Nerve-induced clustering of the nicotinic acetylcholine receptor (AChR) requires rapsyn, a synaptic peripheral membrane protein, as well as protein- tyrosine kinase activity. Here, we show that rapsyn induces the clustering of the synapse-specific receptor-tyrosine kinase MuSK in transfected QT-6 fibroblasts. Furthermore, rapsyn stimulates the autophosphorylation of MUSK, leading to a subsequent MuSK-dependent increase in cellular tyrosine phosphorylation. Moreover, rapsyn-activated MuSK specifically phosphorylated the AChR β subunit, the same subunit that is tyrosine phosphorylated during innervation or agrin treatment of muscle. These results suggest rapsyn may mediate the synaptic localization of MuSK in muscle and that MuSK may play an important role in the agrin-induced clustering of the AChR.
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