Evaluating the GspC protein in substrate specificity of Ralstonia solanacearum type II secretion
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The type II secretion (T2S) system in Ralstonia solanacearum secrets multiple plant cell wall-degrading enzymes and other proteins that are important for pathogenesis. GspC is an essential component of the T2S system, and is one of two proteins thought to help determine substrate specificity. The N-terminal half of GspC is conserved, but the C-terminal half usually has one of two protein-protein interaction domains. However, in R. solanacearum GspC lacks a predicted C-terminal protein-protein interaction domain, and this may contribute to its secreting more proteins than other bacteria. To investigate the role of GspC in the T2S system, I deleted gspC in R. solanacearum GMI1000 and tested various plasmid-borne genes for their ability to restore secretion of polygalacturonase, endoglucanase, pectin methylesterase, and trehalase enzymes. GspC protein from Cupriavidus metallidurans fully restored secretion of the tested enzymes, while Burkholderia thailandensis GspC, which naturally lacks a C-terminal domain, partially restored secretion of some enzymes. These results suggest that general recognition of secreted proteins by the R. solanacearum T2S system lies in both the secreted protein and GspC function.