Functional Analysis of Phosphatidylinositol Transfer Proteins

Abstract

All eukaryotic cells contain a battery of cytosolic proteins that catalyze the energy-independent transfer of PLs, as monomers, between membrane bilayers in vitro (Rueckert and Schmidt, 1990; Wirtz, 1991). These proteins were initially detected in assays designed to identify polypeptides that could be involved in intracellular lipid sorting/ trafficking and, based solely upon this operational definition, such proteins are referred to as phospholipid transfer proteins (PL-TPs). These PL-TPs are, in turn, categorized into three general classes on the basis of their unique catalytic activities which reflect the differing PL headgroup specificities of these proteins. For example, the monospecific PL-TPs, exemplified by the phosphatidylcholine transfer protein, exhibit an absolute specificity for one PL species. On the other hand, the nonspecific transfer proteins are able to mobilize most PL species, glycolipids, and sterols in the in vitro transfer reaction. Finally, the oligospecific transfer proteins, exemplified by the phosphatidylinositol (PI)/phosphatidylcholine (PC) transfer proteins (PI-TPs) are able to transfer only a few PLs. In the case of PI-TPs these substrates are PI and PC.