Regulation of Phospholipase D2: Selective Inhibition of Mammalian Phospholipase D Isoenzymes by α- and β-Synucleins

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Abstract
Two widely expressed mammalian phosphatidylcholine (PC)-specific phospholipases D (PLD), PLD1 and PLD2, have been identified. Recombinantly expressed PLD2 has high basal activity and is insensitive to GTP-binding protein activators of PLD1 [Colley, W. C., et al. (1997) Curr. Biol. 7, 191−201]. To investigate the regulation of PLD2 we isolated PLD2, from mouse brain by immunoaffinity chromatography. The native and recombinant proteins have indistinguishable properties: PLD2 is potently activated by phosphoinositides with a vicinal 4,5-phosphate pair but is not stimulated by guanosine 5‘-O-(3-thio triphosphate)-activated ADP-ribosylation factor-1, Rho family GTP-binding proteins, or protein kinases C-α, or -β1. We used recombinant PLD2 in a reconstitution assay to search for regulators in cell and tissue extracts. Bovine brain contains a heat-stable protein factor that inhibits PLD2 activity in vitro. This factor was purified to homogeneity and identified as a mixture of α- and β-synucleins by microsequencing and Western blotting. Recombinantly expressed α- and β-synucleins inhibit PLD2 activity in vitro (K0.5 10 nM). Inhibition is not overcome by the protein or lipid activators of PLD1. Synucleins have been implicated in Parkinson's and Alzheimer's diseases. Our findings suggest that inhibition of PLD2 may be a function of synucleins. Modulation of PLD2 activity by synucleins may play a role in some aspects of the pathophysiologies that characterize these neurodegenerative diseases.