Rapsyn interacts with the muscle acetylcholine receptor via α-helical domains in the α, β, and ε subunit intracellular loops

At the developing vertebrate neuromuscular junction, the acetylcholine receptor becomes aggregated at high density in the postsynaptic muscle membrane. Receptor localization is regulated by the motoneuron-derived factor, agrin, and requires an intracellular, scaffolding protein called rapsyn. However, it remains unclear where rapsyn binds on the acetylcholine receptor and how their interaction is regulated. In this study, we identified rapsyn's binding site on the acetylcholine receptor using chimeric constructs where the intracellular domain of CD4 was substituted for the major intracellular loop of each mouse acetylcholine receptor subunit. When expressed in heterologous cells, we found that rapsyn clustered and cytoskeletally anchored CD4-α, β and ε subunit loops but not CD4-δ loop. Rapsyn-mediated clustering and anchoring was highest for β loop, followed by ε and α, suggesting that rapsyn interacts with the loops with different affinities. Moreover, by making deletions within the β subunit intracellular loop, we show that rapsyn interacts with the α-helical region, a secondary structural motif present in the carboxyl terminal portion of the subunit loops. When expressed in muscle cells, rapsyn co-immunoprecipitated together with a CD4-α helical region chimera, independent of agrin signaling. Together, these findings demonstrate that rapsyn interacts with the acetylcholine receptor via an α-helical structural motif conserved between the α, β and ε subunits. Binding at this site likely mediates the critical rapsyn interaction involved in localizing the acetylcholine receptor at the neuromuscular junction.