THE NG DOMAIN OF THE PROKARYOTIC SIGNAL RECOGNITION PARTICLE RECEPTOR, FTSY, IS FULLY FUNCTIONAL WHEN FUSED TO AN UNRELATED INTEGRAL MEMBRANE POLYPEPTIDE

Recent studies have revealed that Escherichia coli possesses an essent ial targeting system for integral membrane proteins, similar to the ma mmalian signal recognition particle (SRP) machinery, One essential pro tein in this system is FtsY, a homologue of the alpha-subunit of the m ammalian SRP-receptor (SR-alpha). However, E. coli does not possess a close homologue of the integral membrane protein SR-beta, which anchor s SR-alpha to the membrane. Moreover, although FtsY can be found as a peripheral membrane protein, the majority is found soluble in the cyto plasm. In this study, we obtained genetic and biochemical evidence tha t FtsY must be targeted to the membrane for proper function, We demons trate that the essential membrane targeting activity of FtsY is mediat ed by a 198-residue-long acidic N-terminal domain, This domain can be functionally replaced by unrelated integral membrane polypeptides, thu s avoiding the need for specific FtsY membrane targeting factors, Ther efore, the N terminus of FtsY constitutes an independent domain, which is required only for the targeting of the C-terminal NG domain of Fts Y to the membrane.