Phospholipid transfer proteins: from lipid monolayers to cells.

Eukaryotic cells contain phospholipid transfer proteins that act as carriers of phospholipids between membranes. In mammalian tissues three transfer proteins with different specificities have been identified: the phosphatidylcholine transfer protein (PC-TP), the phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) that transfers all common diacyl-phospholipids and cholesterol. Properties of these transfer proteins have been discussed with a special emphasis on the lipid binding site of bovine liver PC-TP. Application of photoactivatable and fluorescent analogues of PC have indicated that PC-TP contains specific and independent hydrophobic binding sites for the sn-1- and sn-2-fatty acyl chains. Because these sites have different properties, PC-TP can discriminate between positional isomers of PC and displays a distinct preference for those molecular species that carry a polyunsaturated fatty acid chain at the sn-2-position. Recent studies on bovine brain PI-TP have strongly suggested that this protein may be well-suited to maintain the levels of PI in natural membranes. Besides this proposed role, evidence has become available from studies on Swiss mouse 3T3 fibroblasts that, apart from its occurrence in cytosol, PI-TP is present in nuclei.