The Chlamydia Type III Secretion System C-ring Engages a Chaperone-Effector Protein Complex

Abstract

In Gram-negative bacterial pathogens, specialized chaperones bind to secreted effector proteins and maintain them in a partially unfolded form competent for translocation by type III secretion systems/injectisomes. How diverse sets of effector-chaperone complexes are recognized by injectisomes is unclear. Here we describe a new mechanism of effector-chaperone recognition by the Chlamydia injectisome, a unique and ancestral line of these evolutionarily conserved secretion systems. By yeast two-hybrid analysis we identified networks of Chlamydia-specific proteins that interacted with the basal structure of the injectisome, including two hubs of protein-protein interactions that linked known secreted effector proteins to CdsQ, the putative cytoplasmic C-ring component of the secretion apparatus. One of these protein-interaction hubs is defined by Ct260/Mcsc (Multiple cargo secretion chaperone). Mcsc binds to and stabilizes at least two secreted hydrophobic proteins, Cap1 and Ct618, that localize to the membrane of the pathogenic vacuole (“inclusion”). The resulting complexes bind to CdsQ, suggesting that in Chlamydia, the C-ring of the injectisome mediates the recognition of a subset of inclusion membrane proteins in complex with their chaperone. The selective recognition of inclusion membrane proteins by chaperones may provide a mechanism to co-ordinate the translocation of subsets of inclusion membrane proteins at different stages in infection.

Biochemical and Localization Analyses of Putative Type III Secretion Translocator Proteins CopB and CopB2 of Chlamydia trachomatis Reveal Significant Distinctions

“…Previous studies have revealed that CopB is secreted via a T3S mechanism and is detectible by immunolocalization in a pattern consistent with inclusion membrane localization (15). Similarly to Yersinia YopB, CopB interacts with a T3S chaperone (Scc2) that is encoded immediately upstream from its own gene (15,49). Scc2 also interacts with CopD, the putative YopD-like translocator component (49).…”

“…Similarly to Yersinia YopB, CopB interacts with a T3S chaperone (Scc2) that is encoded immediately upstream from its own gene (15,49). Scc2 also interacts with CopD, the putative YopD-like translocator component (49).…”

“…For example, the upstream-encoded T3S chaperone Scc3 does not associate with CopB2 (15) but instead appears to bind another secreted protein, CopN (48,49). CopB2 does not exhibit apparent homology with any YopB family proteins.…”

The type III secretion injectisome, a complex nanomachine for intracellular ‘toxin’ delivery

“…Together these data indicate that there is a C-ring in the injectisome even though it could not be visualized by cryo-EM thus far. It would form a platform at the cytoplasm/inner membrane interface for the recruitment of the ATPase complex (see below) (Jackson and Plano, 2000) and substratechaperone complexes (Morita-Ishihara et al, 2006;Spaeth et al, 2009). …”

Identification and biochemical characterization of the CHLAMYDIA TRACHOMATIS type III secretion chaperone, SLC1, and its role in the translocation of the invasion-associated effector TARP.

“…CdsD has been shown to interact with the cytoplasmic ATPase CdsN and with CdsL, a possible negative regulator of ATPase activity [35]. A complex network of interactions has been shown to occur in vitro between CdsQ, CdsD, CdsN, and CdsL [35,49]. Upon assembly of the inner membrane and C-ring components, the machinery becomes secretion competent and secretes its needle subunits CdsF [27], possibly with the help of the putative molecular ruler protein CdsP [50].…”