INTERACTION OF ESCHERICHIA-COLI FFH 4.5S RIBONUCLEOPROTEIN AND FTSY MIMICS THAT OF MAMMALIAN SIGNAL RECOGNITION PARTICLE AND ITS RECEPTOR/

THE mechanism of-protein translocation across the endoplasmic reticulu m membrane of eukaryotic cells and the plasma membrane of prokaryotic cells are thought to be evolutionarily related(1-7). Protein targeting to the eukaryotic translocation apparatus is mediated by the signal r ecognition particle (SRP), a cytosolic ribonucleoprotein, and the SRP receptor, an endoplasmic reticulum membrane protein(8,9). During targe ting, the 54K SRP subunit (M(r), 54,000; SRP54), a GTP-binding protein (10-12), binds to signal sequences(13,14) and then interacts with the alpha-subunit of the SRP receptor (SR alpha), another GTP-binding prot ein(12,15). Two proteins from Escherichia coli, Ffh and FtsY, structur ally resemble SRP54 and SR alpha(10,11,16). Like SRP54, Ffh is a subun it of a cytosolic ribonucleoprotein that also contains the E. coli 4.5 S RNA(17,18). Although there is genetic and biochemical evidence that the E. coli Ffh/ 4.5S ribonucleoprotein has an SRP-like function(19-21 ), there is no evidence for an SR alpha-like role for FtsY. Here we sh ow that the Ffh/ 4.5S ribonucleoprotein binds tightly to FtsY in a GTP -dependent manner. This interaction results in the stimulation of GTP hydrolysis which can be inhibited by synthetic signal peptides. These properties mimic those of mammalian SRP and its receptor, suggesting t hat the E. coli Ffh/4.5S ribonucleoprotein and FtsY have functions in protein targeting that are similar to those of their mammalian counter parts.