Identification of four new members of the internalin multigene family of Listeria monocytogenes EGD

Abstract

Listeria monocytogenes is a bacterial pathogen that is able to invade nonphagocytic cells. Two surface proteins, internalin, the inlA gene product, and InlB, play important roles in the entry into cultured mammalian cells. These proteins also have extensive sequence similarities. Previously, Southern hybridization predicted the existence of an internalin multigene family. Recently, InlC, a secreted protein of 30 kDa homologous to InlA and InlB, was identified. In this work, we identified and characterized four new members of the internalin multigene family, inlC2, inlD, inlE, and inlF which encode proteins of 548, 567, 499, and 821 amino acids respectively. inlC2, inlD, and inlE are contiguous on the chromosome of L. monocytogenes EGD, whereas inlF is located in a different chromosomal region. These four inl gene products display the principal features of internalin, namely, a signal sequence, two regions of repeats (or LRR and B repeats), and a putative cell wall anchor sequence containing the sorting motif LPXTG. The four inl genes were maximally expressed albeit at a low level during early exponential growth in bacterial medium at 37 degrees C. The role of these inl genes in L. monocytogenes invasion was assessed by constructing isogenic chromosomal deletion mutants and testing them for entry into various nonphagocytic cells. Unexpectedly, the inlC2, inlD, inlE, and inlF null mutants were not affected for entry into any of the cell lines tested, raising the possibility that these genes are needed for an aspect of pathogenicity other than invasion. The identity of such an aspect remains to be determined.