SopE acts as an Rab5-specific nucleotide exchange factor and recruits non-prenylated Rab5 on Salmonella-containing phagosomes to promote fusion with early endosomes

Rab-GTPase regulates the fusion between two specific vesicles. It is well d ocumented that, for their biological function, Rab proteins need to be pren ylated for attachment to the vesicle membrane. In contrast, we showed in th e present investigation that SopE, a type III secretory protein of Salmonel la, translocates onto Salmonella-containing phagosomes (LSP) and mediates t he recruitment of non-prenylated RabB (Rab5:Delta C4) on LSP in GTP form. S imultaneously, SopE present in infected cell cytosol acts as an RabB specif ic exchange factor and converts the inactive Rab-GDP to the G;TP form. The non-prenylated Rab5 subsequently promoted efficient fusion of LSP with earl y endosomes. This is the first demonstration that a prenylation-deficient R ab protein retains biological activity and can promote vesicle fusion, if i t is recruited on the membrane by some other method.