Previously, we have shown that the capping of surface immunoglobulins on murine lymphocytes can be affected by modulating the lipid environment of the surface membrane with free fatty acids. In the present study, murine lymphocytes were depleted of cholesterol by incubation with phospholipid vesicles. As the cellular cholesterol:phospholipid ratio decreased, the capping of the surface immunoglobulin was seen to decrease. This inhibition of capping could not be reversed by calcium and is not accompanied by changes in either the cytoskeletal element alpha-actinin or cellular ATP levels. Incubation of the cholesterol-depleted cells with cholesterol-containing phospholipid vesicles raised both the cholesterol:phospholipid ratio and capping levels to values close to those of untreated control cells. Remarkably, stearic acid, a saturated fatty acid, could also restore the capping levels in the cholesterol-depleted cells. On the basis of the present data and measurements of the fluorescence polarization of the probe diphenyl hexatriene, we propose a model in which the protein(s) involved in capping is located in a gel-like lipid domain, and that removal of cholesterol makes this domain less gel-like and inhibits capping. Restoration of the gel-like nature of this domain by the addition of either cholesterol or stearic acid enables the protein(s) to function normally.

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