Phospholipid transfer proteins: mechanism of action

Membrane properties modulate the activity of a phosphatidylinositol transfer protein from the yeast, Saccharomyces cerevisiae

A phospholipid transfer protein from yeast (Daum, G. and Paltauf, F. (1984) Biochim. Biophys. Acta 794, 385-391) was 2800-fold enriched by an improved procedure. The specificity of this transfer protein and the influence of membrane properties of acceptor vesicles (lipid composition, charge, fluidity) on the transfer activity were determined in vitro using pyrene-labeled phospholipids. The yeast transfer protein forms a complex with phosphatidylinositol or phosphatidylcholine, respectively, and transfers these two phospholipids between biological and/or artificial membranes. The transfer rate for phosphatidylinositol is 19-fold higher than for phosphatidylcholine as determined with 1:8 mixtures of phosphatidylinositol and phosphatidylcholine in donor and acceptor membrane vesicles. If acceptor membranes consist only of non-transferable phospholipids, e.g., phosphatidylethanolamine, a moderate but significant net transfer of phosphatidylcholine occurs. Phosphatidylcholine transfer is inhibited to a variable extent by negatively charged phospholipids and by fatty acids. 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 which is greater than phosphatidylglycerol which is greater than phosphatidylinositol which is greater than cardiolipin which is greater than phosphatidic acid which is greater than fatty acids. Although mitochondrial membranes contain high amounts of negatively charged phospholipids, they serve effectively as acceptor membranes, whereas transfer to vesicles prepared from total mitochondrial lipids is essentially zero. Ergosterol reduces the transfer rate, probably by decreasing membrane fluidity. This notion is supported by data obtained with dipalmitoyl phosphatidylcholine as acceptor vesicle component; in this case the transfer rate is significantly reduced below the phase transition temperature of the phospholipid.

General kinetic model for protein-mediated phospholipid transfer between membranes

Phospholipid transfer protein catalyzes the transfer of phospholipids between bilayer membranes. A general model is developed for describing the kinetics of this process. While previous models derive detailed expressions only for the initial rate of transfer from donor to acceptor membranes, this model takes into account donor-to-donor, acceptor-to-acceptor, and acceptor-to-donor transfers, in addition to the usual donor-to-acceptor transfer. The apparent rate of transfer along any of these specific routes is given as the product of the total rate of transfer (the sum of the rates of transfer along all four routes) and a probability function uniquely defined for each route. The model explains adequately the effects of membrane concentration on phospholipid transfer activity as well as the consequences of varying membrane surface charge and size. Using bovine liver phosphatidylcholine transfer protein, the model is applied to the kinetic analysis of phosphatidylcholine transfer between two populations of small unilamellar vesicles. Rates of protein-catalyzed phosphatidylcholine transfer between vesicles with identical phosphatidic acid content (2 or 6 mol%) are determined experimentally as a function of total vesicle concentration to calculate apparent dissociation constants and maximum rates of transfer; apparent rates of transfer between various combinations of vesicles containing 2 or 6 mol% phosphatidic acid are then deduced from the derived velocity expression. Reasonably good agreement is seen between theoretical apparent rate-vesicle concentration relationships and those measured experimentally. The results support the general treatment of the kinetics of protein-mediated phospholipid transfer and permit an estimation of useful kinetic parameters.

The effect of polyphosphoinositides and phosphatidic acid on the phosphatidylinositol transfer protein from bovine brain: a kinetic study

The phosphatidylinositol transfer protein from bovine brain (PI-TP) has lipid transfer characteristics which make it well suited to maintain phosphatidylinositol (PI) levels in intracellular membranes (Van Paridon, P.A., Gadella, Jr., T.W.J., Somerharju, P.J. and Wirtz, K.W.A. (1987) Biochim. Biophys. Acta 903, 68-77). Using a continuous fluorimetric transfer assay we have investigated in what way phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) affect the transfer activity of this protein in model systems. The effects were analysed by application of a kinetic model which yielded the association constant (K) and dissociation rate constant (k-) for the PI-TP/vesicle complex. Incorporation of PA, PIP and PIP2 into the phosphatidylcholine-containing vesicles increased the association constant solely by diminishing the dissociation rate constant. This effect could be completely accounted for by changes in the membrane surface charge density. In contrast to the inhibitory effect of PA, the inhibition caused by PIP2 was completely abolished by the addition of neomycin, in agreement with the observed preferential binding of this polyamine antibiotic to PIP2. A rise in pH from 5.5 to 8 drastically reduced the association constant for vesicles containing 16 mol% PA (e.g., from 38 to 2 mM-1), without affecting the Vmax. This effect could be mainly attributed to an increase in the negative charge on PI-TP (isoelectric point 5.5), resulting in an enhanced repulsion. Increasing the negative membrane surface charge at pH 7.4 had the opposite effect. This is interpreted to indicate that the membrane interaction site on PI-TP must be positively charged, overcoming the repulsive forces between PI-TP and the vesicle. Addition of PIP2 micelles as a third component in the transfer assay strongly inhibited PI-TP transfer activity. The extent of inhibition suggests a very high affinity of PI-TP for this lipid.

