A ternary complex of AChR, rapsyn, and α-actinin. (A) Live C2C12 myotubes were incubated with B-BTX for 2 hr at 4°C. Surface receptors were purified by streptavidin-coupled agarose beads. AChR-associated proteins were probed with antibodies specific for α-actinin and the AChR β-subunit. (B) Quantitation of blot. The amount of α-actinin associated with surface AChRs was increased upon in agrin-stimulated myotubes. Data represent mean ± SEM of three experiments. **, p < 0.01, two-tailed Student's paired t test. (C) α-Actinin was not associated with AChRs in rapsyn -/- myotubes. Control and rapsyn mutant myotubes were treated with agrin. AChRs were labeled by B-BTX and probed for α-actinin.