Postsynaptic protein required for clustering of nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction. It may link the receptor to the underlying postsynaptic cytoskeleton, possibly by direct association with actin or spectrin (By similarity). Cytoplasmic surface of postsynaptic membranes. A cysteine-rich region homologous to part of the regulatory domain of protein kinase C may be important in interactions of this protein with the lipid bilayer. Ubiquitinated by the BCR(KLHL8) complex, leading to its degradation. Belongs to the RAPsyn family.