Topology and dynamics of phospholipids in membranes

The present lecture summarizes some studies on the transbilayer distribution of phospholipids in biomembranes, and in addition deals with recent investigations on the translocation of phospholipids between the outer and inner compartment. The topology of phospholipids has been studied mainly by chemical labelling, enzymatic modification with phospholipases, and through the use of exchange on transfer proteins (fig.1). In principle the approachis simple, namely a comparison of the action of these tools on intact and open membranes, but many pitfalls can be made. Merits and limitations of the various methods have been discussed in detail in a recent review by Op den Kamp [ 11. In order to obtain unambiguous results the complete action of the impermeable probe should leave the membrane structure intact and not cause phospholipid redistribution. Experience has shown that it is often difficult to meet all prerequisites and for several membranes information about phospholipid topology is either incomplete or contradict0r.y. However, a non-random distribution of phospholipids in the erythrocyte membrane has been established now by many investigators and the overall view is rather uniform.