Cholesterol and other membrane active sterols: from membrane evolution to "rafts".

The appearance of "membrane-active sterols" in biological membranes of eukaryocytes is one of the major steps in membrane evolution. This is exemplified best by membrane-active sterols of mammalia. The effect of membrane-active sterols on controlling membrane permeability by reducing average "fluidity" and free volume is well established. Recently it became clear that cholesterol also has a key role in the lateral organization of membranes and free volume distribution. The latter two parameters seem to be involved in controlling membrane protein activity and "raft" formation. Such an effect allows for the fine tuning of membrane lipid composition, organization/dynamics, and function.