Rotational diffusion of acetylcholine receptors on cultured rat myotubes.

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Abstract
The rotational mobility of acetylcholine receptors (AChR) in the plasma membrane of living rat myotubes in culture is measured in this study by polarized fluorescence recovery after photobleaching (PFRAP). These AChR are known to exist in two distinct classes, evident by labeling with rhodamine .alpha.-bungarotoxin; clustered AChR that are aggregated in a pattern of highly concentrated speckles and streaks, with each cluster occupying an area of .apprx. 1,000 .mu.m2; and nonclustered AChR that appear as diffuse labeling. PFRAP results reported here show that: (a) most clustered AChR (.apprx. 86%) are rationally immobile within a time scale of at least several seconds; and (b) most nonclustered AChR (.apprx. 76%) are rotationally mobile with characteristic times ranging from < 50 ms to 0.1 s. External cross-linking with the tetravalent lectin concanavalin A immobilizes many nonclustered AChR. PFRAP experiments in the presence of carbachol or cytochalasin D show that the restraints to rotational motion in clusters are remarkably immune to treatments that disperse clusters or disrupt cytoplasmic actin. The experiments also demonstrate the feasibility of using PFRAP to measure rotational diffusion on selected microscopic areas of living nondeoxygenated cells labeled with standard fluorescence probes over a very wide range of time scales, and they also indicate what technical improvements would make PFRAP even more practicable.