Lmo1656 is a secreted virulence factor of Listeria monocytogenes that interacts with the sorting nexin 6–BAR complex

International audienceListeria monocytogenes (Lm) is a facultative intracellular bacterial pathogen and the causative agent of listeriosis, a rare but fatal disease. During infection, Lm can traverse several physiological barriers; it can cross the intestine and placenta barrier and, in immunocompromised individuals, the blood– brain barrier. With the recent plethora of sequenced genomes available for Lm, it is clear that the complete repertoire of genes used by Lm to interact with its host remains to be fully explored. Recently, we focused on secreted Lm proteins because they are likely to interact with host cell components. Here, we investigated a putatively secreted protein of Lm, Lmo1656, that is present in most sequenced strains of Lm but absent in the nonpathogenic species Listeria innocua. lmo1656 gene is predicted to encode a small, positively charged protein. We show that Lmo1656 is secreted by Lm. Furthermore, deletion of the lmo1656 gene (lmo1656) attenuates virulence in mice infected orally but not intravenously, suggesting that Lmo1656 plays a role during oral listeriosis. We identified sorting nexin 6 (SNX6), an endosomal sorting component and BAR domain– containing protein, as a host cell interactor of Lmol656. SNX6 colocal-izes with WT Lm during the early steps of infection. This colocalization depends on Lmo1656, and RNAi of SNX6 impairs infection in infected tissue culture cells, suggesting that SNX6 is utilized by Lm during infection. Our results reveal that Lmo1656 is a novel secreted virulence factor of Lm that facilitates recruitment of a specific member of the sorting nexin family in the mam-malian host. The foodborne pathogen Listeria monocytogenes (Lm) 5 can cross several physiological barriers and infect multiple cell types. The pathogenic potential of Lm relies on the ability of this bacterium to cross multiple physiological barriers as well as its ability to enter and replicate within a wide variety of host cell types (for recent reviews, see Refs. 1 and 2). Upon binding to host cell surface receptors, Lm induces its internalization into both professional phagocytes and nonphagocytic cells (for a recent review, see Ref. 2). From there, Lm escapes into the cyto-sol by rupturing its vacuole. Lm is able to evade host cell immune responses (for a recent review, see Ref. 3) and subvert the host cell actin cytoskeleton to drive intra-and intercellular motility (for recent reviews, see Refs. 4 –6). Secreted and surface-exposed Lm proteins can encounter host components and serve as virulence factors. For example, the secreted pore-forming toxin listeriolysin O (LLO) is one of the most well-characterized and potent virulence factors of Lm (for a review, see Ref. 7). Secretion of LLO occurs prior to Lm entry into the host cell. It inserts into the host plasma membrane and makes large pores. The resulting ion flux drives a diverse array of responses within the cell from global deSUMOylation (8) to mitochondrial fragmentation (9). Upon entry, Lm can escape into the host cytosol by lysing the phagosomal membrane through the combined actions of secreted LLO and phospholipases A and B (PlcA and PlcB) (10 –12). Recent work has uncovered novel secreted Lm virulence factors and their binding partners in the host cell. The secreted protein Listeria nuclear targeted protein A (LntA) targets the host epigenetic regulator BAHD1, altering host cell transcription (13). The small secreted protein internalin C (InlC) sequesters Tuba, a Cdc42 guanine exchange factor, to induce relaxation of membrane cortical tension, thereby facilitating increased bacterial cell-to-cell spread (14, 15). InlC also directly binds to host IB kinase , interfering with host innate immunity (16). The recent plethora of genomics data and the rise of bioin-formatics pipelines have enabled the rapid comparison of mu