Gel phase phospholipid in the plasma membrane of sterol-depleted mouse LM cells. Analysis by fluorescence polarization and x-ray diffraction.

Fluorescence polarization of cis- and trans-parinaric acid and high angle x-ray diffraction are used to examine the physical effects of in vitro sterol depletion of LM cell plasma membranes. Measurements on lipids isolated from the normal plasma membrane show that most of the lipid is fluid between 5 and 45 degrees C. When no sterol is present, approximately 15% of the phospholipids undergo gel phase formation, detectable by x-ray measurements, in this temperature range. Partial sterol repletion of the isolated phospholipids serves to lower the onset temperature of the phase transition, as well as to decrease the fraction of lipid undergoing a transition. Intact plasma membranes are sterol depleted by in vitro incubation with egg phosphatidylcholine liposomes. This induces a phase transition with an onset temperature increasingly above the physiological temperature as more sterol is removed from the membranes. The transition can be suppressed by returning sterol to the membranes. These experiments extend earlier work (Rintoul, D. A., Chou, S.-M., and Silbert, D. F. (1979) J. Biol. Chem. 254, 10070-10077) on isolated plasma membrane lipids by showing that sterol suppresses a phase transition in intact membranes as well as in isolated lipids. The correlation between gel phase formation upon sterol loss at the physiological temperature and loss of membrane glucose transport function is discussed.