MuSK glycosylation restrains MuSK activation and acetylcholine receptor clustering.

MuSK, a muscle-specific receptor tyrosine kinase that is activated by agrin, has a critical role in neuromuscular synapse formation. In cultured myotubes, agrin stimulates the rapid phosphorylation of MuSK, leading to MuSK activation and tyrosine phosphorylation and clustering of acetylcholine receptors. Agrin, however, fails to stimulate tyrosine phosphorylation of MuSK that is force-expressed in myoblasts and fibroblasts, indicating that myotubes contain an additional activity that is required for agrin to stimulate MuSK. Certain glycosyltransferases are expressed selectively at synaptic sites in skeletal muscle, raising the possibility that carbohydrate modifications of MuSK, catalyzed by glycosyltransferases expressed selectively in myotubes, may be essential for agrin to bind and activate MuSK. We identifed two N-linked glycosylation sites in MuSK, and we expressed MuSK mutants lacking one or both N-linked sites into MuSK mutant myotubes to determine whether N-linked carbohydrate modifications of MuSK have a role in MuSK activation. We found that N-linked glycosylation restrains ligand-independent tyrosine phosphorylation of MuSK and downstream signaling but is not necessary for agrin to stimulate MuSK.