Developmental regulation of tyrosine phosphorylation of the nicotinic acetylcholine receptor in Torpedo electrocyte

Tyrosine phosphorylation is thought to play a critical role in the clusteri ng of acetylcholine receptors (AChR) at the developing neuromuscular juncti on. Yet, in vitro approaches have led to conflicting conclusions regarding the function of tyrosine phosphorylation of AChR beta subunit in AChR clust ering. In this work, we followed in situ the time course of tyrosine phosph orylation of AChR in developing Torpedo electrocyte. We observed that tyros ine phosphorylation of the AChR beta and delta subunits occurs at a late st age of embryonic development after the accumulation of AChRs and rapsyn in the membrane and the onset of innervation. Interestingly, in the mature pos tsynaptic membrane, we observed two populations of AChR differing both in t heir phosphotyrosine content and distribution. Our data are consistent with the notion that tyrosine phosphorylation of the AChR is related to downstr eam events in the pathway regulating AChR accumulation rather than to initi al clustering events.