Effect of low temperatures on the transfer of phospholipids with various acyl-chain lengths to the plasma membrane of leek cells.

The intracellular transport of lipids with very-long-chain acyl moieties (C20-C24 fatty acid-containing lipids) to the plasma membrane of leek cells follows the vesicular ER-Golgi apparatus-plasma membrane pathway. Here we report on the effect of low temperatures on the vesicular transport of lipids and especially C20-C24 fatty acid-containing lipids to the plasma membrane of leek cells. These lipids, normally transported through the ER-Golgi apparatus-plasma membrane pathway at 24 degrees C, accumulated in the ER and the Golgi apparatus at 12 degrees C with a related deficit in the plasma membrane. As lipids with long chain acyl moieties (C16 and C18 fatty acid-containing lipids) were still transferred to the plasma membrane at 12 degrees C, the data demonstrate a specific effect of low temperatures on the vesicular transfer of C20-C24 fatty acid-containing lipids to the plasma membrane of leek cells. Therefore, evidence is provided for distinct pathways and/or mechanisms transferring lipids to the plasma membrane of leek cells which differ in their sensitivity to low temperatures.

Cell-free transfer of phospholipids between the endoplasmic reticulum and the Golgi apparatus of leek seedlings.

The transfer of lipids between the endoplasmic reticulum and the Golgi apparatus was investigated in vitro using a cell-free system from leek seedlings. Lipids of the donor membranes (endoplasmic reticulum) were radiolabeled either by incubating leek seedlings with [1-14C]acetate or [3H]acetate. Acceptor membranes (Golgi apparatus) were unlabeled and immobilized on nitrocellulose strips. The assay measured the lipid transfer resulting from both an ATP-independent process and an ATP- and cytosol-dependent process. A significant ATP- and cytosol-dependent lipid transfer was observed only in the case of the endoplasmic reticulum as donor and the Golgi apparatus as acceptor. Lipids transferred in an ATP-dependent manner were chiefly phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. The stimulation of lipid transfer by ATP as compared to the ATP-independent process was +79% (PC), +123% (PS) and +69% (PE). On the other hand, PI was not transferred in an ATP-dependent manner (the stimulation by ATP was only 20%). This supports the theory that a sorting of phospholipids takes place in the donor membrane. Moreover, a formation of lysoPC was observed only in the presence of ATP (+330%). The ATP-dependent lipid transfer was inhibited by N-ethylmaleimide, indicating the involvement of cytosolic (but no phospholipid transfer proteins) or membrane proteins in the transfer process. The ATP-dependent transfer of lipids was also diminished at 12 degrees C showing the sensitivity to low temperatures of the transfer of lipids between the endoplasmic reticulum and the Golgi apparatus.