Restricted mobility of membrane constituents in cell-substrate focal contacts of chicken fibroblasts.

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Abstract
We studied the lateral mobility of membrane components in cell-substrate focal contacts using the fluorescence photobleaching recovery method. The measurements were performed on isolated substrate-attached membranes of chicken gizzard fibroblasts. The diffusion coefficients of a fluorescent lipid probe and rhodamine-conjugated surface proteins within contact regions (identified by interference-reflection microscopy) were significantly lower than those measured in nonattached areas along the ventral membrane. Complete recovery of fluorescence after photobleaching of the lipid probe was measured both in focal contacts and in nonattached areas with lateral diffusion coefficient (D) of approximately 10(-8) cm2/s. This indicated that the lipid probe is free to diffuse from and into the contact regions. Rhodamine-labeled surface components (mostly proteins) exhibited almost complete recovery after bleaching (approximately 90%) in unattached regions of the ventral membrane with D congruent to 10(-9 cm2/s. The rhodamine-labeled proteins in focal contacts showed only partial recovery (approximately 50%), suggesting that large proportion of the membrane proteins in cell-substrate contacts are immobile (within the time scale of the experiments, D less than or equal to 5 x 10(-12) cm2/s. The implications of these findings on the molecular dynamics of cell contacts are discussed.