Phospholipid transporter proteins

Figure 26-5. Factors affecting cholesterol balance at the cellular level. Reverse cholesterol transport may be initiated by pre 3 HDL binding to the ABC-1 transporter protein via apo A-l. Cholesterol is then moved out of the cell via the transporter, lipidating the HDL, and the larger particles then dissociate from the ABC-1 molecule. (C, cholesterol CE, cholesteryl ester PL, phospholipid ACAT, acyl-CoA cholesterol acyltransferase LCAT, lecithinicholesterol acyltransferase A-l, apolipoprotein A-l LDL, low-density lipoprotein VLDL, very low density lipoprotein.) LDL and HDL are not shown to scale.

Many of the globulins act as transport proteins. Of particular interest are those proteins which are combined with lipids, themselves synthesized in the liver, to form lipoprotein complexes. High density lipoprotein (HDL), which contains predominantly apoproteins A and C combined with mainly phospholipids (most of the cholesterol found in mature HDL is added later) and very low density lipoprotein... 

In many eukaryotic plasma membranes, PS resides in the inner leaflet (Schroit and Zwaal, 1991 Zachowski, 1993). This transbilayer distribution of membrane hpids is not a static situation but a result of balance between the inward and outward translocation of phospholipids across the membranes. Recent studies showed that the transbilayer lipid asymmetry is regulated by several lipid transporter proteins, such as aminophospholipid translocase (Daleke and Lyles, 2000), ATP-binding cassette transporter family (van Helvoort et al, 1996 Klein et al, 1999), and phospholipid scramblase (Zhou et al, 1997 Zhao et al, 1998). An increment of intracellular due to cell activation, cell injury, and apoptosis affects the activities of these transporters, resulting in exposure of PS (Koopman et al, 1994 Verhoven et al, 1995) and PE (Emoto et al, 1997) on the cell surface. 

As detailed in chapter 17, biological membranes are basically lipid—think fat or oil—in nature with some attached proteins. As such, these thin sheets of phospholipids and proteins are nearly impermeable to charged particles such as sodium, potassium, or chloride ions. While the isolation of the cell interior from the exterior ionic environment is critical in many ways, it is also true that controlled permeability to ions may be critical. In fact, it is the near-impermeability of biological membranes to ions that permits control of ion transport across them by certain, specific proteins. 

The phosphatidylcholine in bile is synthesised in the endoplasmic reticulum of the hepatocyte and must be transported to the canalicular membrane. One possibility involves the nonspecific phosphatidylcholine transfer protein but a mouse null for this protein did not show reduced phosphatidylcholine secretion into bile and there was no compensatory increase in other phospholipids transfer proteins. However, the plasma membrane would receive a ready supply of phospholipid by insertion of vesicles, and the MDR3 protein translocates this molecule from the inner leafiet to the outer surface where there is contact with bile acids, as suggested by Smit and colleagues. The role of this transporter is shown in Figure 2.2. 

Several studies have evaluated the effects of oral di(2-ethylhexyl) adipate on various aspects of hepatic lipid metabolism. Feeding di(2-ethylhexyl) adipate (2% of diet) to male Wistar rats for seven days resulted in increased hepatic fatty acid-binding protein as well as in increased microsomal stearoyl-CoA desaturation activity (Kawashima et al., 1983a,b). Feeding the compound at this dose for 14 days resulted in increased levels of hepatic phospholipids and a decline in phosphatidyl-choline phosphatidylethanolamine ratio (Yanagita et al., 1987). Feeding di(2-ethyl-hexyl) adipate (2% of diet) to male NZB mice for five days resulted in induction of fatty acid translocase, fatty acid transporter protein and fatty acid binding protein in the liver (Motojima et al., 1998). 

Fig. 5.2.1 The major metabolic pathways of the lipoprotein metabolism are shown. Chylomicrons (Chylo) are secreted from the intestine and are metabolized by lipoprotein lipase (LPL) before the remnants are taken up by the liver. The liver secretes very-low-density lipoproteins (VLDL) to distribute lipids to the periphery. These VLDL are hydrolyzed by LPL and hepatic lipase (HL) to result in intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL), respectively, which then is cleared from the blood by the LDL receptor (LDLR). The liver and the intestine secrete apolipoprotein AI, which forms pre-jS-high-density lipoproteins (pre-jl-HDL) in blood. These pre-/ -HDL accept phospholipids and cholesterol from hepatic and peripheral cells through the activity of the ATP binding cassette transporter Al. Subsequent cholesterol esterification by lecithinxholesterol acyltransferase (LCAT) and transfer of phospholipids by phospholipid transfer protein (PLTP) transform the nascent discoidal high-density lipoproteins (HDL disc) into a spherical particle and increase the size to HDL2. For the elimination of cholesterol from HDL, two possible pathways exist (1) direct hepatic uptake of lipids through scavenger receptor B1 (SR-BI) and HL, and (2) cholesteryl ester transfer protein (CfiTP)-mediated transfer of cholesterol-esters from HDL2 to chylomicrons, and VLDL and hepatic uptake of the lipids via the LDLR pathway...

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

To study transport in the absence of complicating metabolic processes, it often is advantageous to work with isolated membrane vesicles rather than with whole cells. Cytoplasmic membrane vesicles can be obtained from either eukaryotic or bacterial cells after homogenization or osmotic lysis. Transport proteins that have been solubilized with detergents also can be reincorporated into synthetic phospholipid vesicles (fig. 17.27). 

Phospholipids are ideal compounds for making membranes because of their amphipathic nature (see chapter 17). The polar head-groups of phospholipids prefer an aqueous environment, whereas the nonpolar acyl substituents do not. As a result, phospholipids spontaneously form bilayer structures (see fig. 17.6), which are a dominant feature of most membranes. The phospholipid bilayer is the barrier of the cell membrane that prevents the unrestricted transport of most molecules other than water into the cell. Entry of other molecules is allowed if a specific transport protein is present in the cell membrane. Similarly, the phospholipid bilayer prevents leakage of metabolites from the cell. The amphipathic nature of phospholipids has a great influence on the mode of their biosynthesis. Thus, most of the reactions involved in lipid synthesis occur on the surface of membrane structures catalyzed by enzymes that are themselves amphipathic. 

How does the cell sort and transport phospholipids from the site of synthesis to other membranes in the cell One view is that phospholipid vesicles that bud from the endoplasmic reticulum are targeted to another membrane where the vesicles fuse with the membrane. Alternatively, phospholipid transfer proteins may be involved. Proteins that transfer phospholipids between membranes in vitro have been known for over 25 years but it has not been demonstrated that they function in this way in vivo. 

Apart from purely artificial vesicles made from phospholipids and proteins of choice, it is also possible to make vesicles from the cytoplasmic membranes of cells, usually by a sonication procedure. Artificial vesicles and vesicles derived from natural membranes have proved very useful in studying transport phenomena across membranes. Vesicles also occur naturally, e.g.. by the budding of the Golgi apparatus in eukaryotic cells (Chap. 1). 

Guan L, Smirnova IN, Verner G, Nagamori S, Kaback HR. Manip- 65. ulating phospholipids for crystallization of a membrane transport protein. Proc. Natl. Acad. Sci. U.S.A. 2006 103 1723-1726. 

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