We have used fluorescence photobleaching and recovery (FPR) to measure the lateral diffusion of mouse H-2 antigens, labeled with fluorescent Fab fragments, in the membrane of cl 1d fibroblasts. Diffusion coefficients, D, vary more than 20-fold from cell to cell, though they vary no more than twofold when measured at different points on a single cell. The fraction of H-2 antigens mobile, R, also varies from cell to cell, and no lateral diffusion of H-2 antigens can be detected in approximately 20% of the cells examined. Treatment of cells with NaCN + NaF, reducing their levels of ATP reduces the proportion of cells in which no lateral diffusion can be detected. The maximum values of D seen in poisoned cells are less than those in controls. Treatment of cells with the divalent inophore, A23187, greatly increases the proportion of cells in which diffusion of H-2 is rapid, D greater than 2 x 10(-9) cm2 s-1. The data obtained on diffusion by FPR can be replotted in the form of an experiment in which lateral diffusion of H-2 antigens is measured in a population of heterokaryons. There is good agreement between this transformation and actual data on heterokaryons. Thus the two methods appear to measure the same transport process.

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