Phospholipid flip-flop in biogenic membranes: what is needed to connect opposite sides (original) (raw)
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Biophysical Journal, 2002
The transbilayer movement of fluorescent phospholipid analogs in liposomes was studied at the lipid phase transition of phospholipid membranes. Two NBD-labeled analogs were used, one bearing the fluorescent moiety at a short fatty acid chain in the sn-2 position (C 6-NBD-PC) and one headgroup-labeled analog having two long fatty acyl chains (N-NBD-PE). The transbilayer redistribution of the analogs was assessed by a dithionite-based assay. We observed a drastic increase of the transbilayer movement of both analogs at the lipid phase transition of DPPC (T c ϭ 41°C) and DMPC (T c ϭ 23°C). The flip-flop of analogs was fast at the T c of DPPC with a half-time (t 1/2) of ϳ6-10 min and even faster at the T c of DMPC with t 1/2 on the order of Ͻ2 min, as shown for C 6-NBD-PC. Suppressing the phase transition by the addition of cholesterol, the rapid transbilayer movement was abolished. Molecular packing defects at the phase transition are assumed to be responsible for the rapid transbilayer movement. The relevance of those defects for understanding of the activity of flippases is discussed.
Transbilayer (flip-flop) lipid motion and lipid scrambling in membranes
FEBS Letters, 2010
This paper reviews the current knowledge on the various mechanisms for transbilayer, or flip-flop, lipid motion in model and cell membranes, enzyme-assisted lipid transfer by flippases, floppases and scramblases is briefly discussed, while non-catalyzed lipid flip-flop is reviewed in more detail. Transbilayer lipid motion may occur as a result of the insertion of foreign molecules (detergents, lipids, or even proteins) in one of the membrane leaflets. It may also be the result of the enzymatic generation of lipids, e.g. diacylglycerol or ceramide, at one side of the membrane. Transbilayer motion rates decrease in the order diacylglycerol ) ceramide ) phospholipids. Ceramide, but not diacylglycerol, can induce transbilayer motion of other lipids, and bilayer scrambling. Transbilayer lipid diffusion and bilayer scrambling are defined as two conceptually and mechanistically different processes. The mechanism of scrambling appears to be related to local instabilities caused by the nonlamellar ceramide molecule, or by other molecules that exhibit a relatively slow flip-flop rate, when asymmetrically inserted or generated in one of the monolayers in a cell or model membrane.
Biochemistry, 2003
Since phospholipid synthesis is generally confined to one leaflet of a membrane, membrane growth requires phospholipid translocation (flip-flop). It is generally assumed that this process is proteinmediated; however, the mechanism of flip-flop remains elusive. Previously, we have demonstrated flop of 2-[6-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]caproyl] (C 6 NBD) phospholipids, induced by the presence of membrane-spanning peptides in vesicles composed of an Escherichia coli phospholipid extract, supporting the hypothesis that the presence of transmembrane stretches of proteins in the bilayer is sufficient to allow phospholipid flip-flop in the inner membrane of E. coli Biochemistry 40, 10500]. Here, we investigated whether the specific phospholipid composition of E. coli is a prerequisite for transmembrane helix-induced flop of phospholipids. This was tested by determining the amount of C 6 NBD-phospholipid that was translocated from the inner leaflet to the outer leaflet of a model membrane in time, using a dithionite reduction assay. The transmembrane peptides GWWL(AL) 8 WWA (WALP23) and GKKL(AL) 8 KKA (KALP23) induced phospholipid flop in model membranes composed of various lipid mixtures. The rate of peptide-induced flop was found to decrease with increasing dioleoylphosphatidylethanolamine (DOPE) content of vesicles composed of DOPE and dioleoylphosphatidylcholine (DOPC), and the rate of KALP23-induced flop was shown to be stimulated by higher dioleoylphosphatidylglycerol (DOPG) content in model membranes composed of DOPG and DOPC. Furthermore, the incorporation of cholesterol had an inhibitory effect on peptide-induced flop. Finally, flop efficiency was strongly dependent on the phospholipid headgroup of the NBD-phospholipid analogue. Possible implications for transmembrane helix-induced flop in biomembranes in general are discussed.
