A membrane component of the endoplasmic reticulum that may be essential for protein translocation

We have purified a glycosylated, membrane-spanning protein of relative molecular mass ~34,000 (M(r) ~ 34 K) from canine microsomes that appears to be essential for protein translocation across the endoplasmic reticulum (ER) as shown by the inhibitory action of antibodies directed against it and of monovalent F(ab)-fragments produced from them. The ER membrane contains at least as many molecules of the 34 K membrane protein as bound ribosomes. The protein can be detected immunologically in tissues of various organisms, indicating a universal function.