Evidence for coupling of membrane targeting and function of the signal recognition particle (SRP) receptor FtsY

Recent studies have indicated that FtsY, the signal recognition particle re ceptor of Escherichia coli, plays a central role in membrane protein biogen esis. For proper function, FtsY must be targeted to the membrane, but its m embrane-targeting pathway is unknown. We investigated the relationship betw een targeting and function of FtsY in vivo, by separating its catalytic dom ain (NG) from its putative targeting domain (A) by three means: expression of split ftsY, insertion of various spacers between A and NG, and separatio n of A and NG by in vivo proteolysis. Proteolytic separation of A and NG do es not abolish function, whereas separation by long linkers or expression o f split ftsY is detrimental. We propose that proteolytic cleavage of FtsY o ccurs after completion of co-translational targeting and assembly of NG. In contrast, separation by other means may interrupt proper synchronization o f co-translational targeting and membrane assembly of NG. The co-translatio nal interaction of FtsY with the membrane was confirmed by in vitro experim ents.