SUBSTITUTION OF 54-HOMOLOG (FFH) IN ESCHERICHIA-COLI WITH THE MAMMALIAN 54-KDA PROTEIN OF SIGNAL-RECOGNITION PARTICLE

The fifty four homologue (Ffh) of Escherichia coli promotes the transl ocation of a subset of periplasmic, membrane and secreted proteins acr oss the cytoplasmic membrane. The ffh erne product is essential for ce ll viability and efficient protein export. Here we show that the mamma lian homologue signal-recognition particle (SRP) 54 kDa is not able to suppress the translocation defect in an Ffh conditional mutant Wam 11 3 [Phillips, G. J. & Silhavy, T. J. (1992) Nature 359, 744-746]. The e xpression of SRP 54 kDa, which is increased when Ffh is suppressed in the Warn 113 strain. causes a pleiotropic defect characterised by cell elongation, and increased accumulation of precursor proteins. The acc umulation of precursors of outer membrane protein A (Omp A) and maltos e-binding protein (MBP), Sec-B dependent pre-proteins, was less than t he Ffh-dependent proteins ribose-binding protein (RBF) and beta-lactam ase. Sec B expression was suppressed by Ffh expression. The recombinan t SRP 54 kDa, which forms a ribonucleoprotein complex in E. coli, was shown to bind to precursor proteins, but is unable to interact with th e filamentous temperature-sensitive Y (Fts Y) membrane receptor of the translocation machinery.