A bacterial type III secretion system inhibits actin polymerization to prevent pore formation in host cell membranes

The bacterial pathogen Yersinia pseudotuberculosis uses type III secretion machinery to translocate Yop effector proteins through host cell plasma mem branes. A current model suggests that a type III translocation channel is i nserted into the plasma membrane, and if Yops are not present to fill the c hannel, the channel will form a pore. We examined the possibility that Yops act within the host cell to prevent pore formation. Yop(-) mutants of Y. p seudotuberculosis were assayed for pore-forming activity in HeLa cells. A Y opE(-) mutant exhibited high levels of pore-forming activity. The GTPase-do wnregulating function of YopE was required to prevent pore formation. YopE( +) bacteria had increased pore-forming activity when HeLa cells expressed a ctivated Rho GTPases. Pore formation by YopE- bacteria required actin polym erization. F-actin was concentrated at sites of contact between HeLa cells and YopE- bacteria. The data suggest that localized actin polymerization, t riggered by the type III machinery, results in pore formation in cells infe cted with YopE- bacteria. Thus, translocated YopE inhibits actin polymeriza tion to prevent membane damage to cells infected with wild-type bacteria.