Receptor tyrosine kinase which plays a central role in the formation and the maintenance of the neuromuscular junction (NMJ), the synapse between the motor neuron and the skeletal muscle. Recruitment of AGRIN by LRP4 to the MUSK signaling complex induces phosphorylation and activation of MUSK, the kinase of the complex. The activation of MUSK in myotubes regulates the formation of NMJs through the regulation of different processes including the specific expression of genes in subsynaptic nuclei, the reorganization of the actin cytoskeleton and the clustering of the acetylcholine receptors (AChR) in the postsynaptic membrane. May also play a role within the central nervous system by mediating cholinergic responses, synaptic plasticity and memory formation (By similarity). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Monomer (By similarity). Homodimer (Probable). Interacts with LRP4; the heterodimer forms an AGRIN receptor complex that binds AGRIN resulting in activation of MUSK (By similarity). Localizes to the postsynaptic cell membrane of the neuromuscular junction. Expressed in muscle, but also in brain. Weakly expressed in kidney, gizzard, intestine and testis. From E7.0 day, expressed in muscle but also in brain and liver. Skeletal myogenesis is a major site of expression during normal embryogenesis. In addition, the ganglia of the developing peripheral nervous system and various embryonic epithelia, including those of kidney, lung and gut are also sites of expression. Declined slowly to adult. Belongs to the protein kinase superfamily. Tyr protein kinase family.