An alternative protein targeting pathway in Escherichia coli: studies on the role of FtsY

In Escherichia coli, a signal recognition particle (SRP) has been identified which binds specifically to the signal sequence of presecretory proteins and which appears to be essential for efficient translocation of a subset of proteins. In this study we have investigated the function of E. coli FtsY which shares sequence similarity with the alpha-subunit of the eukaryotic SRP receptor ('docking protein') in the membrane of the endoplasmic reticulum. A strain was constructed which allows the conditional expression of FtsY. Depletion of FtsY is shown to cause the accumulation of the precursor form of beta-lactamase, OmpF and ribose binding protein in vivo, whereas the processing of various other presecretory proteins is unaffected. Furthermore, FtsY-depleted inverted cytoplasmic membrane vesicles are shown to be defective in the translocation of pre-beta-lactamase using an in vitro import assay. Subcellular 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.