Regulation of phospholipid distribution in the lipid bilayer by flippases and scramblases
Nature Reviews Molecular Cell Biology
Cellular membranes function as permeability barriers that separate cells from the external environment or partition cells into distinct compartments. These membranes are lipid bilayers composed o f g ly ce ro ph os ph olipids, sphingolipids and cholesterol, in which proteins are embedded. G l y ce r o ph o s ph olipids and sphingolipids freely move laterally, whereas t r a ns v e rse m o v em ent between lipid bilayers is limited. Phospholipids are asymmetrically distributed between membrane leaflets but change their location in biological processes, serving as signalling molecules or enzyme activators. Designated proteins-flippases and scramblases-mediate this lipid movement between the bilayers. Flippases mediate the confined localization of specific phospholipids (phosphatidylserine (PtdSer) and phosphatidylethanolamine) to the cytoplasmic leaflet. Scramblases randomly scramble phospholipids between leaflets and facilitate the exposure of PtdSer on the cell surface, which serves as an important signalling molecule and as an 'eat me' signal for phagocytes. Defects in flippases and scramblases cause various human diseases. We herein review the recent research on the structure of flippases and scramblases and their physiological roles. Although still poorly understood, we address the mechanisms by which they translocate phospholipids between lipid bilayers and how defects cause human diseases. Sections The role of P4-type ATPases in lipid 'flipping' between the bilayer. P-type ATPases constitute a large protein family that transport cations and lipids across the membrane 24. They are named 'P-type' because a conserved aspartic acid is transiently modified by phosphorylation during the reaction cycle. A phylogenic analysis divided the family into five subfamilies (P1-P5 types). Similar to other P-type ATPases, P4-ATPases are membrane proteins with cytoplasmic amino Apoptotic bodies
Identification and purification of aminophospholipid flippases
Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids, 2000
Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes : (1) bidirectional, non-energy dependent`scramblases', and energy-dependent transporters that move lipids (2) toward (`flippases') or (3) away from (`floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.
Molecular Mechanism for Lipid Flip-Flops
The Journal of Physical Chemistry B, 2007
Transmembrane lipid translocation (flip-flop) processes are involved in a variety of properties and functions of cell membranes, such as membrane asymmetry and programmed cell death. Yet, flip-flops are one of the least understood dynamical processes in membranes. In this work, we elucidate the molecular mechanism of pore-mediated transmembrane lipid translocation (flip-flop) acquired from extensive atomistic molecular dynamics simulations. On the basis of 50 successful flip-flop events resolved in atomic detail, we demonstrate that lipid flip-flops may spontaneously occur in protein-free phospholipid membranes under physiological conditions through transient water pores on a time scale of tens of nanoseconds. While the formation of a water pore is induced here by a transmembrane ion density gradient, the particular way by which the pore is formed is irrelevant for the reported flip-flop mechanism: the appearance of a transient pore (defect) in the membrane inevitably leads to diffusiVe translocation of lipids through the pore, which is driven by thermal fluctuations. Our findings strongly support the idea that the formation of membrane defects in terms of water pores is the rate-limiting step in the process of transmembrane lipid flip-flop, which, on average, requires several hours. The findings are consistent with available experimental and computational data and provide a view to interpret experimental observations. For example, the simulation results provide a molecular-level explanation in terms of pores for the experimentally observed fact that the exposure of lipid membranes to electric field pulses considerably reduces the time required for lipid flip-flops.
Tracking down lipid flippases and their biological functions
Journal of Cell Science, 2004
Eukaryotic cells are compartmentalized into distinct organelles by lipid bilayers. Each membrane is composed of hundreds of lipids, and there are dramatic differences between organellesfor example, sphingomyelin and sterols are highly enriched at the plasma membrane ( ). Even within a membrane, lipids are not randomly distributed. Lipids are asymmetrically arranged between the two leaflets of the plasma membrane, and this was first thought to be a static characteristic of membranes . However, it is now clear that the lipid topology results from a continuous bi-directional movement of lipids between the two leaflets -so-called 'flip-flop' -in which specific membrane proteins have an essential role. Recently, several 'lipid flippases' have been identified. Besides maintaining an asymmetric lipid arrangement, the physiological functions of which are still relatively unclear, these appear to regulate other, more dynamic events, such as the budding of intracellular transport vesicles.