The signal sequence receptor has a second subunit and is part of a translocation complex in the endoplasmic reticulum as probed by bifunctional reagents

Dirk Görlich, Siegfried Prehn, Enno Hartmann, Joachim Herz, Albrecht Otto, Regine Kraft, Martin Wiedmann, Siegne Knespel, Bernhard Dobberstein, Tom A. Rapoport

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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. 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 ER membrane.

Original languageEnglish (US)
Pages (from-to)2283-2294
Number of pages12
JournalJournal of Cell Biology
Issue number6 PART 1
Publication statusPublished - Dec 1990


ASJC Scopus subject areas

  • Cell Biology

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