Catalytic domain of the Protein Tyrosine Kinase, Muscle-specific kinase
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Musk is a receptor PTK (RTK) containing an extracellular region with four immunoglobulin-like domains and a cysteine-rich cluster, a transmembrane segment, and an intracellular catalytic domain. Musk is expressed and concentrated in the postsynaptic membrane in skeletal muscle. It is essential for the establishment of the neuromuscular junction (NMJ), a peripheral synapse that conveys signals from motor neurons to muscle cells. Agrin, a large proteoglycan released from motor neurons, stimulates Musk autophosphorylation and activation, leading to the clustering of acetylcholine receptors (AChRs). To date, there is no evidence to suggest that agrin binds directly to Musk. Mutations in AChR, Musk and other partners are responsible for diseases of the NMJ, such as the autoimmune syndrome myasthenia gravis. The Musk subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.