INTERNALIN OF LISTERIA-MONOCYTOGENES WITH AN INTACT LEUCINE-RICH REPEAT REGION IS SUFFICIENT TO PROMOTE INTERNALIZATION

Listeria monocytogenes can use two different surface proteins, interna lin (In1A) and In1B, to invade mammalian cells. The exact role of thes e invasiveness factors in vivo remains to be determined. In cultured c ells, In1A is necessary to promote Listeria entry into human epithelia l cells, such as Caco-2 cells, whereas In1B is necessary to promote Li steria internalization in several other cell types, including hepatocy tes, fibroblasts, and epithelioid cells, such as Vero, HeLa, CHO, or H ep-2 cells, We have recently reported that the In1A receptor on Caco-2 cells is the cell adhesion molecule E-cadherin and demonstrated that nonpermissive fibroblasts become permissive for internalin-mediated en try when transfected with the gene coding for LCAM, the chicken homolo g of the human E-cadherin gene. In this study, we demonstrate for the first time that the internalin protein alone is sufficient to promote internalization into cells expressing its receptor, Indeed, internalin confers invasiveness to both Enterococcus faecalis and internalin-coa ted latex beads, As shown by transmission electron microscopy, these b eads were phagocytosed via a ''zipper'' mechanism similar to that obse rved during the internalin-E-cadherin-mediated entry of Listeria. More over, a functional analysis of internalin demonstrates that its amino- terminal region, encompassing the leucine-rich repeat (LRR) region and the inter-repeat (IR) region, is necessary and sufficient to promote bacterial entry into cells expressing its receptor, Several lines of e vidence suggest that the LRR region would interact directly with E-cad herin, whereas the LR region would be required for a proper folding of the LRR region.