An invasion-associated Salmonella protein modulates the actin-bundling activity of plastin

The entry of Salmonella typhimurium into nonphagocytic cells requires a panel of bacterial effector proteins that are delivered to the host cell via a type III secretion system. These proteins modulate host-cell signal-transduction pathways and the actin cytoskeleton to induce membrane ruffling and bacterial internalization. One of these bacterial effecters, termed SipA, is an actin-binding protein that is required for efficient Salmonella entry into host cells. We report here that SipA forms a complex with T-plastin on bacterial infection. Formation of such a complex, which requires the presence of F-actin, results in a marked increase in the actin-bundling activity of T-plastin. We also report that T-plastin is recruited to S. typhimurium-induced membrane ruffles by a CDC42-dependent signaling process and is required for bacterial entry, We propose that modulation of the actin-bundling activity of T-plastin by SipA results in the stabilization of the actin filaments at the point of bacterial-host cell contact, which leads to more efficient Salmonella internalization.