ABSTRACT
The asymmetric disposition of membrane proteins was the first to be observed. Proteins that span the membrane have a specific orientation. Research on phospholipid asymmetry was initiated after observing that some of the labeling reagents used to explore protein asymmetry also reacted with certain phospholipids. A different approach to studying the asymmetric distribution of phospholipids has been the use of phospholipid exchange proteins. By incubating intact cells or sealed membrane vesicles with liposomes containing a given radioactive phospholipid in the presence of the specific exchange protein, it is possible to measure the size of the exchangeable pool of this lipid. The factors determining phospholipid asymmetry are not well understood, but the existence of an outward as well as an inward motion has been proposed. Phospholipid asymmetry is probably related to the topology of membrane proteins and to the structural integrity of the cytoskeleton.
