PROTEIN-CATALYZED TRANSPORT OF ETHER PHOSPHOLIPIDS

The protein-catalyzed transfer of alkenylacyl-, alkylacyl-, or diacyl-glycerophosphocholines, carrying a pyrenedecanoyl residue as a fluorogenic acyl chain, was studied using unilamellar bilayer vesicles as donor and acceptor membranes in a fluorescence assay. Specific phospholipid transfer proteins, such as phosphatidylinositol transfer protein from yeast and phosphatidylcholine transfer protein from bovine liver showed higher transfer rates with ether lipid substrates. Transfer rates for alkylacyl- and alkenylacyl-glycerophosphocholine as compared to the diacyl analog were rather similar in the presence of non-specific lipid transfer proteins from maize or from bovine liver, respectively. When vesicles of fluorogenic compounds were titrated with the yeast phosphatidylinositol transfer protein, only a 15-20% higher binding affinity for alkenylacyl- and alkylacyl-glycerophosphocholine than for diacyl-glycerophosphocholine was observed. Thus the marked differences of transfer rates measured with this transfer protein cannot be attributed to different binding affinities for the respective phospholipid subclasses. A possible explanation for differences in transfer rates could be differences in the organization of the phospholipid subclasses at the hydrophobic / hydrophilic interface of bilayer membranes.