Dynamic Transbilayer Lipid Asymmetry
Abstract
Cells have thousands of different lipids. In the plasma membrane, and in membranes of the late secretory and endocytotic pathways, these lipids are not evenly distributed over the two leaflets of the lipid bilayer. The basis for this transmembrane lipid asymmetry lies in the fact that glycerolipids are primarily synthesized on the cytosolic and sphingolipids on the noncytosolic surface of cellular membranes, that cholesterol has a higher affinity for sphingolipids than for glycerolipids. In addition, P4-ATPases, “flippases,” actively translocate the aminophospholipids phosphatidylserine and phosphatidylethanolamine to the cytosolic surface. ABC transporters translocate lipids in the opposite direction but they generally act as exporters rather than “floppases.” The steady state asymmetry of the lipids can be disrupted within seconds by the activation of phospholipases and scramblases. The asymmetric lipid distribution has multiple implications for physiological events at the membrane surface. Moreover, the active translocation also contributes to the generation of curvature in the budding of transport vesicles.
A lipid bilayer consisting of phosphatidylcholine (PC) with one saturated and one unsaturated acyl chain is stable, flexible, and semipermeable. It is the simplest model of a biomembrane. In such membranes, PC with a spin label on its choline headgroup diffused rapidly in the plane of the membrane with a diffusion coefficient of 1.8 µm/sec (Devaux and McConnell 1972). In contrast, PC movement between leaflets, “flip-flop,” was slow with a half-time of >6 h at 30°C (Kornberg and McConnell 1971). Similar half-times for PC flip-flop were measured in erythrocyte membranes, a mammalian plasma membrane with a complex lipid composition (Rousselet et al. 1976; Renooij and Van Golde 1977; van Meer et al. 1980). Interestingly, the erythrocyte membrane maintains an asymmetric lipid distribution across the lipid bilayer with all of its phosphatidylserine (PS) and most of its phosphatidylethanolamine (PE) in the cytosolic leaflet (Bretscher 1972; Verkleij et al. 1973). A critical discussion of these early data and the techniques used can be found in (Op den Kamp 1979).
It was then observed that the enrichment of aminophospholipids in the cytosolic leaflet is maintained by an ATP-consuming translocator that flips these lipids from the outer leaflet across the lipid bilayer (Seigneuret and Devaux 1984). The flippase was later identified as a P4-ATPase (Tang et al. 1996; Soupene and Kuypers 2006). Around the same time it was found that an ABC transporter, ABCB4, was involved in transporting PC into the bile (Smit et al. 1993), and studies on the closely related ABCB1 proved that these transporters can translocate lipids across the plasma membrane onto acceptors in the extracellular space (van Helvoort et al. 1996). Finally, evidence was provided for passive, bidirectional movement of lipids across the ER membrane and under some conditions across the plasma membrane, in which cases the responsible proteins have not yet been unequivocally identified (Sanyal and Menon 2009; Bevers and Williamson 2010). Thus, we now have a general picture of how lipid asymmetry is generated, maintained, and disrupted. However, there are still important gaps in our knowledge. For example, the transbilayer orientation of the sterols that make up one-third of the lipids in eukaryotic plasma membranes has still not been resolved satisfactorily. Moreover, we do not understand mechanistically how translocators and exporters work and how their activity is regulated.