Dual 4- and 5-phosphatase activities regulate SopB-dependent phosphoinositide dynamics to promote bacterial entry.
Salmonella are able to invade non-phagocytic cells such as intestinal epithelial cells by modulating the host actin cytoskeleton to produce membrane ruffles. Two type III effector proteins SopB and SopE play key roles to this modulation. SopE is a known guanine nucleotide exchange factor (GEF) capable of activating Rac1 and CDC42. SopB is a phosphatidylinositol 4-phosphatase and 5-phosphatase promoting membrane ruffles and invasion of Salmonella through undefined mechanisms. Previous studies have demonstrated that the 4-phosphatase activity of SopB is required for PtdIns-3-phosphate (PtdIns(3)P) accumulation and SopB-mediated invasion. We show here that both the 4-phosphatase as well as the 5-phosphatase activities of SopB are essential in ruffle formation and subsequent invasion. We found that the 5-phosphatase activity of SopB is likely responsible for generating PtdIns-3,4-bisphosphate (PtdIns(3,4)P(2)) and subsequent recruitment of sorting nexin 9 (SNX9), an actin modulating protein. Intriguingly, the 4-phosphatase activity is responsible for the dephosphorylation of PtdIns(3,4)P(2) into PtdIns(3)P. Alone, neither activity is sufficient for ruffling but when acting in conjunction with one another, the 4-phosphatase and 5-phosphatase activities led to SNX9-mediated ruffling and Salmonella invasion. This work reveals the unique ability of bacterial effector protein SopB to utilize both its 4- and 5-phosphatase activities to regulate phosphoinositide dynamics to promote bacterial entry.