Phospholipid-exchange proteins

The method of introduction of the fluorophore into the membrane is also important. Many probes are introduced into preexisting vesicles, natural membranes, or whole cells by the injection of a small volume of organic solvent containing the fluorophore. For DPH, tetrahydrofuran is commonly used, while methanol is often employed for other probes. The amount of solvent used should be the absolute minimum possible to avoid perturbation of the lipids, since the solvent will also partition into the membrane. With lipid vesicles this potential problem can be avoided by mixing the lipids and fluorophore followed by evaporation of the solvent and codispersing in buffer. For fluorophores attached to phospholipids, this is the only way to get the fluorophore into the bilayer with natural membranes, phospholipid exchange proteins or other techniques may have to be employed. 

D10. DiCorleto, P. E., Warach, J. B., and Zilversmit, D. B., Purification and characterization of two phospholipid exchange proteins from bovine heart. /. Biol. Chem. 254, 7795-7802 (1979). 

T2. Tall, A. R, Abreu, E., and Shuman, J., Separation of a plasma phospholipid transfer protein from cholesterol ester/phospholipid exchange protein. J. Biol. Chem. 258, 2174-2180 (1983). 

W15. Wilson, D. B., Ellsworth, J. L., and Jackson, R. L., Net transfer of phosphatidylcholine from plasma low density lipoproteins to sphingomyelin-apolipoprotein A-II complexes by bovine liver and human plasma phospholipid exchange proteins. Biochim. Biophys. Acta 620, 550-561 (1980). 

W21. Wirtz, K. W. A., and Zilversmit, D. B., Partial purification of phospholipid exchange protein from beef heart. FEBS Lett. 7, 44-46 (1970). 

Two mechanisms have been proposed to explain the transport of phospholipids from the ER to other cellular membranes protein-mediated transfer and a vesicular process. Several experiments have demonstrated that water-soluble proteins, known as phospholipid exchange proteins, can bind to specific phospholipid molecules and transfer them to another bilayer. Vesicular transport of phospholipids and membrane proteins in structures known as transition vesicles from the ER to the Golgi complex is not clearly understood. However, evidence of transfer of luminal material from the ER to the Golgi cistemae clearly supports vesicular transport. 

Phospholipid translocator proteins, phospholipid exchange proteins, and transition vesicles are involved in the complicated process of membrane synthesis and delivery of membrane components to their cellular destinations. 

DiCorleto and Zilversmit (1977) reported that the phospholipid exchange proteins from either beef heart or beef liver were not able to catalyze the exchange of phosphatidylcholine between phosphatidylcholine multi-lamellar vesicles and small unilamellar vesicles. However, by adding acidic phospholipids to the multilamellar vesicles, protein-enhanced exchange was observed. This assay is versatile and can be easily performed. Both substrates are well-defined particles in which the composition can be readily manipulated. The separation of acceptor and donor membranes by centrifugation is rapid and nearly complete with 98—99% of the multilamellar vesicles being sedimented and with 90% of the small unilamellar vesicles remaining in the supernatant. 

Transfer Activity of the Bovine Brain Phospholipid Exchange Protein with Acceptor Vesicles of Varying Composition 1... 

In Table 4 the amino acid composition of purified SCP2 is also compared with the amino acid composition of a protein described as a nonspecific phospholipid exchange protein (CM2) [49]. The amino acid compositions of SCP2 and CM2 nearly identical. The correlation coefficient is 0.992 [21]. 

The mode of action of lipid transfer proteins is not entirely elucidated. However, it is assumed that a complex between phospholipid and protein is formed. This complex interacts with the various membranes the transfer of the phospholipid molecule from the complex to the membrane leads to a net transfer of phospholipid. However, an exchange of the phospholipid molecule bound to the complex with the phospholipids of the membranes leads to an overall-processes of exchange of phospholipids this explains why these proteins were previously called phospholipid exchange proteins". 

Schwitzguebel J.P., Nguyen T.D., and Siegenthaler P.A. (1984) Are phospholipid exchange proteins present in the stroma from higher plant chloroplasts in Structure Function and Metabolism of... 

Tanaka T. and Yamada M., 1982 - Properties of phospholipid exchange proteins from germinated castor bean endosperms in Biochemistry and Metabolism of Plant Lipids, J.F.G.M. and P.J.C.Kulper Eds, Elsevier, p. 99-106. 

Phospholipids may be transported within cells by binding to specific phospholipid exchange proteins (PLEP) whose function may be to introduce lipids into cell membranes and organelles remote from their sites of synthesis. For example, mitochondria do not possess enzymes for the complete biosynthesis of phospholipids de novo and may acquire their characteristic lipid composition by exchange from endoplasmic reticulum via the PLEP. Carrier proteins also exist for sterols, including vitamin D, and for retinol. These are referred to in the appropriate sections. 

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