REGULATION BY THE RIBOSOME OF THE GTPASE OF THE SIGNAL-RECOGNITION PARTICLE DURING PROTEIN TARGETING

THE signal-recognition particle (SRP) is important for the targeting o f many secretory and membrane proteins to the endoplasmic reticulum (E R). Targeting is regulated by three GTPases, the 54K subunit of SRP (S RP54), and the alpha- and beta-subunits of the SRP receptor(1). When a signal sequence emerges from the ribosome, SRP interacts with it and targets the resulting complex to the ER membrane by binding to the SRP receptor. Subsequently, SRP releases the signal sequence into the tra nslocation channel(2,3). Here we use a complex of a ribosome with a na scent peptide chain, the SRP and its receptor, to investigate GTP bind ing to SRP54, and GTP hydrolysis. Our findings indicate that a ribosom al component promotes GTP binding to the SRP54 subunit of SRP, GTP-bou nd SRP54 is essential for high-affinity interaction between SRP and it s receptor in the ER membrane. This interaction induces the release of the signal sequence from SRP, the insertion of the nascent polypeptid e chain into the translocation channel, and GTP hydrolysis. The contri bution of the ribosome had previously escaped detection because only s ynthetic signal peptides were used in the analysis(4).