Phospholipid flippase

A second lipid flippase that moves lyso-PE across the inner membrane was recently discovered (E.M. Harvat, 2005). This protein belongs to the major facilitator superfamily 

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Romsicki, Y., Sharom, F. J., Phospholipid flippase activity of the reconstituted P-glycoprotein multidrug transporter, Biochemistry 2001, 40, 6937-6947. 

A EXPERIMENTAL FIGURE 18-5 In vitro fluorescence quenching assay can detect phospholipid flippase activity of... 

FIGURE9.il Phospholipids can be flipped across a bilayer membrane by the action of flippase proteins. Wlien, by normal diffusion through the bilayer, the lipid encounters a flippase, it can be moved quickly to the other face of the bilayer. 

Proteins that can flip phospholipids from one side of a bilayer to the other have also been identified in several tissues (Figure 9.11). Called flippases, these proteins reduce the half-time for phospholipid movement across a membrane from 10 days or more to a few minutes or less. Some of these systems may operate passively, with no required input of energy, but passive transport alone cannot establish or maintain asymmetric transverse lipid distributions. However, rapid phospholipid movement from one monolayer to the other occurs in an ATP-dependent manner in erythrocytes. Energy-dependent lipid flippase activity may be responsible for the creation and maintenance of transverse lipid asymmetries. 

The mechanisms involved in the establishment of lipid asymmetry are not well understood. The enzymes involved in the synthesis of phospholipids are located on the cytoplasmic side of microsomal membrane vesicles. Translocases (flippases) exist that transfer certain phospholipids (eg, phosphatidylcholine) from the inner to the outer leaflet. Specific proteins that preferentially bind individual phospholipids also appear to be... 

FIGURE 11-16 Motion of single phospholipids in a bilayer, (a) Movement from one leaflet to the other is very slow, unless (b) catalyzed by a flippase in contrast, lateral diffusion within the leaflet (c) is very rapid and requires no protein catalysis. 

Analysis of the complete yeast genome revealed the presence of 16 proteins that clearly belong to the P-type ATPase family. More detailed sequence analysis suggests that 2 of these proteins transport H+ ions, 2 transport Ca +, 3 transport Na+, and 2 transport metals such as Cu2+. In addition, 5 members of this family appear to participate in the transport of phospholipids with amino acid head groups. These latter proteins assist in the maintenance of membrane asymmetry by transporting lipids such as phosphatidyl serine from the outer to the irmer leaflet of the bilayer membrane (Figure 13.6). Such enzymes have been termed "flippases."... 

Figure 13.6. P-Type ATPases Can Transport Lipids. Flippases are enzymes that maintain membrane asymmetry by "flipping" phospholipids (displayed with a red head group) from the outer to the inner layer of the membrane.

Figure 4.9 Hydrolysis of phenol esters bound to the outer surface of phospholipid vesicles is finished after 3 minutes. The same reaction on the inner surface takes hours. Flippases (see Figure 4.8) accelerate the latter reaction.

If recently synthesized phospholipid molecules remained only on the cytoplasmic surface of the ER, a monolayer would form. Unassisted bilayer transfer of phospholipid, however, is extremely slow. (For example, half-lives of 8 days have been measured across artificial membrane.) A process known as phospholipid translocation is now believed to be responsible for maintaining the bilayer in membranes (Figure 12F). Transmembrane movement of phospholipid molecules (or flip-flop), which may occur in as little as 15 seconds, appears to be mediated by phospholipid translocator proteins. One protein (sometimes referred to as flippase) that transfers choline-containing phospholipids across the ER membrane has been identified. Because the hydrophilic polar head group of a phospholipid molecule is probably responsible for the low rate of spontaneous translocation, an interaction between flippase and polar head groups is believed to be involved in phosphatidylcholine transfer. Translocation results in a higher concentration of phosphatidylcholine on the lumenal side of the ER membrane than that... 

Membrane phospholipids are synthesized on the cytoplasmic side of SER membrane. Because the polar head groups of phospholipid molecules make transport across the hydrophobic core of a membrane an unlikely event, a translocation mechanism is used to transfer phospholipids across the membrane to ensure balanced growth. Choline-containing phospholipids are found in high concentration on the lumenal side of ER membrane because a prominent phospholipid translocator protein called flippase preferentially transfers this class of molecule. 

Flippases Move Phospholipids from One Membrane Leaflet to the Opposite Leaflet... 

Although the mechanisms employed to generate and maintain membrane phospholipid asymmetry are not well understood. It Is clear that flippases play a key role. These Integral membrane proteins facilitate the movement of phospholipid molecules from one leaflet to the other (see Figure... 

Most membrane phospholipids are preferentially distributed in either the exoplasmlc or the cytosolic leaflet. This asymmetry results in part from the action of flippases such as ABCB4, a phosphatidylcholine fllppase contributing to the generation of bile in the liver. 

Figure 18-11 outlines the major transport proteins that mediate the secretion and movement of bile components. Three ABC proteins move phospholipids, cholesterol, and bile acids across the apical surface of liver cells into small ductules (step H]). One of these proteins, the ABCB4 flippase, flips phosphatidylcholine from the cytosolic leaflet to the... 

PS is decarboxylated by PS decarboxylase to yield the zwitterionic PE. This inner membrane enzyme has a subunit molecular mass of 36 kDa. PS decarboxylase has a pyruvate prosthetic group that participates in the reaction by forming a Schiff base with PS. Overproduction of the enzyme 30-50-fold by plasmid-bome copies of the psd gene has no effect on membrane phospholipid composition, indicating that the level of this enzyme does not regulate the amount of PE in the membrane. The majority of the PE is found in the periplasmic leaflet of the inner membrane, and there is a rapid flipping from the inner to outer leaflet by the MsbA lipid flippase (Section 7). 

The membranes of cells are generally asymmetric, in that the lipids and proteins that inhabit the membrane are not evenly distributed across both the leaflets of the bilayer. To maintain this necessary membrane asymmetry, transverse diffusion of phospholipids (flip-flop. Figure 6a) in cellular membranes is accelerated by translocase enzymes like the flippases. These enzymes overcome the energy barrier for the passage of polar headgroups through the apolar center of the membrane and maintain asymmetry by the consumption of adenosine triphosphate (ATP). ... 

Early examples of synthetic flippases were lipidated polymers, which used bilayer distortion to bring about lipid flip-flop. In contrast to these mechanical flippases, synthetic species that apply the principles of molecular recognition to create phospholipid complexes capable of transverse diffusion have been shown to enhance lipid flip-flop in model membrane systems. Boon and Smith generated asymmetric bilayers by adding synthetic NBD phospholipids to the outer leaflet of POPC vesicles and then determined the rate of flip-flop to the inner leaflet... 

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