REGULATION OF THE INTERACTION OF NICOTINIC ACETYLCHOLINE-RECEPTORS WITH THE CYTOSKELETON BY AGRIN-ACTIVATED PROTEIN-TYROSINE KINASE

Agrin induces the accumulation of nicotinic acetylcholine receptors (A ChRs) in the myofiber membrane at synaptic sites in vertebrate skeleta l muscle and causes an increase in tyrosine phosphorylation of the ACh R beta subunit. To examine further the mechanism of agrin-induced AChR phosphorylation and the relationship between changes in protein phosp horylation and AChR aggregation, the effect of the protein tyrosine ph osphatase inhibitor sodium pervanadate was tested on chick myotubes in culture. Pervanadate caused an increase in the phosphotyrosine conten t of a variety of proteins, including the AChR. Pervanadate also preve nted agrin-induced AChR aggregation and slowed the rate at which AChRs were extracted from intact myotubes by mild detergent treatment. The rate at which phosphorylation of the AChR beta subunit and receptor de tergent extractability changed following pervanadate-induced phosphata se inhibition was increased by agrin, indicating that agrin activates a protein tyrosine kinase rather than inhibiting a protein tyrosine ph osphatase. The present results, taken together with previous findings on the inhibition of agrin-induced AChR aggregation by protein kinase inhibitors, demonstrate that protein tyrosine phosphorylation regulate s the formation and stability of AChR aggregates, apparently by streng thening the interaction between AChRs and the cytoskeleton.