CELL-FREE TRANSFER OF PHOSPHOLIPIDS BETWEEN THE ENDOPLASMIC-RETICULUMAND 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 lee k seedlings. Lipids of the donor membranes (endoplasmic reticulum) wer e radiolabeled either by incubating leek seedlings with [1-C-14]acetat e or [H-3]acetate. Acceptor membranes (Golgi apparatus) were unlabeled and immobilized on nitrocellulose strips. The assay measured the lipi d transfer resulting from both an ATP-independent process and an ATP- and cytosol-dependent process. A significant ATP- and cytosol-dependen t lipid transfer was observed only in the case of the endoplasmic reti culum as donor and the Golgi apparatus as acceptor. Lipids transferred in an ATP-dependent manner were chiefly phosphatidylcholine, phosphat idylethanolamine and phosphatidylserine. The stimulation of lipid tran sfer 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%). Thi s supports the theory that a sorting of phospholipids takes place in t he donor membrane. Moreover, a formation of lysoPC was observed only i n the presence of ATP (+330%). The ATP-dependent lipid transfer was in hibited by N-ethylmaleimide, indicating the involvement of cytosolic ( but no phospholipid transfer proteins) or membrane proteins in the tra nsfer process. The ATP-dependent transfer of lipids was also diminishe d at 12 degrees C showing the sensitivity to low temperatures of the t ransfer of lipids between the endoplasmic reticulum and the Golgi appa ratus.