The subsynaptic 43-kDa protein is concentrated at developing nerve-muscle synapses in vitro

A 43-kDa peripheral membrane protein is known to copurify with acetylcholine receptor (AcChoR)-rich membranes isolated from the electric organ of Torpedo californica. Immunoelectron microscopy and crosslinking studies have demonstrated that this 43-kDa protein is closely associated with the cytoplasmic domain(s) of the AcChoR and suggest that the 43-kDa protein could regulate the distribution of the AcChoR in the postsynaptic membrane. This paper demonstrates that this postsynaptic protein appears at developing neuromuscular synapses in Xenopus nerve/muscle cocultures as early as AcChoRs become clustered at synaptic sites. Moreover, this protein is concentrated at AcChoR clusters that occur on noninnervated muscle cells. The close spatial and temporal relationship of this subsynaptic protein and AcChoR clusters is consistent with a role for the 43-kDa protein in the formation and/or stabilization of AcChoR clusters.