Properties of Phospholipid Transfer Proteins

Abstract

Owing to the extremely low critical micelle concentration of natural phospholipids, spontaneous transfer of monomer phospholipid molecules between membrane surfaces through an aqueous phase is very slow (Martin and MacDonald, 1976; Thilo, 1977). Movement of phospholipids would be restricted to the membrane bilayer if it were not for the ability of phospholipid transfer proteins to shuttle phospholipids between membranes (for reviews see Dawson, 1973; Wirtz, 1974; Zilversmit and Hughes, 1976; Kader, 1977). Since the original observation in 1968 that phospholipid transfer activity is present in the membrane-free cytosol of rat liver (Wirtz and Zilversmit, 1968), it has become clear that the cytosol of eukaryotic cells contains a number of phospholipid transfer proteins of different specificity. So far three distinct classes of transfer proteins have been purified to homogeneity: (1) the phosphatidylcholine transfer protein from bovine and rat liver (Kamp et al.,1973; Poorthuis et al.,1980); (2) the phosphatidylinositol transfer proteins from bovine brain and heart (Helmkamp et al., 1974; DiCorleto et al.,1979); (3) the nonspecific phospholipid transfer proteins from rat and bovine liver (Bloj and Zilversmit, 1977; Crain and Zilversmit, 1980a) and rat hepatoma (Dyatlovitskaya et al.,1978). In this series, the phosphatidylcholine transfer proteins are specific for phosphatidylcholine whereas the phosphatidylinositol transfer proteins display a dual specificity with a preference for phosphatidylinositol and, to a lesser extent, phosphatidylcholine. The nonspecific proteins transfer all common diacyl phospholipids as well as cholesterol and are very likely identical to the sterol carrier protein 2 recently isolated from rat liver (Noland et al., 1980). Less well characterized transfer proteins have also been found for other typical membrane components such as phosphatidylglycerol (Van Golde et al., 1980), cholesterol (Erickson et al.,1978), and glucosylceramide (Metz and Radin, 1980). This paper presents a summary on the characterization and properties of the transfer proteins purified to date.