Effect of Sterol Depletion on LM Cell Sterol Mutants. Changes in the Lipid Composition of the Plasma Membrane and Their Effects on 3-O-methlglucose Transport

With the use of two sterol biosynthetic mutants, LM cells with various sterol levels were obtained after extended periods of growth with suboptimal sterol concentrations. As was found in the preceding paper, sterol depletion resulted in reduction of the apparent Vmax values for 3-O-methylglucose transport with no change in the apparent Km values. Analysis of the lipid compositions of isolated plasma membranes and crude endoplasmic reticulum membranes showed that depletion of the cellular sterol levels led to a reduction in the sterol content of both membranes. Furthermore, there were significant changes in the fatty acid composition of the phospholipids of both membranes. The amount of 18:1 of the plasma membrane phospholipids increased dramatically accompanied by a marked decrease in 16:0. The loss of transport activity could be reproduced in vitro by removing cholesterol from cells with the use of liposomes containing phosphatidylcholine. Furthermore, activity was restored to normal values when the cells were subsequently incubated with liposomes composed of phospholipids plus cholesterol. These findings indicate that 1) an inter-relation exists between the fatty acyl group structure of phospholipids and the sterol content of LM cell membranes which may serve to maintain membrane function and 2) that the reversible inactivation of a surface membrane transport system is directly related to reversible changes in the properties of membrane lipid.