The InIB protein of Listeria monocytogenes is sufficient to promote entry into mammalian cells.

InIB is one of the two Listeria monocytogenes invasion proteins required for bacterial entry into mammalian cells. Entry into human epithelial cells such as Caco-2 requires InIA, whereas InIB is needed for entry into cultured hepatocytes and some epithelial or fibroblast cell lines such as Vero, HEp-2 and HeLa cells. InIB-mediated entry requires tyrosine phosphorylation, cytoskeletal rearrangements and activation of the host protein phosphoinositide (PI) 3-kinase, probably in response to engagement of a receptor. In this study, we demonstrate for the first time that InIB is sufficient to promote internalization. Indeed, coating of normally non-invasive bacteria or inert latex beads with InIB leads to internalization into mammalian cells. In addition, a soluble form of InIB also appears to promote uptake of non-invasive bacteria, albeit at a very low level. Similar to entry of L. monocytogenes, uptake of InIB-coated beads required tyrosine phosphorylation in the host cell, PI 3-kinase activity and cytoskeletal reorganization. Taken together, these data indicate that InIB is sufficient for entry of L. monocytogenes into host cells and suggest that this protein is an effector of host cell signalling pathways.