Fig. 9. Model of events that precede the SRP–SR interaction. In an initial step, SRP54/Ffh binds to targeting signals and SRα/FtsY interacts with lipids and membrane proteins (perhaps SRβ in the ER and translocon components in the bacterial inner membrane). The N domain participates in or senses each binding reaction and changes conformation. Communication of this conformational change across the N–GTPase domain interface activates or ‘primes’ SRP54/Ffh and SRα/FtsY. Interaction between the primed GTPases leads to stable GTP binding and release of the nascent chain. The bacterial SRP pathway is illustrated here.