Subnanosecond polarized fluorescence photobleaching: rotational diffusion of acetylcholine receptors on developing muscle cells.

Polarized fluorescence recovery after photobleaching (PFRAP) is a technique for measuring the rate of rotational motion of biomolecules on living, nondeoxygenated cells with characteristic times previously ranging from milliseconds to many seconds. Although very broad, that time range excludes the possibility of quantitatively observing freely rotating membrane protein monomers that typically should have a characteristic decay time of only several microseconds. This report describes an extension of the PFRAP technique to a much shorter time scale. With this new system, PFRAP experiments can be conducted with sample time as short as 0.4 microseconds and detection of possible characteristic times of less than 2 microseconds. The system is tested on rhodamine-alpha-bungarotoxin-labeled acetylcholine receptors (AChRs) on myotubes grown in primary cultures of embryonic rat muscle, in both endogenously clustered and nonclustered regions of AChR distribution. It is found that approximately 40% of the AChRs in nonclustered regions undergoes rotational diffusion fast enough to possibly arise from unrestricted monomer Brownian motion. The AChRs in clusters, on the other hand, are almost immobile. The effects of rat embryonic brain extract (which contains AChR aggregating factors) on the myotube AChR were also examined by the fast PFRAP system. Brain extract is known to abolish the presence of endogenous clusters and to induce the formation of new clusters. It is found here that rotational diffusion of AChR in the extract-induced clusters is as slow as that in endogenous clusters on untreated cells but that rotational diffusion in the nonclustered regions of extract-treated myotubes remains rapid.