SopB- and SifA-dependent shaping of the Salmonella-containing vacuole proteome in the social amoeba Dictyosteliumdiscoideum

Abstract

The ability of Salmonella to survive and replicate within mammalian host cells involves the generation of a membranous compartment known as the Salmonella-containing vacuole (SCV). Salmonella employs a number of effector proteins that are injected into host cells for SCV formation using its type-three secretion systems encoded in SPI-1 and SPI-2 (T3SSSPI-1 and T3SSSPI-2, respectively). Recently, we reported that S. Typhimurium requires T3SSSPI-1 and T3SSSPI-2 to survive in the model amoeba Dictyostelium discoideum. Despite these findings, the involved effector proteins have not been identified yet. Therefore, we evaluated the role of two major S. Typhimurium effectors SopB and SifA during D. discoideum intracellular niche formation. First, we established that S. Typhimurium resides in a vacuolar compartment within D. discoideum. Next, we isolated SCVs from amoebae infected with wild type or the {Delta}sopB and {Delta}sifA mutant strains of S. Typhimurium, and we characterized the composition of this compartment by quantitative proteomics. This comparative analysis suggests that S. Typhimurium requires SopB and SifA to modify the SCV proteome in order to generate a suitable intracellular niche in D. discoideum. Accordingly, we observed that SopB and SifA are needed for intracellular survival of S. Typhimurium in this organism. Thus, our results provide insight into the mechanisms employed by Salmonella to survive intracellularly in phagocytic amoebae. ImportanceThe molecular mechanisms involved in Salmonella survival to predation by phagocytic amoebae, such as D. discoideum, remains poorly understood. Although we established that S. Typhimurium requires two specialized type-three secretion systems to survive in D. discoideum, no effector protein has been implicated in this process so far. Here, we confirmed the presence of a membrane-bound compartment containing S. Typhimurium in D. discoideum, and purified the D. discoideum SCV to characterize the associated proteome. In doing so, we established a key role for effector proteins SopB and SifA in remodeling the protein content of the SCV that ultimately allow the intracellular survival of S. Typhimurium in D. discoideum. We also discuss similarities and differences with the proteomes of the human SCV. These findings contribute to unravel the mechanisms used by Salmonella to survive in the environment exploiting phagocytic amoebae as a reservoir.