Salmonella species are among the most common foodborne pathogens that incur significant burden on the healthcare system worldwide. As an intracellular pathogen, Salmonella utilises two type III secretion systems (SPI-1 and SPI-2) to inject virulence effector proteins into host cells to mediate invasion and subsequent intracellular replication via subverting a series of critical host cell biochemical and physiological events. It has become clear that a common strategy employed by Salmonella is to hijack key intracellular vesicle transport regulators, Rab GTPases, to contribute to its pathogenesis.
Within the Salmonella SPI-2 effector cohort, SseK1, SseK2 and SseK3 have been identified as three homologues of NleB1, an arginine N-acetylglucosamine (GlcNAc) transferase of enteropathogenic Escherichia coli that blocks protective cell death by modifying death domain containing proteins in host cells. Although these effectors have been studied previously, detailed virulence mechanisms and host targets of these SseK proteins still remain to be explored and verified.
Preliminary work from our laboratory revealed that SseK3 modifies several Rab GTPases during Salmonella infection of mice macrophages. Here, using co-transfection and immunoprecipitation experiments, we confirmed the modification of Rab1, Rab5 and Rab11 by SseK3. Three arginine residues were confirmed as SseK3 modification sites on Rab1; and the modification exerted on Rab1 by SseK3 was independent of the GTP/GDP – nucleotide binding states of Rab1. Rab1 regulates vesicular protein transport from endoplasmic reticulum to the Golgi apparatus. Importantly, a secreted alkaline phosphatase reporter assay indicated that the expression of both SseK3 and SseK2 impede the secretory pathway in HEK293T cells. Further experiments are currently being completed to understand the detailed mechanisms underlying the SseKs activity in host cells during infection.