Empty Site Forms of the SRP54 and SRα GTPases Mediate Targeting of Ribosome–Nascent Chain Complexes to the Endoplasmic Reticulum

AbstractThe SRP54 and SRα subunits of the signal recognition particle (SRP) and the SRP receptor (SR) undergo a tightly coupled GTPase cycle that mediates the signal sequence–dependent attachment of ribosomes to the Sec61 complex. Here, we show that SRP54 and SRα are in the empty site conformation prior to contact between the SRP–ribosome complex and the membrane-bound SR. Cooperative binding of GTP to SRP54 and SRα stabilizes the SRP–SR complex and initiates signal sequence transfer from SRP54 to Sec61α. The GTP-bound conformations of SRα and SRP54 perform distinct roles, with SRα performing a predominant role in complex stabilization. Hydrolysis by both SRP54 and SRα is a prerequisite for dissociation of the SRP–SR complex