Correlation of cholesterol to phospholipid content in membranes of growing mycoplasmas

Abstract

The mechanism by which cells control the amount of exogenous cholesterol incorporated into their plasma membrane is still undefined. Mycoplasmas may serve as excellent models for studying this problem since they cannot synthesize cholesterol and depend on the growth medium for its supply [l] . That a control mechanism for cholesterol uptake does operate in mycoplasmas is quite obvious since their membrane cholesterol content shows significant differences even when the organisms are grown with the same amount of exogenous cholesterol. Thus, the cholesterol content in Acholeplasma ~aidlaw~i membranes did not exceed 10% of the totaI membrane lipids while that in Mycoplasma hominis reached almost 40% when grown with excessive amounts of exogenous cholesterol [2, 31. Do the differences in the cholesterol uptake capacity of the various mycoplasm~ result from differences in membrane phospholipid or protein composition, or are they caused by differences in the molecular organization of these components in the membrane? Our recent studies [4,5] on the uptake of cholesterol by isolated mycoplasma membranes indicated that the lipid domain of the membrane is the main element responsible for cholesterol uptake, while membrane proteins contribute little to cholesterol binding. Yet, these studies also indicated that the control mechanism of cholesterol uptake operates faultily once the membrane is isolated from the cell