GTP HYDROLYSIS BY COMPLEXES OF THE SIGNAL RECOGNITION PARTICLE AND THE SIGNAL RECOGNITION PARTICLE RECEPTOR

Translocation of proteins across the endoplasmic reticulum membrane is a GTP-dependent process. The signal recognition particle (SRP) and th e SRP receptor both contain subunits with GTP binding domains. One GTP -dependent reaction during protein translocation is the SRP receptor-m ediated dissociation of SRP from the signal sequence of a nascent poly peptide. Here, we have assayed the SRP and the SRP receptor for GTP bi nding and hydrolysis activities. GTP hydrolysis by SRP was not detecte d, so the maximal GTP hydrolysis rate for SRP was estimated to be <0.0 02 mol GTP hydrolyzed x mol of SRP-1 x min-1. The intrinsic GTP hydrol ysis activity of the SRP receptor ranged between 0.02 and 0.04 mol GTP hydrolyzed x mol of SRP receptor-1 x min-1. A 40-fold enhancement of GTP hydrolysis activity relative to that observed for the SRP receptor alone was obtained when complexes were formed between SRP and the SRP receptor. GTP hydrolysis activity was inhibited by GDP, but not by AT P. Extended incubation of the SRP or the SRP receptor with GTP resulte d in substoichiometric quantities of protein-bound ribonucleotide. SRP -SRP receptor complexes engaged in GTP hydrolysis were found to contai n a minimum of one bound guanine ribonucleotide per SRP-SRP receptor c omplex. We conclude that the GTP hydrolysis activity described here is indicative of one of the GTPase cycles that occur during protein tran slocation across the endoplasmic reticulum.