Sterols in Mycoplasma Membranes

Abstract

Mycoplasmas are distinguishable from other prokaryotes by their lack of a cell wall and intracytoplasmic membranes. Hence, mycoplasma cells have a single membrane—the plasma membrane. The requirement of mycoplasmas for sterols, unique among prokaryotes, shows that sterols function as regulators of membrane fluidity during changes in growth temperature or alterations in the fatty acid composition of membrane lipids. The sterol-requiring mycoplasmas incorporate large quantities of cholesterol into their plasma membrane from their host or from the serum component of artificial growth media. Cholesterol uptake by isolated mycoplasma membranes and intact cells is a physical adsorption process, independent of growth and metabolic activity. Cholesterol incorporated by resting cells has an identical disposition and function in the membrane as cholesterol incorporated during growth. The major factor that determines the effectiveness of serum lipoproteins and lipid vesicles as cholesterol donors is the molar ratio of free cholesterol to phospholipid in the donor. In sterol-requiring mycoplasmas the contact appears to be tighter, or more prolonged, as it suffices for transfer of some of the lipoprotein phospholipid and esterified cholesterol to the membrane. The sterol-requiring mycoplasmas possess on their surface receptors, of a protein nature, responsible for better contact of the lipoprotein particle with the membrane. The presence of sterols in membrane helps in regulation of membrane fluidity. The structural features of the sterol molecule essential for modulating membrane fluidity include a planar (trans-fused) tetracyclic ring system, an equatorial hydroxyl group at position C-3, and a branched aliphatic side-chain at least eight carbon atoms long. The small amounts of cholesterol concentrate in specific areas in the membrane, and increase the viscosity in these microdomains.