RECRUITMENT OF A NICOTINIC ACETYLCHOLINE-RECEPTOR MUTANT LACKING CYTOPLASMIC TYROSINE RESIDUES IN ITS BETA-SUBUNIT INTO AGRIN-INDUCED AGGREGATES

During synaptogenesis at the vertebrate skeletal neuromuscular junctio n, acetylcholine receptors (AChRs) form high-density aggregates apposi te the presynaptic terminal in response to nerve-derived agrin. Agrin has been shown to stimulate tyrosine phosphorylation of a muscle-speci fic receptor tyrosine kinase MuSK and of the AChR beta subunit, and ty rosine kinase inhibitors and a tyrosine kinase-deficient mutant of MuS K prevent AChR aggregation. To evaluate the role of tyrosine phosphory lation of the AChR beta subunit in receptor aggregation, we replaced a ll three putative cytoplasmic tyrosine residues of the AChR beta subun it with phenylalanine residues and expressed the mutant receptors in c ultured myotubes. Upon agrin treatment, transfected myotubes formed AC hR aggregates that contained receptors with mutant beta subunits. Thus , AChRs can be recruited into agrin-induced specializations by protein -protein interactions that do not depend on tyrosine phosphorylation o f the AChR beta subunit.