The Signal Sequence Receptor Has a Second Subunit and IsPart of a Translocation Complex in the Endoplasmic Reticulum as Probed by Bifunctional Reagents
Bifunctional cross-linking reagents were used to probe the protein environment in the ER membrane of the signal sequence receptor (SSR), a 34-kD integral membrane glycoprotein (Wiedmann, M., T. V. Kurzchalia, E. Hartmarm, and T. A. Rapoport. 1987. Nature [Lond.]. 328:830-833). The proximity of several polypeptides was demonstrated. A 22-kD glycoprotein was identified tightly bound to the 34-kD SSR even after membrane solubilization. The 34-kD polypeptide, now termed otSSR, and the 22-kD polypeptide, the #SSR, represent a heterodimer. We report on the sequence of the/3SSR, its membrane topology, and on the mechanism of its integration into the membrane. Cross-linking also produced dimers of the a-subunit of the SSR indicating that oligomers of the SSR exist in the ER membrane. Various bifunctional cross-linking reagents were used to study the relation to ER membrane proteins of nascent chains of preprolactin and/3-1actamase at different stages of their translocation through the membrane. The predominant cross-linked products obtained in high yields contained the aSSR, indicating in conjunction with previous results that it is a major membrane protein in the neighborhood of translocating nascent chains of secretory proteins. The results support the existence of a translocon, a translocation complex involving the SSR, which constitutes the specific site of protein translocation across the ER membrane
