The Effect of Cholesterol on a Five Component Mitochondria-Like Membrane

Abstract

Cholesterol is known to affect biophysical quantities in one and two component membranes, overall increasing membrane thickness and orientational order and decreasing membrane fluidity. Although these effects are useful in the plasma membrane of cells where strength is a desired property, there is evidence suggesting that the appearance of cholesterol in other cellular membranes has adverse effects. Indeed, cholesterol is found at elevated levels in the mitochondria membranes of cancer cells and is associated with chemotherapy resistance. At the molecular level, the link between cancer and cholesterol seems to be that cholesterol interferes with apoptosis, or programmed cell death, specifically by inhibiting the insertion of the pro-apoptotic protein Bax in the mitochondria outer membrane. We studied the effects of cholesterol on a five component mitochondria-like membrane, in order to determine which of these are relevant for Bax membrane insertion. As expected we found, using x-ray and neutron scattering, that upon cholesterol addition: (1) the thickness of the mitochondria-like membrane increases, (2) the area per phospholipid decreases, and (3) the orientational order of the membrane increases. Interestingly, our data indicate that the ordering effect of cholesterol is less efficient for the five component mitochondria-like membrane than for a single component membrane. Finally, we determined that in spite of the relatively high degree of unsaturation of the lipids in the mitochondria-like membrane, cholesterol adopts a canonical orientation. At higher cholesterol concentrations, cholesterol's polar hydroxyl group moves outwards and comes in proximity with the phospholipid's carbonyl group, allowing hydrogen bonding between the two types of molecules. Any of the above effects could in principle be responsible for cholesterol's inhibition of Bax insertion, thus follow up studies are required to confirm which, or what combination of them are relevant for apoptosis.