Different exported proteins in E. coli show differences in the temporal mode of processing in vivo

A number of exported proteins in E. coli, both periplasmic proteins and proteins of the outer membrane, were examined to determine when removal of the “signal sequence” occurs in vivo. One protein was processed entirely cotranslationally (amp C β-lactamase) and one was processed entirely post-translationally (TEM β-lactamase). The others (maltose-binding protein, arabinose-binding protein, omp A protein, lam B protein and alkaline phosphatase) showed both modes of processing, although the amount of cotranslational processing varied considerably among the individual proteins of this class. When processing occurred cotranslationally, the proteolytic removal of the “signal” was a late event. For four of the proteins studied, processing was initiated only after the polypeptides had been elongated to approximately 80% of their full length.