Design, synthesis, and characterization of bis-phosphatidylcholine, a mechanistic probe of phosphatidylcholine transfer protein catalytic activity

The design, synthesis, and characterization of 1-(17,18-dithiatetratriacontandioyl)-bis(2-hexadecanoyl-sn-glycero -3- phosphocholine) is described. Bis-phosphatidylcholine is a dimeric phospholipid comprised of two glycerophosphocholine groups linked together by a disulfide bond at the distal ends of the sn-1 fatty acyl chains. Electron microscopy and [14C]glucose trapping studies indicate that hydrated dispersions of bis-phosphatidylcholine form closed, spherical structures which have diameters in the range of 125-500 nm. Sensitivity to phospholipase hydrolysis suggests that this bipolar lipid is organized in a membrane such that the two polar head groups of the molecular are oriented at the same surface of the membrane. Using conditions in which bovine liver phosphatidylcholine transfer protein transfers both unsaturated and saturated diacyl phosphatidylcholines between fluid phosphatidylcholine vesicles, no transfer of the bipolar phospholipid is observed. The lack of activity toward bis-phosphatidylcholine suggests that this molecule may be a useful tool for elucidating the role of membrane phosphatidylcholine in the catalytic mechanism of the phosphatidylcholine transfer protein.

Secretion and membrane recycling in plant cells: novel intermediary structures visualized in ultrarapidly frozen sycamore and carrot suspension-culture cells

Freeze-fracture electron microscopy of propane-jet-frozen samples has been employed to investigate vesicle-mediated secretion and membrane recycling events in carrot (Daucus carota L.) and sycamore maple (Acer pseudoplatanus L.) suspension-culture cells. Stabilization of the cells by means of ultrarapid freezing has enabled us to preserve the cells in a turgid state and to visualize new intermediate membrane configurations related to these events. Indeed, many of the observed membrane configurations, such as flattened membrane vesicles with slit-shaped membrane fusion sites and horseshoe-shaped membrane infoldings, appear to result from the action of turgor forces on the plasma membrane. Individual cells exhibited great variations in numbers and types of membrane configurations postulated to be related to secretion and membrane-recycling events. In the majority of cells, the different membrane profiles displayed a patchy distribution, and within each patch the membrane configurations tended to be of the same stage. This result indicates that secretory events are triggered in domains measuring from 0.1 to about 10 μm in diameter. Based on an extensive analysis of the different membrane configurations seen in our samples, we have formulated the following model of vesicle-mediated secretion in plant cells: Fusion of a secretory vesicle with the plasma membrane leads to the formation of a single, narrow-necked pore that increases in diameter up to about 60 nm. During discharge, the vesicle is flattened, forming a disc-shaped structure perpendicular to the plane of the plasma membrane. As the vesicle is flattened, the pore is converted to a slit, the maximum length of which coincides with the diameter of the flattened vesicle. The flattened vesicle then tips over and concomitantly the plasma-membrane slit becomes curved into a horseshoe-shaped configuration as it extends along the outer margins of the tipped-over vesicle. Some coated pits are present interspersed between the above-mentioned structures, but their numbers appear insufficient to account for an exclusively endocytotic mechanism of membrane recycling. Instead, our micrographs are more consistent with a mixed mode of recycling of membrane components to the cortical endoplamic reticulum and to Golgi cisternae that involves both internalization of membrane by endocytosis and of individual lippid molecules by unknown mechanisms (lipid exchange proteins?). To this end, overall flattening out of the horseshoe-shaped membrane infoldings is accompanied by a retraction and reduction in size of their central, tongue-like structure.