The type III secretion apparatus encoded on the low-calcium response (LCR) virulence plasmid, pCD1 in strain KIM5, of Y. pestis is a conserved virulence mechanism that is absolutely required for virulence of Y. pestis. YscF is a surface localized protein required both to secrete Yops and to translocate toxins into eukaryotic cells. YscF polymerization is possibly required for a YscF needle to puncture eukaryotic cell membranes. It is also likely that YscF and its homologs function to provide a base that a translocon complex is built upon, or that YscF builds a conduit from the bacterium to the eukaryotic membrane - this suggestion seems more likely given that other proteins such as YopB, YopD, and LcrV are also required for translocation into eukaryotic cells. YscF is thought to be a surface-expressed protein in Y. pestis and is required for virulence. YscF has been found to function as a protective antigen against experimental plague infection (Matson et al., 2005).
