Lateral motion of fluorescently labeled acetylcholine receptors in membranes of developing muscle fibers

The authors have made direct, quantitative measurements of the lateral motion and age-dependent distribution of acetylcholine receptors (AChR) on the surface of rat myotubes in primary culture. AChR were fluorescently marked with tetramethylrhodamine-labeled α-bungarotoxin and AChR lateral motion was measured by the fluorescence photobleaching recovery technique. The authors found two coexisting distinct classes of AChR: mobile, uniformly distributed AChR that appear on all myotubes shortly after fusion from myoblasts; and immobile, dense, highly granular AChR in patches of 10-60 μm size that appear shortly after fusion and disappear after myotubes have become extensively interconnected. In addition, evidence of turnover of AChR labeled with tetramethylrhodamine-α-bungarotoxin is seen in the gradual internalization of surface fluorescence within 36 hr after labeling. The relevance of these results to an understanding of the membrane dynamics and localization of muscle AChR is discussed.