Surface proteins of Gram-positive bacteria play important roles during the pathogenesis of human infections and require sortase for anchoring to the cell-wall envelope. Sortase cleaves surface proteins at the LPXTG motif and catalyzes the formation of an amide bond between the carboxyl group of threonine (T) and the amino group of cell-wall crossbridges. The NMR structure of sortase reveals a unique ␤-barrel structure, in which the active-site sulfhydryl of cysteine-184 is poised for ionization by histidine-120, presumably enabling the resultant thiolate to attack the LPXTG peptide. Calcium binding near the active site stimulates catalysis, possibly by altering the conformation of a surface loop that recognizes newly translocated polypeptides. The structure suggests a mechanistic relationship to the papain͞cathepsin proteases and should facilitate the design of new antiinfective agents.

SUMMARY RING1B, a Polycomb Group (PcG) protein, binds methylated chromatin through its association with another PcG protein called Polycomb (Pc). However, RING1B can associate with nonmethylated chromatin suggesting an alternate mechanism for RING1B interaction with chromatin. Here, we demonstrate that two proteins with little sequence identity between them, the Pc cbox domain and RYBP, bind the same surface on the C-terminal domain of RING1B (C-RING1B). Pc cbox and RYBP each fold into a nearly identical, intermolecular beta sheet with C-RING1B and a loop structure which are completely different in the two proteins. Both the beta sheet and loop are required for stable binding and transcription repression. Further, a mutation engineered to disrupt binding on the Drosophila dRING1 protein prevents chromatin association and PcG function in vivo. These results suggest that PcG targeting to different chromatin locations relies, in part, on binding partners of C-RING1B that are diverse in sequence and structure.

Tyrosine hydroxylase (TyrH) catalyzes the hydroxylation of tyrosine to form 3,4-dihydroxyphenylalanine in the biosynthesis of the catecholamine neurotransmitters. The activity of the enzyme is regulated by phosphorylation of serine residues in a regulatory domain and by binding of catecholamines to the active site. Available structures of TyrH lack the regulatory domain, limiting the understanding of the effect of regulation on structure. We report the use of NMR spectroscopy to analyze the solution structure of the isolated regulatory domain of rat TyrH. The protein is composed of a largely unstructured N-terminal region (residues 1-71) and a well-folded C-terminal portion (residues 72-159). The structure of a truncated version of the regulatory domain containing residues 65-159 has been determined and establishes that it is an ACT domain. The isolated domain is a homodimer in solution, with the structure of each monomer very similar to that of the core of the regulatory domain of phenylalanine hydroxylase. Two TyrH regulatory domain monomers form an ACT domain dimer composed of a sheet of eight strands with four α-helices on one side of the sheet. Backbone dynamic analyses were carried out to characterize the conformational flexibility of TyrH65-159. The results provide molecular details critical for understanding the regulatory mechanism of TyrH.

Many virulence factors in GramSurface proteins on bacteria are frequently virulence factors, promoting bacterial adhesion, resistance to phagocytic killing, and host cell invasion during infection. In Gram-positive bacteria these proteins are often covalently anchored to the cell wall by sortase enzymes, a family of novel cysteine transpeptidases (1-3). The sortase A protein (SrtA) 2 from Staphylococcus aureus has been characterized extensively (4) and anchors proteins bearing a cell wall sorting signal that consists of a conserved LPXTG motif (where X is any amino acid), a hydrophobic domain, and a tail of mostly positively charged residues (4 -6). SrtA cleaves in between the threonine and glycine of the LPXTG motif (7) and catalyzes the formation of a peptide bond between the carboxylgroup of the threonine and the amine-group of the cell-wall precursor lipid II (7-9). The lipid II-linked protein is then incorporated into the peptidoglycan of the cell wall via the transglycosylation and transpeptidation reactions of bacterial cell-wall synthesis. Sortases represent an attractive target for new anti-infective agents, because they are widely distributed among a variety of bacterial pathogens (10, 11) (e.g. Bacillus anthracis, Listeria monocytogenes, Streptococcus pneumoniae, and Streptococcus pyogenes), and have been shown to be required for virulence (12-16).The catalytic domain of SrtA (SrtA ⌬N59 , residues 60 -206) adopts a conserved eight-stranded ␤-barrel fold (17, 18). The active site is organized around the catalytically essential side chain of Cys-184, whose thiolate nucleophilically attacks the threonine carbonyl carbon within the LPXTG sorting signal, forming a thioester linkage between the enzyme and substrate (19). In addition to Cys-184, the hydrophilic side chains of His-120 and Arg-197 are absolutely required for catalysis (20 -22). These residues likely participate in general acid/base catalysis, and one of them must activate the thiol for nucleophilic attack, because it is protonated at neutral pH (23). The indole ring of Trp-194 partially shields the cysteine thiol from the solvent, and its mutation to alanine reduces enzyme activity 4-fold through an unknown mechanism (20). Using NMR and crystallography, the LPXTG sorting signal binding site has recently been localized to a surface formed by strands ␤4 and ␤7, and to a proximal loop that connects strands ␤6 to ␤7(the ␤6/␤7 loop) (18,22). Substrate binding may occur through an induced-fit mechanism involving conformational changes in the ␤6/␤7 loop, because it is disordered in the absence of the sorting signal substrate (17,18). Ca 2ϩ stimulates the activity of SrtA ⌬N59 in vitro (17) and may enable S. aureus to increase the rate of surface protein anchoring as it encounters elevated concentrations of this ion at sites of infection. Because many surface proteins function as virulence factors, the stimulatory effect of Ca 2ϩ likely plays an important role in the infection process. Previously we showed that Ca 2ϩ bound to an ordered pocket positio...

Background:The mechanism by which Sterile Alpha Motifs (SAMs) self-associate and polymerize to control protein function is unknown. Results: SAM polymerization in Polyhomeotic, a Polycomb group protein, is controlled by an unstructured linker sequence in Polyhomeotic. Conclusion: Polyhomeotic growth suppressive function is enhanced by increasing SAM polymerization. Significance: Functions of other SAM domain-containing proteins could be manipulated through their unstructured linkers.