Protein transport across the ER membrane.

Protein transport across the endoplasmic reticulum (ER) membrane may be divided into two phases: an initiation or targeting cycle, which has been fairly well characterized, and the actual transfer of the polypeptide chain through the membrane, the mechanism of which is still unknown. In this review, the initiation cycle is discussed with emphasis on the mechanism of signal sequence recognition by the 54 kDa polypeptide of the signal recognition particle (SRP) and on the efficiency of targeting of nascent chains. Recent results are reviewed suggesting the transfer of the polypeptide chain by means of a translocation complex, a constituent of which appears to be the signal sequence receptor protein (SSR).