The signal recognition particle (SRP)-mediated elongation arrest of the synthesis of nascent secretory proteins can be released by salt-extracted rough microsomal membranes (Walter, P., and G. Blobel, 1981, J. Cell Biol, 91:557-561). Both the arrest-releasing activity and the signal peptidase activity were solubilized from rough microsomal membranes using the nonionic detergent Nikkol in conjunction with 250 mM KOAc. Chromatography of this extract on SRP-Sepharose separated the arrest-releasing activity from the signal peptidase activity. Further purification of the arrest-releasing activity using sucrose gradient centrifugation allowed the identification of a 72,000-dalton polypeptide as the protein responsible for the activity. Based upon its affinity for SRP, we refer to the 72,000-dalton protein as the SRP receptor. A 60,000-dalton protein fragment (Meyer, D. I., and B. Dobberstein, 1980, J. Cell Biol., 87:503-508) that had been shown previously to reconstitute the translocation activity of protease-digested membranes, was shown here by peptide mapping and immunological criteria to be derived from the SRP receptor. Findings that are in part similar, and in part different from these reported here and in our preceding paper were made independently (Meyer, D. I., E. Krause, and B. Dobberstein, 1982, Nature (Lond.). 297:647-650) and the term "docking protein" was proposed for the SRP receptor. A lower membrane content of both SRP and the SRP receptor than that of membrane bound ribosomes suggests that the SRP-SRP receptor interaction may exist transiently during the formation of a ribosome-membrane junction and during translocation.

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