RAPSYN CARBOXYL TERMINAL DOMAINS MEDIATE MUSCLESPECIFIC KINASE–INDUCED PHOSPHORYLATION OFTHE MUSCLE ACETYLCHOLINE RECEPTORY. LEE, J. RUDELL, S. YECHIKHOV, R. TAYLOR,S. SWOPE AND M. FERNS*Departments of Anesthesiology and Physiology and Membrane Biol-ogy, One Shields Avenue, University of California Davis, Davis, CA95616, USAAbstract—At the developing vertebrate neuromuscular junc-tion, postsynaptic localization of the acetylcholine receptor(AChR) is regulated by agrin signaling via the muscle specifickinase (MuSK) and requires an intracellular scaffolding pro-tein called rapsyn. In addition to its structural role, rapsyn isalso necessary for agrin-induced tyrosine phosphorylation ofthe AChR, which regulates some aspects of receptor local-ization. Here, we have investigated the molecular mechanismby which rapsyn mediates AChR phosphorylation at the ro-dent neuromuscular junction. In a heterologous COS cellsystem, we show that MuSK and rapsyn induced phosphor-ylation of  subunit tyrosine 390 (Y390) and ␦ subunit Y393,as in muscle cells. Mutation of  Y390 or ␦ Y393 did not inhibitMuSK/rapsyn-induced phosphorylation of the other subunitin COS cells, and mutation of  Y390 did not inhibit agrin-induced phosphorylation of the ␦ subunit in Sol8 musclecells; thus, their phosphorylation occurs independently,downstream of MuSK activation. In COS cells, we furthershow that MuSK-induced phosphorylation of the  subunitwas mediated by rapsyn, as MuSK plus rapsyn increased Y390 phosphorylation more than rapsyn alone and MuSKalone had no effect. Intriguingly, MuSK also induced tyrosinephosphorylation of rapsyn itself. We then used deletion mu-tants to map the rapsyn domains responsible for activation ofcytoplasmic tyrosine kinases that p...
