (NMR), and computational approaches. Results The PSIP1-PWWP domain binds H3K36me3 peptide and DNA with low affinity, through distinct, adjacent binding surfaces. PWWP binding to H3K36me nucleosomes is enhanced approximately 10,000-fold compared to a methylated peptide. Based on mutational analyses and NMR data, we derive a structure of the complex showing that the PWWP domain is bound to H3K36me nucleosomes through simultaneous interactions with both methylated histone tail and nucleosomal DNA. Conclusion Concerted binding to the methylated histone tail and nucleosomal DNA underlies the high- affinity, specific recognition of H3K36me nucleosomes by the PSIP1-PWWP domain. We propose that this bipartite binding mechanism is a distinctive and general property in the recognition of histone modifications close to the nucleosome core.

SummaryTet proteins oxidize 5-methylcytosine (mC) to generate 5-hydroxymethyl (hmC), 5-formyl (fC), and 5-carboxylcytosine (caC). The exact function of these oxidative cytosine bases remains elusive. We applied quantitative mass-spectrometry-based proteomics to identify readers for mC and hmC in mouse embryonic stem cells (mESC), neuronal progenitor cells (NPC), and adult mouse brain tissue. Readers for these modifications are only partially overlapping, and some readers, such as Rfx proteins, display strong specificity. Interactions are dynamic during differentiation, as for example evidenced by the mESC-specific binding of Klf4 to mC and the NPC-specific binding of Uhrf2 to hmC, suggesting specific biological roles for mC and hmC. Oxidized derivatives of mC recruit distinct transcription regulators as well as a large number of DNA repair proteins in mouse ES cells, implicating the DNA damage response as a major player in active DNA demethylation.

J couplings are essential for measuring RDCs (residual dipolar couplings), now routinely used to deduce molecular structure and dynamics of glycans and proteins. Accurate measurement of 1JCH is critical for RDCs to reflect the true structure and dynamics in the molecule of interest. We report noticeable discrepancies between 1JCH values measured with HSQC type pulse sequences in the 1H dimension from those measured in the 13C dimension for 17 sugars and show that these discrepancies arise from strong scalar coupling. In order to determine how to minimize errors in measuring 1JCH, we analyze the strong coupling effects in detail using the product operator-formalism and spectral simulations based on the solution of the Liouville equation (not considering relaxation effects) in the presence of strong coupling. We report that the apparent 1JCH measured with 2D HSQC-based sequences in either dimension can be in error by up to 4 Hz and that the values measured in the 1H dimension can disagree with those in the 13C dimension by up to 7 Hz. We demonstrate that spectral simulations can reproduce the errors induced by strong coupling. and that these can be used to extract true 1JCH values. We find that the 1JCH values measured using a modified Z-filtered coupled HSQC are still affected by strong coupling. We conclude that spectral simulation yields accurate 1JCH with errors as low as 1% in the presence of strong coupling.

J couplings are essential for measuring RDCs (residual dipolar couplings), now routinely used to deduce molecular structure and dynamics of glycans and proteins. Accurate measurement of 1 J CH is critical for RDCs to reflect the true structure and dynamics in the molecule of interest. We report noticeable discrepancies between 1 J CH values measured with HSQC type pulse sequences in the 1 H dimension from those measured in the 13 C dimension for 17 sugars and show that these discrepancies arise from strong scalar coupling. In order to determine how to minimize errors in measuring 1 J CH , we analyze the strong coupling effects in detail using the product operator-formalism and spectral simulations based on the solution of the Liouville equation (not considering relaxation effects) in the presence of strong coupling. We report that the apparent 1 J CH measured with 2D HSQC-based sequences in either dimension can be in error by up to 4 Hz and that the values measured in the 1 H dimension can disagree with those in the 13 C dimension by up to 7 Hz. We demonstrate that spectral simulations can reproduce the errors induced by strong coupling and that these can be used to extract true 1 J CH values. We find that the 1 J CH values measured using a modified Z-filtered coupled HSQC are still affected by strong coupling. We conclude that spectral simulation yields accurate 1 J CH with errors as low as 1% in the presence of strong coupling.

Leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), one of the most widely spread immune receptors, attenuates immune cell activation when bound to specific sites in collagen. The collagen-binding domain of LAIR-1 is homologous to that of glycoprotein VI (GPVI), a collagen receptor crucial for platelet activation. Because LAIR-1 and GPVI also display overlapping collagen-binding specificities, a common structural basis for collagen recognition would appear likely. Therefore, it is crucial to gain insight into the molecular interaction of both receptors with their ligand to prevent unwanted cross-reactions during therapeutic intervention. We determined the crystal structure of LAIR-1 and mapped its collagen-binding site by nuclear magnetic resonance (NMR) titrations and mutagenesis. Our data identify R59, E61, and W109 as key residues for collagen interaction. These residues are strictly conserved in LAIR-1 and GPVI alike; however, they are located outside the previously proposed GPVI collagen-binding site. Our data provide evidence for an unanticipated mechanism of collagen recognition common to LAIR-1 and GPVI. This fundamental insight will contribute to the exploration of specific means of intervention in collagen-induced signaling in immunity and hemostasis.

Pax-8 is a transcription factor belonging to the PAX genes superfamily and its crucial role has been proven both in embryo and in the adult organism. Pax-8 activity is regulated via a redoxbased mechanism centered on the glutathionylation of specific cysteines in the N-terminal region (Cys45 and Cys57). These residues belong to a highly evolutionary conserved DNA binding site: the Paired Box (Prd) domain. Crystallographic protein-DNA complexes of the homologues Pax-6 and Pax-5 showed a bipartite Prd domain consisting of two helix-turn-helix (HTH) motifs separated by an extended linker region. Here, by means of nuclear magnetic resonance, we show for the first time that the HTH motifs are largely defined in the unbound Pax-8 Prd domain. Our findings contrast with previous induced fit models, in which Pax-8 is supposed to largely fold upon DNA binding. Importantly, our data provide the structural basis for the enhanced chemical reactivity of residues Cys45 and Cys57 and explain clinical missense mutations that are not obviously related to the DNA binding interface of the paired box domain. Finally, sequence conservation suggests that our findings could be a general feature of the Pax family transcription factors.

Trimethylation of lysine residue K4 of histone H3 (H3K4me3) strongly correlates with active promoters for RNA polymerase II-transcribed genes. Several reader proteins, including the basal transcription factor TFIID, for this nucleosomal mark have been identified. Its TAF3 subunit specifically binds the H3K4me3 mark via its conserved plant homeodomain (PHD) finger. Here, we report the solution structure of the TAF3-PHD finger and its complex with an H3K4me3 peptide. Using a combination of NMR, mutagenesis, and affinity measurements, we reveal the structural basis of binding affinity, methylation-state specificity, and crosstalk with asymmetric dimethylation of R2. A unique local structure rearrangement in the K4me3-binding pocket of TAF3 due to a conserved sequence insertion underscores the requirement for cation-π interactions by two aromatic residues. Interference by asymmetric dimethylation of arginine 2 suggests that a H3R2/K4 “methyl-methyl” switch in the histone code dynamically regulates TFIID-promoter association.