INTERNALIN MUST BE ON THE BACTERIAL SURFACE TO MEDIATE ENTRY OF LISTERIA-MONOCYTOGENES INTO EPITHELIAL-CELLS

Entry of Listeria monocytogenes into cultured epithelial cells require s production of internalin, a protein with features characteristic of some Gram-positive bacterial surface proteins, in particular an LPXTG motif preceding a hydrophobic sequence and a few basic residues at its C-terminal end. By immunofluorescence and immunogold labelling, we sh ow that in wild-type L. monocytogenes, internalin is present on the ce ll surface and has a polarized distribution similar to that of ActA, a nother surface protein of L. monocytogenes involved in actin assembly. Through a genetic analysis, we establish that the C-terminal region o f internalin is necessary for cell-surface association, and that altho ugh internalin is partially released in the culture medium, its locati on on the bacterial surface is required to promote entry. Finally, usi ng a 'domain-swapping' strategy - replacement of the cell wall anchor of InlA by the membrane anchor of ActA - we show that the reduced abil ity to adhere and enter cells of strains expressing InlA-ActA correlat es with a lower amount of surface-exposed internalin. Taken together, these results suggest that internalin exposed on the bacterial surface mediates direct contact between the bacterium and the host cell.