Rapsyn as a signaling and scaffolding molecule in neuromuscular junction formation and maintenance.

The acetylcholine receptor (AChR) is highly concentrated at the neuromuscular junction (NMJ), ensuring efficient signal transmission from motoneurons to muscle fibers. This requires the agrin-LRP4-MuSK signaling as well as rapsyn, a peripheral, intracellular protein that is enriched at the NMJ. Mutations of rapsyn have been associated with NMJ diseases including congenital myasthenia syndromes. Rapsyn is a prototype of synaptic adaptor proteins that is thought to bind and anchor neurotransmitter receptors to the postsynaptic membrane. In accord, it interacts with the AChR and a plethora of proteins that associate or regulate the cytoskeleton. Rapsyn also interacts with signaling molecules. Recent studies show that it possesses E3 ligase activity that is required for NMJ formation, revealing a novel function of this classic adaptor protein. Identifying rapsyn as a signaling molecule provides a handle in studies of mechanisms of NMJ formation, maintenance, aging and disorders.