title: Identification of a motif in the acetylcholine receptor beta subunit whose phosphorylation regulates rapsyn association and postsynaptic receptor localization.
abstract: At the neuromuscular junction, the acetylcholine receptor (AChR) is specifically clustered in the postsynaptic membrane via interactions with rapsyn and other scaffolding proteins. However, it remains unclear where these proteins bind on the AChR and how the interactions are regulated. Here, we define a phosphorylation-dependent binding site on the receptor that mediates agrin-induced clustering. Using chimeric proteins in which CD4 is fused to the large intracellular loop of each of the AChR subunits we found that agrin induced clustering of only chimeras containing the beta subunit loop. By making deletions in the beta loop we defined a 20 amino-acid sequence that is sufficient for clustering. The sequence contains a conserved tyrosine (Y390) whose phosphorylation is induced by agrin and whose mutation abolished clustering of beta loop chimeras and their ability to inhibit agrin-induced clustering of the endogenous AChR. Phosphorylation of the AChR beta subunit is correlated with increased rapsyn/AChR binding, suggesting that the effect of betaY390 phosphorylation on clustering is mediated by rapsyn. Indeed, we found that rapsyn associated with CD4-beta loop chimeras in a phosphorylation-dependent manner, and that agrin increased the ratio of rapsyn binding to wild type AChR but not to AChR-beta(3F/3F), which lacks beta loop tyrosine phosphorylation sites. Together, these findings suggest that agrin-induced phosphorylation of the beta subunit motif increases the stoichiometry of rapsyn binding to the AChR, thereby helping to stably cluster the receptor and anchor it at high density in the postsynaptic membrane.
authors.affiliation: Department of Anesthesiology and Physiology, University of California, Davis, Davis, California 95616, USA.
authors.forename: Lucia S
authors.initials: LS
authors.lastname: Borges
authors.forename: Sergey
authors.initials: S
authors.lastname: Yechikhov
authors.forename: Young I
authors.initials: YI
authors.lastname: Lee
authors.forename: John B
authors.initials: JB
authors.lastname: Rudell
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Friese
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Burden
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ferns
mesh_terms: D018171:Agrin / Q000494:pharmacology*; D020816:Amino Acid Motifs / Q000502:physiology; D000704:Analysis of Variance; D000818:Animals; D002038:Bungarotoxins / Q000378:metabolism; D002461:Cell Line, Transformed; D005786:Gene Expression Regulation / Q000187:drug effects; D047468:Immunoprecipitation; D051379:Mice; D008958:Models, Molecular; D032342:Muscle Cells / Q000187:drug effects* / Q000502:physiology; D009124:Muscle Proteins / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D011485:Protein Binding / Q000187:drug effects; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000378:metabolism; D011978:Receptors, Nicotinic / Q000378:metabolism*; D014162:Transfection / Q000379:methods; D014443:Tyrosine / Q000378:metabolism
chemical_list: D018171:Agrin; D002038:Bungarotoxins; D009124:Muscle Proteins; D021122:Protein Subunits; D011978:Receptors, Nicotinic; C051609:peripheral membrane protein 43K; D014443:Tyrosine