[Show abstract][Hide abstract] ABSTRACT: Many Gram-negative bacteria use a type III secretion (T3S) system to directly inject effector molecules into eucaryotic cells in order to establish a symbiotic or pathogenic relationship with their host. The translocation of many T3S proteins requires specialized chaperones from the bacterial cytosol. SycD belongs to a class of T3S chaperones that assists the secretion of pore-forming translocators and, specifically chaperones the translocators YopB and YopD from enteropathogenic Yersinia enterocolitica. In addition, SycD is involved in the regulation of virulence factor biosynthesis and secretion. In this study, we present two crystal structures of Y. enterocolitica SycD at 1.95 and 2.6 A resolution, the first experimental structures of a T3S class II chaperone specific for translocators. The fold of SycD is entirely alpha-helical and reveals three tetratricopeptide repeat-like motifs that had been predicted from amino acid sequence. In both structures, SycD forms dimers utilizing residues from the first tetratricopeptide repeat motif. Using site-directed mutagenesis and size exclusion chromatography, we verified that SycD forms head-to-head homodimers in solution. Although in both structures, dimerization largely depends on the same residues, the two assemblies represent alternative dimers that exhibit different monomer orientations and overall shape. In these two distinct head-to-head dimers, both the concave and the convex surface of each monomer are accessible for interactions with the SycD binding partners YopB and YopD. A SycD variant carrying two point mutations in the dimerization interface is properly folded but defective in dimerization. Expression of this stable SycD monomer in Yersinia does not rescue the phenotype of a sycD null mutant, suggesting a physiological relevance of the dimerization interface.
[Show abstract][Hide abstract] ABSTRACT: Pathogenic Yersinia species use a type III secretion (TTS) system to deliver a number of cytotoxic effector proteins directly into the mammalian host cell. To ensure effective translocation, several such effector proteins transiently bind to specific chaperones in the bacterial cytoplasm. Correspondingly, SycT is the chaperone of YopT, a cysteine protease that cleaves the membrane-anchor of Rho-GTPases in the host. We have analyzed the complex between YopT and SycT and determined the structure of SycT in three crystal forms. Biochemical studies indicate a stoichometric effector/chaperone ratio of 1:2 and the chaperone-binding site contains at least residues 52-103 of YopT. The crystal structures reveal a SycT homodimer with an overall fold similar to that of other TTS effector chaperones. In contrast to the canonical five-stranded anti-parallel beta-sheet flanked by three alpha-helices, SycT lacks the dimerization alpha-helix and has an additional beta-strand capable of undergoing a conformational change. The dimer interface consists of two beta-strands and the connecting loops. Two hydrophobic patches involved in effector binding in other TTS effector chaperones are also found in SycT. The structural similarity of SycT to other chaperones and the spatial conservation of effector-binding sites support the idea that TTS effector chaperones form a single functional and structural group.
Protein Science 09/2005; 14(8):1993-2002. DOI:10.1110/ps.051474605 · 2.85 Impact Factor