Transport of phosphatidylserine

Emoto, K., Kuge, O., Nishijima, M., and Umeda, M., 1999, Isolation ofa Chinese hamster ovary ceh mutant defective in intramitochondrial transport of phosphatidylserine. Proc. Natl. Acad. Sci. USA, 96 12400-12405. 

REGULATION OF PHOSPHATIDYLSERINE BIOSYNTHESIS AND TRANSPORT IN MAMMALIAN CELLS... 

Osteoblasts secrete osteoid, a matrix rich in type I collagen fibers and vesicles. Precipitation of calcium phosphate is inhibited by a high concentration of pyrophosphate in stromal interstitial fluids, and a high concentration also of albumin and citrate in blood plasma. Pyrophosphate is derived from (1) transport out of the cytosol, and (2) synthesis from nucleoside triphosphates in the stromal interstitial fluid that permeates the osteoid matrix. Precipitation occurs only when calcium and phosphate ions are taken up into vesicles whose inner membrane is composed of phosphatidylserine. The high concentration of calcium and phosphate ions in the vesicle is mediated by annexin and type HI Pi Na-dependent transporters. This overwhelms the pyrophosphate and nucleation occurs. As the precipitate grows and ruptures the membrane, tissue-nonspecific alkaline phosphatase is activated to remove pyrophosphate from the osteoid matrix fluid so that calcium phosphate precipitates around phosphorylated serine residues within the collagen fibers. 

Membrane lipids, and particularly cholesterol, are instrumental not only in the control of diffusion across biological membranes but also in the determination of the activity of membrane-bound enzymes, their modulation by hormones and other agents, and the determination of membrane fluidity (for original references, see [4,6]). It is generally accepted that incorporation of cholesterol in a lipid bilayer membrane tends to decrease significantly the permeability of these membranes to water. Movement of water across these membranes occurs primarily by dissolution in the membrane matrix. The decrease in the rate of water transport as a result of cholesterol incorporation is due mainly to a decrease in membrane fluidity. As a general rule, it is found that the presence of cholesterol in membranes or the incorporation of cholesterol into dispersions composed of phosphatidylserine or ganglioside lead to a decrease in the fluidity of the hydrocarbon chains of lipid membranes which are in the liquid-crystalline state [4,20]. 

In our laboratory, studies of lipid transfer in leek seedlings in vivo, have already shown the existence of a vesicular process for the transfer of phospholipids and particularly of very long chain fatty acid-containing lipids [6]. This process follows the vesicular endoplasmic reticulum- Golgi apparatus- plasma membrane pathway. Using the cell-free system developed by Morre and coworkers, we have reconstituted in vitro the vesicular transfer of some phospholipids between the ER and the GA. This transfer is ATP and cytosol-dependent, is N Ethyl Maleimide and temperature sensitive and specific for the ER as donor and the GA as acceptor. The phospholipids transferred via an ATP-dependent manner in vitro between the ER and the GA were phosphatidylcholine (PC +79%), phosphatidylethanolamine (PE +67%) and phosphatidylserine (PS +123%) [7]. All those results are in favour of a vesicular transport of phospholipids between the ER and the GA of leek seedlings, and brought us to purify these transition vesicles issued from the ER. 

Many of the proteins of membranes are enzymes. For example, the entire electron transport system of mitochondria (Chapter 18) is embedded in membranes and a number of highly lipid-soluble enzymes have been isolated. Examples are phosphatidylseiine decarboxylase, which converts phosphatidylserine to phosphatidylethanolamine in biosynthesis of the latter, and isoprenoid alcohol phosphokinase, which participates in bacterial cell wall synthesis (Chapter 20). A number of ectoenzymes are present predominantly on the outsides of cell membranes.329 Enzymes such as phospholipases (Chapter 12), which are present on membrane surfaces, often are relatively inactive when removed from the lipid environment but are active in the presence of phospholipid bilay-ers.330 33 The distribution of lipid chain lengths as well as the cholesterol content of the membrane can affect enzymatic activities.332... 

Fig. 5.17 Permeability coefficients, P°, for doxorubicin transport across model membranes composed of different phospholipids. PC, phosphatidylcholine PE, phos-phatidylethanolamine PS, phosphatidylserine SM, sphingomyelin. (Reprinted from Fig. 3 of ref. 115 with permission from the American Chemical Society.)...

Nucleation of calcium phosphate precipitation within the matrix vesicles is mediated by phosphatidylserine, which comprises about 8% of the phospholipids of the inner cytosolic membrane surface (Fig. 9.5a). Calbindin in the vesicle (Fig. 9.5b) may also contribute. Rapid mineral growth within the vesicle keeps the concentration of dissolved calcium and inorganic phosphate ions so low that additional Ca2+ and Pi ions spontaneously enter from the extracellular fluid via their respective transporters. Attached type II and type X collagens from cartilage in the growth plate enhance calcium ion transport and calcification during endochondral ossification (Fig. 9.5b). 

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