A tetrameric complex of membrane proteins in the endoplasmic reticulum

Authors


  • Note. The novel DNA sequence data for TRAPγ (previously SSRγ) and TRAPδ (previously SSRδ) published here have been deposited with the EMBL sequence databank and are available under the accession numbers RNSSRGAM Z14030 and RNSSRDSUB Z19087, respectively.

Correspondence to T. A. Rapoport, Max-Delbrück-Center for Molecular Medicine, Robert-Rössle-Str. 10, O-1100 Berlin, Germany.
Fax: +49 30 9406 3363.

Abstract

The translocation site (translocon), at which nascent polypeptides pass through the endoplasmic reticulum membrane, contains a component previously called ‘signal sequence receptor’ that is now renamed as ‘translocon-associated protein’ (TRAP). Two glycosylated subunits of the TRAP complex have been identified before (α and β subunits). We now show that the TRAP complex is actually comprised of four membrane proteins (α, β, γ, δ), present in a stoichiometric relation, which are genuine neighbours in intact microsomes. The amino acid sequences of the additional, non-glycosylated subunits were deduced from cloning of the corresponding cDNAs. The δ subunit spans the membrane only once and has its major portion, containing a disulfide bridge, at the lumenal side. The γ subunit is predicted to span the membrane four times.

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