Essential features of the pathway for protein translocation across the Escherichia coli cytoplasmic membrane.

Abstract

The translocation of proteins across the cytoplasmic membrane of E. coli is largely posttranslational. That is, for most proteins, a substantial proportion of the polypeptide chain is synthesized before any translocation is observed (Josefsson and Randall 1981a,b). Genetic and biochemical studies of E. coli have revealed the existence of 7 Sec proteins that play a role in this posttranslational process (Fig. 1). In addition, several other gene products have been implicated as possibly functioning in a cotranslational pathway. These latter components are 4.5 S RNA; Ffh, a homolog of the 54-kD subunit of eukaryotic signal recognition particle (SRP); and FtsY, a membrane protein that may act as a receptor for a complex of the former two molecules (Poritz et al. 1990; Luirink et al. 1992; Phillips and Silhavy, 1992; Miller et al. 1994).