Shape of the fluidity gradient in the plasma membrane of living HeLa cells.

The shape of the fluidity gradient of the outer hemi- leaflet of the plasma membrane of living HeLa cells was deter- mined using a series of n-(9-anthroyloxy) fatty acid probes where n = 2, 3, 6, 7, 9, 10, 11, 12, and 16. Fluorescence uptake and steady-state anisotropy values were obtained with a flow cytometer capable of continuous recording over time of vertical and horizontal emission intensities, and of the output of these intensities as calculated anisotropy values. The fluorescence up- take of all of the membrane probes was rapid up to about 15 min. The magnitudes of the uptake of fluorescence were, for the n-(9-anthroyloxy) series, in the order 2>3>6>7>9>10> 11 = 12 = 16. Anisotropy values were constant from 5 to 30 min after addition of the various probes, and the magnitudes were in the order 7 > 6 > 9 = 10 > 2 = 3 > 11 > 12 > 16, indicative of the shape of the fluidity gradient. No differences were noted be- tween the values obtained with 12-(9-anthroyloxy) stearic acid and 12- (9-anthroyloxy) oleate. The kinetics of anisotropy exhi- bited by those probes with the anthroyloxy group in positions deeper than 9, where initially higher values declined until equi- librium was reached, were probably indicative of an energy bar- rier at the approximate depth sensed by 7AS. -Collins, J. M., R. N. Dominey, and W. M. Grogan. Shape of the fluidity grad- ient in the plasma membrane of living HeLa cells. J Lipid Res. 1990. 31: 261-270.