InIB: an invasion protein of Listeria monocytogenes with a novel type of surface association

Listeria monocytogenes is an intracellular bacterial pathogen that expresses several surface proteins critical for the infectious process, Such proteins include InIA (internalin) and InIB, involved in bacterial entry into the host cell, and ActA, required for bacterially induced actin-based motility, Although the molecular mechanisms of attachment of InIA and ActA have been characterized, essentially nothing is known about how InIB is anchored to the bacterial surface, Using a genetic approach, we demonstrate that the last 232 amino acids of InIB are both necessary and sufficient for anchoring this protein to the bacterial surface, An InIB mutant protein deleted for the last 232 amino acids was secreted and not detected at the cell surface, A 'domain-swapping' strategy in which these 232 amino acids were used to replace the normal cell wall-anchoring domain of InIA resulted in a chimeric protein that was anchored to the cell surface and able to confer entry, Interestingly, surface association of InIB also occurred when InIB was added externally to bacteria, suggesting that association may be able to occur after secretion, This association was productive for invasion, as it conferred bacterial entry into host cells, The C-terminal anchoring region in InIB contains 80-amino-acid repeats beginning with the sequence GW that is also present in a newly identified surface-associated bacteriolysin of L. monocytogenes, called Ami, Addition of GW repeats to the C-terminal of InIB improves anchoring of the protein to the cell surface, These and other data suggest that such 'GW' repeats may constitute a novel motif for cell-surface anchoring in Listeria and other Gram-positive bacteria, This motif may have important consequences for the release of surface proteins involved in interactions with eukaryotic cells.