Role of the endoplasmic reticulum chaperone calnexin in subunit folding and assembly of nicotinic acetylcholine receptors.
The nicotinic acetylcholine receptor (AChR) is a pentameric complex assembled from four different gene products by mechanisms that are inadequately understood. In this study we investigated the role of the endoplasmic reticulum (ER)-resident molecular chaperone calnexin in AChR subunit folding and assembly. We have shown that calnexin interacts with nascent AChR alpha-subunits (AChR-alpha) in muscle cell cultures and in COS cells transfected with mouse AChR-alpha. In chick muscle cells maximal association of labeled alpha-subunits with calnexin was observed immediately after a 15-min pulse with [35S]methionine/cysteine and subsequently declined with a t1/2 of approximately 20 min. The decrease in association with calnexin was concomitant with the folding of the alpha-subunit to achieve conformational maturation shortly before assembly. Brefeldin A did not inhibit AChR subunit assembly or the dissociation of calnexin from the assembling subunits, confirming that the ER is the site of AChR assembly and that calnexin dissociation is not affected under conditions in which the exit of assembled AChR from the ER is blocked. These results indicate that calnexin participates directly in the molecular events that lead to AChR assembly.
Gelman
M S
MS
Department of Pharmacological Sciences, State University of New York at Stony Brook 11794, USA.
Chang
W
W
Thomas
D Y
DY
Bergeron
J J
JJ
Prives
J M
JM
D000818:Animals; D020126:Brefeldin A; D002135:Calcium-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D037281:Calnexin; D002478:Cells, Cultured; D002642:Chick Embryo; D003517:Cyclopentanes / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D007700:Kinetics; D046911:Macromolecular Substances; D051379:Mice; D008961:Models, Structural; D018832:Molecular Chaperones / Q000378:metabolism*; D008970:Molecular Weight; D018482:Muscle, Skeletal / Q000378:metabolism*; D011485:Protein Binding; D017510:Protein Folding; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000096:biosynthesis* / Q000737:chemistry* / Q000302:isolation & purification; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000302:isolation & purification; D014162:Transfection
D002135:Calcium-Binding Proteins; D003517:Cyclopentanes; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D011500:Protein Synthesis Inhibitors; D011978:Receptors, Nicotinic; D011994:Recombinant Proteins; D037281:Calnexin; D020126:Brefeldin A