Gongyi Zhang

Although urea and guanidine hydrochloride are commonly used to denature proteins, the molecular underpinnings of this process have remained unclear for a century. To address this question, crystal structures of β-catenin were determined at various urea concentrations. These structures contained at least 105 unique positions that were occupied by urea molecules, each of which interacted with the protein primarilyviahydrogen bonds. Hydrogen-bond competition experiments showed that the denaturing effects of urea were neutralized when polyethylene glycol was added to the solution. These data suggest that urea primarily causes proteins to unfold by competing and disrupting hydrogen bonds in proteins. Moreover, circular-dichroism spectra and nuclear magnetic resonance (NMR) analysis revealed that a similar mechanism caused protein denaturation in the absence of urea at pH levels greater than 12. Taken together, the results led to the conclusion that the disruption of hydrogen bonds is a general mechanism of unfolding induced by urea, high pH and potentially other denaturing agents such as guanidine hydrochloride. Traditionally, the disruption of hydrophobic interactions instead of hydrogen bonds has been thought to be the most important cause of protein denaturation.

The Jumonji C domain is a catalytic motif that mediates histone lysine demethylation. The Jumonji C-containing oxygenase JMJD2A specifically demethylates tri- and dimethylated lysine-9 and lysine-36 of histone 3 (H3K9/36me3/2). Here we present structures of the JMJD2A catalytic core complexed with methylated H3K36 peptide substrates in the presence of Fe(II) and N -oxalylglycine. We found that the interaction between JMJD2A and peptides largely involves the main chains of the enzyme and the peptide. The peptide-binding specificity is primarily determined by the primary structure of the peptide, which explains the specificity of JMJD2A for methylated H3K9 and H3K36 instead of other methylated residues such as H3K27. The specificity for a particular methyl group, however, is affected by multiple factors, such as space and the electrostatic environment in the catalytic center of the enzyme. These results provide insights into the mechanisms and specificity of histone demethylation.

The sarA locus in Staphylococcus aureus controls the expression of many virulence genes. The sarA regulatory molecule, SarA, is a 14.7-kDa protein (124 residues) that binds to the promoter region of target genes. Here we report the 2.6 Å-resolution x-ray crystal structure of the dimeric winged helix SarA protein, which differs from the published SarA structure dramatically. In the crystal packing, multiple dimers of SarA form a scaffold, possibly via divalent cations. Mutations of individual residues within the DNA-binding helix–turn–helix and the winged region as well as within the metal-binding pocket implicate basic residues R84 and R90 within the winged region to be critical in DNA binding, whereas acidic residues D88 and E89 (wing), D8 and E11 (metal-binding pocket), and cysteine 9 are essential for SarA function. These data suggest that the winged region of the winged helix protein participates in DNA binding and activation, whereas the putative divalent cation binding pocket is only involved in gene function.

The expression of virulence determinants in Staphylococcus aureus is controlled by global regulatory loci (e.g., sarA and agr ). One of these determinants, protein A ( spa ), is activated by sarS , which encodes a 250-residue DNA-binding protein. Genetic analysis indicated that the agr locus likely mediates spa repression by suppressing the transcription of sarS . Contrary to SarA and SarR, which require homodimer formation for proper function, SarS is unusual within the SarA protein family in that it contains two homologous halves, with each half sharing sequence similarity to SarA and SarR. Here we report the 2.2 Å resolution X-ray crystal structure of the SarS protein. SarS has folds similar to those of SarR and, quite plausibly, the native SarA structure. Two typical winged-helix DNA-binding domains are connected by a well-ordered loop. The interactions between the two domains are extensive and conserved. The putative DNA-binding surface is highly positively charged. In contrast, negatively charged patches are located opposite to the DNA-binding surface. Furthermore, sequence alignment and structural comparison revealed that MarR has folds similar to those of SarR and SarS. Members of the MarR protein family have previously been implicated in the negative regulation of an efflux pump involved in multiple antibiotic resistance in many gram-negative species. We propose that MarR also belongs to the winged-helix protein family and has a similar mode of DNA binding as SarR and SarS and possibly the entire SarA protein family member. Based on the structural differences of SarR, SarS, and MarR, we further classified these winged-helix proteins to three subfamilies, SarA, SarS, and MarR. Finally, a possible transcription regulation mechanism is proposed.

The expression of virulence determinants in Staphylococcus aureus is controlled by global regulatory loci (e.g., sarA and agr ). The sar ( Staphylococcus accessory regulator) locus is composed of three overlapping transcripts ( sarA P1, P3, and P2, transcripts initiated from the P1, P3, and P2 promoters, respectively), all encoding the 124-aa SarA protein. The level of SarA, the major regulatory protein, is partially controlled by the differential activation of the sarA promoters. We previously partially purified a 13.6-kDa protein, designated SarR, that binds to the sarA promoter region to down-modulate sar A transcription from the P1 promoter and subsequently SarA expression. SarR shares sequence similarity to SarA, and another SarA homolog, SarS. Here we report the 2.3 Å-resolution x-ray crystal structure of the dimeric SarR-MBP (maltose binding protein) fusion protein. The structure reveals that the SarR protein not only has a classic helix–turn–helix module for DNA binding at the major grooves, but also has an additional loop region involved in DNA recognition at the minor grooves. This interaction mode could represent a new functional class of the “winged helix” family. The dimeric SarR structure could accommodate an unusually long stretch of ≈27 nucleotides with two or four bending points along the course, which could lead to the bending of DNA by 90° or more, similar to that seen in the catabolite activator protein (CAP)–DNA complex. The structure also demonstrates the molecular basis for the stable dimerization of the SarR monomers and possible motifs for interaction with other proteins.