AN ALTERNATIVE PROTEIN TARGETING PATHWAY IN ESCHERICHIA-COLI - STUDIES ON THE ROLE OF FTSY

In Escherichia coli, a signal recognition particle (SRP) has been iden tified which binds specifically to the signal sequence of presecretory proteins and which appears to be essential for efficient translocatio n of a subset of proteins. In this study we have investigated the func tion of E. coli FtsY which shares sequence similarity with the alpha-s ubunit of the eukaryotic SRP receptor ('docking protein') in the membr ane of the endoplasmic reticulum. A strain was constructed which allow s the conditional expression of FtsY. Depletion of FtsY is shown to ca use the accumulation of the precursor form of beta-lactamase, OmpF and ribose binding protein in vivo, whereas the processing of various oth er presecretory proteins is unaffected. Furthermore, FtsY-depleted inv erted cytoplasmic membrane vesicles are shown to be defective in the t ranslocation of pre-beta-lactamase using an in vitro import assay. Sub cellular localization studies revealed that FtsY is located in part at the cytoplasmic membrane with which it seems peripherally associated. These observations suggest that FtsY is the functional E.coli homolog of the mammalian SRP receptor.