AChR phosphorylation and aggregation induced by an agrin fragment that lacks the binding domain for...

Agrin induces both phosphorylation and aggregation of nicotinic acetylcholine receptors (AChRs) when added to myotubes in culture, apparently by binding to a specific receptor on the myotube surface. One such agrin receptor is alpha-dystroglycan, although binding to alpha-dystroglycan appears not to mediate AChR aggregation. To determine whether agrin-induced AChR phosphorylation is mediated by alpha-dystroglycan or by a different agrin receptor, fragments of recombinant agrin that differ in affinity for alpha-dystroglycan were examined for their ability to induce AChR phosphorylation and aggregation in mouse C2 myotubes. The carboxy-terminal 95 kDa agrin fragment agrin-c95(A0B0), which binds to alpha-dystroglycan with high affinity, failed to induce AChR phosphorylation and aggregation. In contrast, agrin-c95(A4B8) which binds less strongly to alpha-dystroglycan, induced both phosphorylation and aggregation, as did a small 21 kDa fragment of agrin, agrin-c21(B8), that completely lacks the binding domain for alpha-dystroglycan. We conclude that agrin-induced AChR phosphorylation and aggregation are triggered by an agrin receptor that is distinct from alpha-dystroglycan.