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, F. Hartmann, 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 αSSR, and the 22-kD polypeptide, the βSSR, represent a heterodimer. We report on the sequence of the βSSR, its membrane topology, and on the mechanism of its integration into the membrane. Cross-linking also produced dimers of the α-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 β-lactamase at different stages of their translocation through the membrane. The predominant cross-linked products obtained in high yields contained the αSSR, 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 FR membrane.