Figure 2. A. Transmission electron micrograph of Salmonella-induced membrane ruffling in polarized Caco-2 epithelial cells. B. Salmonella invasion into host epithelial cells. Salmonella secrete virulence proteins, including SopE and SptP, into host cells by the type III-secretion system. SopE functions as a guanidine exchange factor for small GTP-binding proteins, probably mediating the exchange of GDP for GTP on a Rho subfamily member, CDC42. SptP is a tyrosine phosphatase required for invasion, probably by disrupting the cytoskeleton. Invasion also stimulates phospholipase C (PLC) activity, leading to inositol triphosphate (IP3) and Ca2+ fluxes, which in turn may be involved in cytoskeletal rearrangements leading to membrane ruffling and Salmonella internalization.