Phosphatidylinositol Transfer Protein Function in Yeast
All eukaryotic cells contain a set of cytosolic proteins that act as diffusible carriers for the energy-independent transport of phospholipid monomers between membrane bilayers in vitro (Rueckert and Schmidt, 1990; Wirtz, 1991). These phospholipid transfer proteins (PL-TPs) are classified into three general categories on the basis of their ligand specificity; i.e. which phospholipids (PLs) serve as substrates for these proteins in the cell-free PL-transfer reaction. These categories include the nonspecific PL-TPs, the phosphatidylcholine transfer proteins, and the phosphatidylinositol (PI)/phosphatidylcholine (PC) transfer proteins. Members of the last class of PL-TPs are frequently referred to as the phosphatidylinositol (PI) transfer proteins in recognition of the fact that PI is clearly the preferred ligand of these PI-TPs in vitro. The mechanisms by which PL-TPs catalyze phospholipid transfer in vitro have been the subject of detailed biochemical analysis. Although there has been much discussion about the possible involvement of PL-TPs in various aspects of intracellular trafficking of lipids in cells, very little information has been forthcoming with respect to the question of what precise cellular functions PL-TPs fulfill in vivo. The primary reasons for this lack of progress have been several-fold. First, it is essentially impossible, even with today’s technology, to measure phospholipid transfer in vivo. Thus, one is left with an in vitro assay that sets the general premise from which models for in vivo function have been extrapolated. While this conceptual arrangement is reasonable in cases where one is analyzing authentic enzymatic activities, there has been considerable discussion as to whether PL-TPs truly fit the criteria for consideration as enzymes that catalyze PL transfer in vitro.
KeywordsTransfer Protein Golgi Membrane Phospholipid Transfer Protein Cell BioI Phosphatidylcholine Biosynthesis
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