Subunit structure of a mammalian ER/Golgi SNARE complex.
SNAP receptor (SNARE) complexes bridge opposing membranes to promote membrane fusion within the secretory and endosomal pathways. Because only the exocytic SNARE complexes have been characterized in detail, the structural features shared by SNARE complexes from different fusion steps are not known. We now describe the subunit structure, assembly, and regulation of a quaternary SNARE complex, which appears to mediate an early step in endoplasmic reticulum (ER) to Golgi transport. Purified recombinant syntaxin 5, membrin, and rbet1, three Q-SNAREs, assemble cooperatively to create a high affinity binding site for sec22b, an R-SNARE. The syntaxin 5 amino-terminal domain potently inhibits SNARE complex assembly. The ER/Golgi quaternary complex is remarkably similar to the synaptic complex, suggesting that a common pattern is followed at all transport steps, where three Q-helices assemble to form a high affinity binding site for a fourth R-helix on an opposing membrane. Interestingly, although sec22b binds to the combination of syntaxin 5, membrin, and rbet1, it can only bind if it is present while the others assemble; sec22b cannot bind to a pre-assembled ternary complex of syntaxin 5, membrin, and rbet1. Finally, we demonstrate that the quaternary complex containing sec22b is not an in vitro entity only, but is a bona fide species in living cells.
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Department of Biology and Program in Biomedical Sciences, University of Michigan, Ann Arbor, Michigan 48109-1048, USA.
Joglekar
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AP
Williams
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Hay
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JC
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002845:Chromatography; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000737:chemistry*; D004926:Escherichia coli / Q000378:metabolism; D005982:Glutathione Transferase / Q000378:metabolism; D006056:Golgi Apparatus / Q000737:chemistry; D006801:Humans; D007700:Kinetics; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008954:Models, Biological; D011233:Precipitin Tests; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D050765:Qa-SNARE Proteins; D050766:Qb-SNARE Proteins; D050767:Qc-SNARE Proteins; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D050600:SNARE Proteins; D014162:Transfection; D033921:Vesicular Transport Proteins
C098783:BET1 protein, rat; C491344:GOSR2 protein, human; C491343:Gosr2 protein, rat; D008565:Membrane Proteins; D050765:Qa-SNARE Proteins; D050766:Qb-SNARE Proteins; D050767:Qc-SNARE Proteins; D011993:Recombinant Fusion Proteins; D050600:SNARE Proteins; D033921:Vesicular Transport Proteins; D005982:Glutathione Transferase