The tetratricopeptide repeat domains of rapsyn bind directly to cytoplasmic sequences of the muscle-specific kinase.

Clustering of acetylcholine receptors at the developing vertebrate neuromuscular junction is initiated by neural agrin, which stimulates the activity of the muscle-specific kinase (MuSK). Acetylcholine receptor clustering is also dependent on the postsynaptic scaffolding protein, rapsyn, which binds to acetylcholine receptors. Here, we address the possibility that MuSK and rapsyn bind directly to each other by coexpressing sequences of the cytoplasmic domain of MuSK with rapsyn in COS-7 cells and assaying for codistribution and biochemical interaction. Sequences constituting the bulk of the kinase domain can interact with rapsyn. This interaction is mediated by the tetratricopeptide repeat domains, but not the coiled coil or zinc finger domains, of rapsyn. This interaction does not require tyrosine phosphorylation of the MuSK sequences. Binding is direct, as indicated by blot overlay and surface plasmon resonance experiments. The sequence of the cytoplasmic domain of MuSK that most effectively codistributes with rapsyn confers the ability of an otherwise inactive receptor tyrosine kinase, TrkA, to associate with rapsyn. Our results support a model in which the tetratricopeptide repeat domains of rapsyn bind directly to the cytoplasmic portion of MuSK, which could thereby serve as an initial scaffold for the clustering of acetylcholine receptors.