Analysis of T3SS-independent Autonomous Internalization of the Bacterial Effector Protein SspH1 from Salmonella typhimurium

Previous studies identified the effector protein YopM of Yersinia enterocolitica as a novel bacterial cell-penetrating protein. YopM’s ability to translocate across the host cell plasma membrane independently of Yersinia’s type III secretion system (T3SS) is mediated by its two N-terminal α-helices. The SspH1 effector protein of Salmonella typhimurium shares significant homology in sequence and structure with YopM, which prompted us to investigate potential cell-penetrating abilities of this effector protein. For this, we recombinantly expressed SspH1 in Escherichia coli and analyzed a potential T3SS-independent translocation of the protein by cell fractionation of HeLa cells, immunofluorescence microscopy and FACS analyses. The functionality of the recombinant protein as an E3 ubiquitin- ligase was determined using in vitro -ubiquitination assays. Additionally, an effect of the recombinant protein on the expression of pro-inflammatory cytokines was analyzed by quantitative real time PCR. In this study, we could show that the SspH1 effector protein of Salmonella typhimurium is able to translocate autonomously into eukaryotic cells without requiring additional factors. Furthermore, we could show that recombinant SspH1 is a functional E3 ubiquitin ligase that is able to undergo auto-ubiquitination following T3SS-independent translocation and to reduce the expression of Interleukin-8 in IL-1β stimulated cells.