Interaction between the protein InIB of Listeria monocytogenes and lipoteichoic acid:: a novel mechanism of protein association at the surface of Gram-positive bacteria

InIB is a Listeria monocytogenes protein that is sufficient to promote entry in a variety of mammalian cells. The last 232-amino-acid domain (Csa) of InIB has been shown to mediate attachment on the listerial surface, although its sequence does not suggest any known mechanism of association to the bacterial surface. InIB is present both on the bacterial surface and in culture supernatants. As has been recently demonstrated, both forms of InIB, soluble and surface-bound, can trigger signalling in host cells. To elucidate the specific role of each of the two forms, it was important to understand how InIB associates with the bacterial surface. Using microscopy, we find evidence that InIB is partially buried in the cell wall layer, and using fractionation experiments we demonstrate that InIB associates with the bacterial cytoplasmic membrane. Moreover, using purified lipoteichoic acid (LTA) and the three polypeptides InIB, Csa, or InIB Delta Csa (InIB lacking the last 232 amino acids), we demonstrate that LTA is a ligand for the Csa domain of InIB, These results provide the first evidence at an interaction between lipoteichoic acids and a bacterial protein involved in adhesion and signalling, and highlight a new mechanism of protein association on the surface of Gram-positive bacteria.