Membrane targeting of Ribosomes and their release require distinct and separable functions of ftsy

The mechanism underlying the interaction of the Escherichia coli signal recognition particle ( SRP) receptor FtsY with the cytoplasmic membrane is not fully understood. We investigated this issue by utilizing active ( NG + 1) and inactive ( NG) mutants of FtsY. In solution, the mutants comparably bind and hydrolyze nucleotides and associate with SRP. In contrast, a major difference was observed in the cellular distribution of NG and NG + 1. Unlike NG + 1, which distributes almost as the wild-type receptor, the inactive NG mutant accumulates on the membrane, together with ribosomes and SRP. The results suggest that NG function is compromised only at a later stage of the targeting pathway and that despite their identical behavior in solution, the membrane-bound NG-SRP complex is less active than NG + 1-SRP. This notion is strongly supported by the observation that lipids stimulate the GTPase activity of NG + 1-SRP, whereas no stimulation is observed with NG-SRP. In conclusion, we propose that the SRP receptor has two distinct and separable roles in ( i) mediating membrane targeting and docking of ribosomes and ( ii) promoting their productive release from the docking site.