LcrV, a substrate for Yersinia enterocolitica type III secretion, is required for toxin targeting into the cytosol of HeLa cells

Pathogenic Yersinia species employ type III machines to transport virulence factors across the bacterial envelope. Some substrates for the type III ma chinery are secreted into the extracellular medium, whereas others are targ eted into the cytosol of host cells. We found that during infection of tiss ue culture cells, yersiniae secrete small amounts of LcrV into the extracel lular medium. Knockout mutations of lcrV abolish Yersinia targeting and red uce expression of the lcrGVHyopBD operon. In contrast, a block in LcrV secr etion does not affect targeting, but results in premature expression and se cretion of Yop proteins into the extracellular medium. LcrV-mediated activa tion of the type III pathway is thought to occur by sequestration of the re gulatory factor LcrG, presumably via the formation of LcrV LcrG complexes. These results suggest that intrabacterial LcrV regulates the expression and targeting of Yop proteins during Yersinia infection, whereas secreted LcrV is required to ensure specificity of Yop injection into eukaryotic cells.