Rapsyn 

Rapsyn is a 43 kDa cytoplasmic membrane-associated protein that tightly associates with the intracellular domain of acetylcholine receptors (AChR) and is detected at neuromuscular junctions (NMJ) as soon as AChRs cluster. When rapsyn is knocked out in mice, they don't undergo AChR clustering.

References

The Neuron Ch 18

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