Muscle-specific tyrosine kinase activation by a peptide-based dimerizer is orientation dependent.

Muscle-specific receptor tyrosine kinase (MuSK) is a single-pass transmembrane protein expressed on skeletal muscle. MuSK is activated by binding of nerve-derived agrin with the help of muscle coreceptor LRP4, leading to the clustering of acetylcholine receptors (AChR), which is required for the formation and maintenance of functional neuromuscular junctions. The structural mechanism of MuSK activation by physiological and artificial agonistic agents has remained elusive. In this study, we isolated a 27-residue linear peptide (L1) that binds human MuSK with high affinity. Genetic fusion of L1 to either the N or C termini of the human IgG Fc resulted in two different versions of MuSK dimerizers, denoted as L1-Fc and Fc-L1. Only Fc-L1 activated MuSK on myotubes and induced AChR clustering. Crystallographic analysis of MuSK-L1 interactions revealed that MuSK activation requires a particular dimeric conformation, pointing toward the importance of the lateral size of the receptor complex at the muscle cell surface.