The PDZ domain of EpsC is required for extracellular secretion of VesB by the Type II secretion system in Vibrio cholerae.

The Type II Secretion System (T2SS) of the Gram-negative pathogen Vibrio cholerae is considered a virulence factor primarily because it facilitates the secretion of cholera toxin upon arrival of this pathogen in the human intestinal lumen, resulting in the profuse watery diarrhea associated with infection. Results from transposon mutagenesis screens suggest that the V. cholerae eps genes that code for the T2SS are essential. The deletion of these genes impairs the secretion of the T2SS's nearly two dozen protein substrates, leading to the selection of suppressor mutations, extracytoplasmic stress, and poor growth of the mutant bacteria. Here, we explore the role of the C-terminal PDZ domain of the T2SS component EpsC, and why transposon insertions in pdz do not affect bacterial viability unlike insertions in most other eps genes. Our data show that the deletion of pdz does not affect the growth of V. cholerae, and we demonstrate via mass spectrometry that the serine protease VesB is the only known T2SS substrate fully reliant on PDZ for secretion. We confirm this finding with immuno- and enzyme assays and show that the immunoglobulin (Ig)-like fold of VesB contains PDZ-dependent secretion information that can be grafted onto β-lactamase, causing this normally periplasmic protein to be secreted in a PDZ-dependent manner. This work adds to a growing body of evidence about substrate selection by the T2SS and reinforces the view that the C-terminal domain of EpsC and its homologs are required for the secretion of only a subset of T2SS substrates within a given species.

Importance: The T2SS is common in Gram-negative pathogens, facilitating the secretion of various toxins and enzymes; however, the mechanisms of substrate selection and secretion remain poorly understood. Here, we demonstrate that the C-terminal PDZ domain of the T2SS "clamp" protein EpsC is only required for secretion of VesB in V. cholerae and that VesB's Ig-fold contains PDZ-dependent secretion information that can be functionally grafted onto a non-secreted periplasmic protein.