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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.

In Escherichia coli, components of a signal recognition particle (SRP) and its receptor have been identified which appear to be essential for efficient translocation of several proteins. In this study we use cross‐linking to demonstrate that E. coli SRP interacts with a variety of nascent presecretory proteins and integral inner membrane proteins. Evidence is presented that the interaction is correlated with the hydrophobicity of the core region of the signal sequence and thereby with its ability to promote transport in vivo. A second E. coli component, which is identified as trigger factor, can be efficiently cross‐linked to all tested nascent chains derived from both secreted and cytosolic proteins. We propose that SRP and trigger factor act as secretion‐specific and general molecular chaperone respectively, early in protein synthesis.