Identification and characterization of a membrane component essential for the translocation of nascent proteins across the membrane of the endoplasmic reticulum

When rough microsomes are subjected to limited proteolysis and high salt, a soluble fraction can be separated from the membrane. Neither fraction alone is capable of vectorially translocating nascent peptides. When the soluble extract is recombined with the residual membrane fraction, translocating activity is restored. Standard biochemical techniques were used to identify and characterize the active component derived by treating rough microsomes with elastase and high salt. The active factor is a peptide fragment with an apparent molecular weight of 60,000. It represents the cytoplasmic domain of a larger membrane protein. The fragment is basic and has at least one accessible sulfhydryl group. These characteristics facilitated its purification and identification as a membrane component required for translocation of nascent peptides across microsomal membranes.