Molecular Mechanisms of Protein Sorting by the Type II Secretion System

The gram-negative pathogen Vibrio cholerae is the causative agent of the acute diarrheal disease cholera. The ability of V. cholerae to cause disease and form multicellular communities in the form of biofilm is dependent on secreted and surface localized factors. Several of these proteins are secreted by the type II secretion system (T2SS), a translocation system that transports folded proteins through the bacterial outer membrane. With the aid of N-terminal signal peptides, the T2S substrates translocate across the inner membrane via the Sec or Tat pathways. Following removal of the signal peptides and folding the proteins engage with the multicomponent T2S complex for outer membrane translocation. Most T2S substrates are released to the extracellular space once transported through the outer membrane; however, a subset of proteins remain surface associated or may reattach to the bacterial cell surface following extracellular release. Cholera toxin, the main virulence factor of V. cholerae is a completely secreted substrate, while the trypsin-like protease VesB is primarily found on the cell surface. VesB belongs to a unique class of extracellular enzymes that have a C-terminal extension consisting of two prominent glycines and a hydrophobic helix followed by positively charged residues (GlyGly-CTERM domain), a tripartite sequence that resembles the sortase targeting motif of surface anchored proteins in Gram positive bacteria. As VesB is transported through the cell envelope, the GlyGly-CTERM domain is cleaved off by rhombosortase, a newly discovered member of the intramembrane rhomboid protease family, and the posttranslationally modified VesB is localized to the cell surface. When VesB is produced in the absence of rhombosortase or without the GlyGly-CTERM domain, it is no longer detected on the V. cholerae cell surface. The experiments described in this proposal are designed to test the hypothesis that the T2S system, in collaboration with rhombosortase, promotes the maturation and surface localization of GlyGly-CTERM containing proteins thus contributing to biofilm formation and pathogenesis of V. cholerae. Specifically, this proposal will determine the mechanism of surface anchoring of VesB and other proteins with GlyGly-CTERM extensions, reveal the relationship between rhombosortase and the T2S apparatus, and define the role(s) of GlyGly-CTERM proteins, which include several proteases, in biofilm formation and pathogenesis. The findings will facilitate understanding of the function and specificity of rhombosortase as well as the broader class of medically relevant rhomboid proteases and may identify ways to manipulate the T2S/rhombosortase system for preventative and/or therapeutic use.