FIG. 4.

STI 571 inhibits late AChR β and MuSK phosphorylation induced by agrin. C2 myotubes were treated as described in the legend to Fig. 2, but STI 571 was used to inhibit Abl kinases. Phosphotyrosine blots were reprobed with antibodies against AChR α subunits (A) or MuSK (B). Quantitation (means ± SEM from at least eight experiments) revealed a pronounced reduction by STI 571 in phosphorylation of AChR β subunits and MuSK at 40 min of agrin treatment (**, significant difference from corresponding inhibitor-free sample, P < 0.01; *, P < 0.05; two-tailed paired t test). Tox-P, precipitation by biotin-α-BT; pTyr-blot, phosphotyrosine immunoblot; IP, immunoprecipitation.