Membrane Properties Modulate the Activity of a Yeast Phosphatidylinositol/Phosphatidylcholine Transfer Protein

Summary

The substrate specificity and the influence of membrane properties of donor and acceptor vesicles (lipid composition, charge effects, fluidity) on the activity of a 2800-fold enriched phosphatidyl inositol/phosphatidylcholine transfer protein (PI/PC-TP) from yeast was tested. A fluorescence assay employing pyrene-labeled phospholipids was used for transfer measurementsin vitro. The rate of transfer catalyzed by this protein is markedly higher for phosphatidyl inositol than for phosphatidylcholine. Although yeast cells contain neither alkylacyl glycerophospholipids nor plasmalogens (alkenylacyl glycerophospholipids) these subclasses of phospholipids are readily transferred by the yeast PI/PC-TP; the transfer rate decreases in the order alkylacyl glycerophospholipid > plasmalogen > diacyl glycerophospholipid. A moderate but significant net transfer of phosphatidylcholine is observed when acceptor membranes consist only of non-transferable phospholipids, e.g. phosphatidylethanolamine.

The yeast PI/PC-TP forms complexes with phosphatidyl inositol and phosphatidylcholine, respectively, the affinity for phosphatidyl inositol being considerably higher. This result correlates with the preferential transfer of phosphatidyl inositol as mentioned above.

Phosphatidylcholine transfer catalyzed by the yeast PI/PC-TP is inhibited by negatively charged phospholipids. Differences in the accessibility of the charged groups of lipids to the transfer protein might account for the different inhibitory effects, which occur in the order phosphatidylserine > phosphatidylglycerol > phosphatidyl inositol >cardiolipin > phosphatidic acid. Mitochondrial membranes containing high quantities of negatively charged phospholipids do not show the same inhibitory effect as vesicles prepared from phospholipids extracted from these membranes. This observation points to a compensation of the negative charge of phospholipids by membrane proteins. Ergosterol, which decreases the membrane fluidity, reduces the rate of phosphatidylcholine transfer. The notion that membrane fluidity modulates the transfer rate, is also supported by experiments using acceptor vesicles consisting of dipalmitoyl phosphatidylcholine: the rate of phosphatidylcholine transfer was significantly reduced when assays were carried out below the phase transition temperature of the acceptor vesicle phospholipid.