THE SIGNAL SEQUENCE RECEPTOR, UNLIKE THE SIGNAL RECOGNITION PARTICLE RECEPTOR, IS NOT ESSENTIAL FOR PROTEIN TRANSLOCATION

Detergent extracts of canine pancreas rough microsomal membranes were depleted of either the signal recognition particle receptor (SR), whic h mediates the signal recognition particle (SRP)-dependent targeting o f the ribosome/nascent chain complex to the membrane, or the signal se quence receptor (SSR), which has been proposed to function as a membra ne bound receptor for the newly targeted nascent chain and/or as a com ponent of a multi-protein translocation complex responsible for transf er of the nascent chain across the membrane. Depletion of the two comp onents was performed by chromatography of detergent extracts on immuno affinity supports. Detergent extracts lacking either SR or SSR were re constituted and assayed for activity with respect to SR dependent elon gation arrest release, nascent chain targeting, ribosome binding, secr etory precursor translocation, and membrane protein integration. Deple tion of SR resulted in the loss of elongation arrest release activity, nascent chain targeting, secretory protein translocation. and membran e protein integration, although ribosome binding was unaffected. Full activity was restored by addition of immunoaffinity purified SR before reconstitution of the detergent extract. Surprisingly, depletion of S SR was without effect on any of the assayed activities, indicating tha t SSR is either not required for translocation or is one of a family o f functionally redundant components.