Cholesterol
Cholesterol occurs naturally in eukaryote cell membranes where it is bio-synthesised from mevalonate via a squalene cyclisation of terpenoids. It associated preferentially with sphingolipids (see diagram) in cholesterol-rich lipid rafts areas of the membranes in eukaryotic cells. Hopanoids serve a similar function in prokaryotes.
Cell membranes require high levels - typically an average of 20% cholesterol molecular in the whole membrane, increasing locally in raft areas up to 50% cholesterol (- % is molecular ratio) Formation of lipid rafts promotes aggregation of peripheral and transmembrane proteins including docking of SNARE and VAMP proteins
Read more about this topic: Membrane Lipids