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

Abstract

Salt-extracted [dog pancreas] 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. This arrest-releasing activity is a function of an integral microsomal membrane protein, termed the SRP receptor. An attempt was made to solubilize the arrest-releasing activity of the SRP receptor by mild protease digestion of K-RM using either trypsin or elastase. 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 the arrest-releasing activity could not be proteolytically severed from K-RM and the release of the elongation arrest thereby uncoupled from the process of chain translocation, the arrest-releasing activity could be proteolytically dissected and reconstituted. Furthermore, the arrest-releasing activity of the SRP receptor can be inactivated by alkylation of K-RM with N-ethylmaleimide.