Here we show that scavenger receptor class B type I is present in the small-intestine brush border membrane where it facilitates the uptake of dietary cholesterol from either bile salt micelles or phospholipid vesicles. This receptor can also function as a port for several additional classes of lipids, including cholesteryl esters, triacylglycerols, and phospholipids. It is the first receptor demonstrated to be involved in the absorption of dietary lipids in the intestine. In liver and steroidogenic tissues, the physiological ligand of this receptor is high-density lipoprotein. We show that binding of high-density lipoprotein and apolipoprotein A-I to the brush border membrane-resident receptor inhibits uptake of cholesterol (sterol) into the brush border membrane from lipid donor particles. This finding lends further support to the conclusion that scavenger receptor BI catalyzes intestinal cholesterol uptake. Our findings suggest new therapeutic approaches for limiting the absorption of dietary cholesterol and reducing hypercholesterolemia and the risk of atherosclerosis.

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A novel plant phospholipase D (PLD; EC 3.1.4.4) activity, which is dependent on phosphatidylinositol 4,5-bisphosphate (PIP 2 ) and nanomolar concentrations of calcium, has been identified in Arabidopsis. This report describes the cloning, expression, and characterization of an Arabidopsis cDNA that encodes this PLD. We have designated names of PLD␤ for this PIP 2 -dependent PLD and PLD␣ for the previously characterized PIP 2 -independent PLD that requires millimolar Ca 2؉ for optimal activity. The PLD␤ cDNA contains an open reading frame of 2904 nucleotides coding for a 968-amino acid protein of 108,575 daltons. Expression of this PLD␤ cDNA clone in Escherichia coli results in the accumulation of a functional PLD having PLD␤, but not PLD␣, activity. The activity of the expressed PLD␤ is dependent on PIP 2 and submicromolar amounts of Ca 2؉ , inhibited by neomycin, and stimulated by a soluble factor from plant extracts. Sequence analysis reveals that PLD␤ is evolutionarily divergent from PLD␣ and that its N terminus contains a regulatory Ca 2؉ -dependent phospholipid-binding (C2) domain that is found in a number of signal transducing and membrane trafficking proteins.Phospholipase D (PLD; EC 3.1.4.4) 1 -catalyzed hydrolysis of glycerophospholipids produces phosphatidic acid (PA) and a hydrophilic constituent. This activity was first identified in plants and since has been found in animals and microorganisms. PLD in plants was originally proposed to be important in phospholipid catabolism, initiating a lipolytic cascade in membrane deterioration during senescence and stress injuries (1, 2). Recent studies in plants, animals, and yeast indicate that PLD hydrolysis plays a pivotal role in transmembrane signaling and cellular regulation (3-9). Activation of PLD has been proposed to mediate many cellular processes including cell proliferation, membrane trafficking, meiosis, and responses to external and internal stimuli. It has been suggested that multiple forms of PLD are involved in these diverse cellular processes since several studies have shown the presence of PLD variants that are expressed differently (9 -12). In castor bean (9) and rice (12), one PLD variant is constitutive whereas the appearance of other variants is associated with specific conditions such as rapid growth, wounding, and senescence. A distinct property shared by these variants is their in vitro requirement of millimolar Ca 2ϩ concentrations for optimal activity. Further analyses of the castor bean PLD variants have led to the suggestion that the catalytic activity of these variants results from the same gene product (9 -11).A recent study has provided important evidence for the presence of two plant PLDs that are derived from different gene products and regulated distinctly (13). One PLD requires polyphosphoinositides and submicromolar concentrations of Ca 2ϩ for activity and the other is PIP 2 -independent and is most active in the presence of millimolar amounts of Ca 2ϩ . The latter is the prevalent form of PLD that has been purified and cha...

Multiple molecular forms of phospholipase D (PLD; EC 3.1.4.4) were identified and partially characterized in endosperm of germinated seeds and leaves of castor bean (Ricinus communis L. var Hale). The different PLD forms were resolved by nondenaturing polyacrylamide gel eledrophoresis, isoelectric focusing, and sizeexclusion chromatography. PLD was deteded with both a PLD adivity assay and immunoblots with PLD-specific antibodies. There were three major forms of PLD, designated types 1,2, and 3, based on their mobility during nondenaturing polyacrylamide gel electrophoresis. Molecular masses of the PLD variants were estimated at 330, 230, and 270 k D for the types 1, 2, and 3, respedively. lsoeledric points of the native type 1, 2, and 3 PLDs were approximately 6.2,4.9, and 4.8. Under the in vitro assay conditions used, the three forms of PLD exhibited the same substrate specificity, hydrolyzing phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PC) but not phosphatidylserine (PS) and phosphatidylinositol (Pl). The three forms of PLD differed in their substrate preferences, and the order of activities was: PLD 1, PE > PC = PC; PLD 2, PE > PC > PC; PLD 3, PE = PC = PC. The K,,, values of PLDs 1,2, and 3 for PC were 1.92,2.62, and 5.18 mM, respectively. These PLDs were expressed differentially following seed germination and during leaf development. Type 1 was found in the early stages of seedling growth and in young leaves, type 2 was present in all the tissues and growth stages examined, and type 3 was expressed in senescent tissues. The PLDs shifted from largely cytosolic to predominantly membrane-associated forms during leaf development. The present studies demonstrate the structural heterogeneity of plant PLD and growth stage-specific expression of different molecular forms. The possible role for the occurrence of multiple molecular forms of PLD in cellular metabolism i s discussed.

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