Fluorescence photobleaching recovery (FPR) denotes a method for measuring two-dimensional lateral mobility of fluorescent particles, for example, the motion of fluorescently labeled molecules in approximately 10 mum2 regions of a single cell surface. A small spot on the fluorescent surface is photobleached by a brief exposure to an intense focused laser beam, and the subsequent recovery of the fluorescence is monitored by the same, but attenuated, laser beam. Recovery occurs by replenishment of intact fluorophore in the bleached spot by lateral transport from the surrounding surface. We present the theoretical basis and some practical guidelines for simple, rigorous analysis of FPR experiments. Information obtainable from FPR experiments includes: (a) identification of transport process type, i.e. the admixture of random diffusion and uniform directed flow; (b) determination of the absolute mobility coefficient, i.e. the diffusion constant and/or flow velocity; and (c) the fraction of total fluorophore which is mobile. To illustrate the experimental method and to verify the theory for diffusion, we describe some model experiments on aqueous solutions of rhodamine 6G.

The first order electric field correlation function of laser light scattered by polydisperse solutions of macromolecules can be written as a sum or distribution of exponentials, with decay rates proportional to the diffusion coefficients of the solute molecules. It is shown that the logarithm of this correlation function is formally equivalent to a cumulant generating function. A method is described by which the distribution function of the decay rates (and thus the extent of polydispersity) can be characterized, in a light scattering experiment, by calculation of the moments or cumulants. The systematic and random statistical errors in the calculated cumulants are discussed.

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We have developed an effective experimental system for the characterization of molecular and structural mobility. It incorporates a modified fluorescence microscope geometry and a variety of analytical techniques to measure effective diffusion coefficients ranging over almost six orders of magnitude, from less than 10(-11) cm2/s to greater than 10(-6) cm2/s. Two principal techniques, fluorescence correlation spectroscopy (FCS) and fluorescence photobleaching recovery (FPR), are employed. In the FPR technique, translational transport rates are measured by monitoring the evolution of a spatial inhomogeneity of fluorescence that is produced photochemically in a microscopic volume by a short burst of intense laser radiation. In contrast, FCS uses laser-induced fluorescence to probe the spontaneous concentration fluctuations in microscopic sample volumes. The kinetics are analyzed by computing time-correlation functions of the stochastic fluctuations of the measured fluorescence intensity. The optical system and digital photocount correlator designed around a dedicated minicomputer are described and discussed. The general power of these techniques is demonstrated with examples from studies conducted on bulk solutions, lipid bilayer membranes, and mammalian cell plasma membranes.

We 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 a-bungarotoxin and AChR lateral motion was measured by the fluoresence photobleaching recovery technique. We found two coexisting distinct classes of AChR: (i) mobile, uniformly distributed AChR that appear on all myotubes shortly after fusion from myoblasts; and (ii) immobile, dense, highly granular AChR in patches of 10-60 Atm size that appear shortly after fusion and disappear after myotubes have become extensively interconnected. In addition, evidence of turnover of AChR labeled with tetramethylrhodamine-a-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.An important feature of the differentiation of skeletal muscle is the appearance of acetylcholine receptors (AChR) on the surface of myotubes (1, 2). In primary tissue culture this event occurs shortly after mononuclear myoblasts fuse to form multinucleated myotubes (3, 4). Study of the membrane incorporation, motion, and turnover of AChR may lead to an understanding of the mechanisms that initiate receptor biosynthesis and determine the eventual localization of receptors in vivo at neuromuscular junctions. We describe here direct, quantitative observations of lateral motion of AChR on the myotube surface.We have used fluorescently labeled a-bungarotoxin (atBgt) as a marker of AChR on viable rat myotubes and the fluorescence photobleaching recovery (FPR) technique (5, 6) to measure lateral motion. We have also mapped the distribution and observed the turnover of the fluorescently labeled aBgt bound to the myotube.Previous studies have shown that the distribution of AChR on muscle is highly nonuniform (7,8), but it is not yet understood how the nonuniformity is achieved or maintained. In normal adult skeletal muscle, AChR are clustered at the neuromuscular junction, although after denervation, AChR appear over the entire surface of the muscle fiber (7). In primary cultures of chick myotubes, both a low density uniform distribution and high density "clusters" have been revealed by optical autoradiography with '25I-labeled neurotoxins that specifically bind to AChR (2-4). The time course of synthesis and turnover of AChR in cultured myotubes has been studied with radioactive snake toxins (9, 10) and with radioactive amino acid precursors (11).FPR, the fluorescence technique that we used in this study to measure lateral AChR motion, and related experimental variations of FPR, have been successfully applied previously (refs. 12-17) to study the lateral motion of proteins and lipids on the surface of tissue culture cells and erythrocytes. Fluorescent marking of AChR (18), although less sensitive than radioactive marking due to a higher ...