A TETRAMERIC COMPLEX OF MEMBRANE-PROTEINS IN THE ENDOPLASMIC-RETICULUM

The translocation site (translocon), at which nascent polypeptides pas s through the endoplasmic reticulum membrane, contains a component pre viously called 'signal sequence receptor' that is now renamed as 'tran slocon-associated protein' (TRAP). Two glycosylated subunits of the TR AP complex have been identified before (alpha and beta subunits). We n ow show that the TRAP complex is actually comprised of four membrane p roteins (alpha, beta, gamma, delta), present in a stoichiometric relat ion, which are genuine neighbours in intact microsomes. The amino acid sequences of the additional, non-glycosylated subunits were deduced f rom cloning of the corresponding cDNAs. The delta subunit spans the me mbrane only once and has its major portion, containing a disulfide bri dge, at the lumenal side. The gamma subunit is predicted to span the m embrane four times.