The signal sequence receptor has a second subunit and is part 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 24-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 alpha SSR, and the 22-kD polypeptide, the beta SSR, represent a heterodimer. We report on the sequence of the beta SSR, its membrane topology, and on the mechanism of its integration into the membrane. Cross-linking also produced dimers of the alpha-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 beta-lactamase at different stages of their translocation through the membrane. The predominant cross-linked products obtained in high yields contained the alpha SSR, 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.
Görlich
D
D
Zentralinstitut für Molekularbiologie, Akademie der Wissenchaften, Berlin-Buch, Federal Republic of Germany.
Prehn
S
S
Hartmann
E
E
Herz
J
J
Otto
A
A
Kraft
R
R
Wiedmann
M
M
Knespel
S
S
Dobberstein
B
B
Rapoport
T A
TA
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002135:Calcium-Binding Proteins; D002846:Chromatography, Affinity; D003001:Cloning, Molecular; D003432:Cross-Linking Reagents / Q000494:pharmacology*; D004247:DNA / Q000235:genetics; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism*; D008961:Models, Structural; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010446:Peptide Fragments / Q000302:isolation & purification; D010957:Plasmids; D014176:Protein Biosynthesis; D011487:Protein Conformation; D011499:Protein Processing, Post-Translational; D012333:RNA, Messenger / Q000235:genetics; D011956:Receptors, Cell Surface / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D014158:Transcription, Genetic; D001618:beta-Lactamases / Q000235:genetics
D002135:Calcium-Binding Proteins; D003432:Cross-Linking Reagents; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D010446:Peptide Fragments; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; C066973:signal sequence receptor; D004247:DNA; D001618:beta-Lactamases