Protein translocation across the endoplasmic reticulum. I. Detection in the microsomal membrane of a receptor for the signal recognition particle.

TLDR

Salt‑extracted microsomal membranes contain an SRP‑mediated elongation‑arrest‑releasing activity attributed to an integral membrane protein, the SRP receptor, and this release precedes translocation of secretory proteins across the membrane and signal‑peptide removal. The authors attempted to solubilize the SRP‑receptor arrest‑releasing activity by mild trypsin or elastase digestion of K‑RM. Neither trypsin nor elastase supernatants alone released the SRP arrest, but combining them with the trypsinized membrane fraction restored activity; although the arrest‑releasing function could not be separated from translocation, it could be proteolytically dissected and reconstituted, and it is inactivated by N‑ethylmaleimide alkylation.

Abstract

Salt-extracted microsomal membranes (K-RM) contain an activity that is capable of releasing the signal recognition particle (SRP)-mediated elongation arrest of the synthesis of secretory polypeptides (Walter, P., and G. Blobel, 1981, J. Cell Biol., 91:557-561). This arrest-releasing activity was shown to be a function of an integral microsomal membrane protein, termed the SRP receptor (Gilmore, R., P. Walter, and G. Blobel, 1982, J. Cell Biol., 95:470-477). We attempted to solubilize the arrest-releasing activity of the SRP receptor by mild protease digestion of K-RM using either trypsin or elastase. We found, however, that neither a trypsin, nor an elastase "solubilized" supernatant fraction exhibited the arrest-releasing activity. Only when either the trypsin- or elastase-derived supernatant fraction was combined with the trypsinized membrane fraction, which by itself was also inactive, was the arrest-releasing activity restored. Release of the elongation arrest was followed by the translocation of the secretory protein across the microsomal membrane and the removal of the signal peptide. Thus, although we have been unable to proteolytically sever the arrest-releasing activity from K-RM and thereby to uncouple the release of the elongation arrest from the process of chain translocation, we have been able to proteolytically dissect and reconstitute the arrest-releasing activity. Furthermore, we found that the arrest-releasing activity of the SRP receptor can be inactivated by alkylation of K-RM with N-ethylmaleimide.