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 one strategy employed by Salmonella to survive intracellularly is the subversion of key intracellular vesicle transport regulators, the Rab GTPases.
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.
Work from our laboratory revealed that SseK3 modifies several Rab GTPases during Salmonella infection of murine macrophages. We confirmed the modification of Rab1, Rab5 and Rab11 by SseK3 in immunoprecipitation experiments. Interestingly, SseK3 targeted three arginine residues (R74, R82, R111) for modification on Rab1, while R74 and R82 localized to the catalytically important switch II region. Rab1 mediates vesicle transport from endoplasmic reticulum to the Golgi apparatus, and SseK3 co-localized with Rab1 on the Golgi of host cells. The expression of SseK3 resulted in reduced secretion of secreted embryonic alkaline phosphatase (SEAP) in transfected cells, which is largely dependent on its glycosyltransferase activity. However, SseK3 only exhibited a modest effect on SEAP secretion during infection on a HeLa229 cell line. The cytokine secretion profile was examined on infected Raw264.7 cells, and inhibition of IL-1 and GM-CSF was only observed when SseK3 was over-expressed. Our results suggest that SseK3 may contribute to Salmonella infection by targeting the activity of key Rab GTPases.