Structural determinants of PLD2 inhibition by α-synuclein

The presynaptic protein α-synuclein has been implicated in both neuronal plasticity and neurodegenerative disease, but its normal function remains unclear. We described the induction of an amphipathic α-helix at the N terminus (exons 2-4) of α-synuclein upon exposure to phospholipid vesicles, and hypothesized that lipid-binding might serve as a functional switch by stabilizing α-synuclein in an active (α-helical) conformation. Others have shown that α and β-synucleins inhibit phospholipase D (PLD), an enzyme involved in lipid-mediated signaling cascades and vesicle trafficking. Here, we report that all three naturally occurring synuclein isoforms (α, β, and γ-synuclein) are similarly effective inhibitors of PLD2 in vitro, as is the Parkinson's disease-associated mutant A30P. The PD-associated mutant A53T, however, is a more potent inhibitor of PLD2 than is wild-type α-synuclein. We analyze mutations of the α-synuclein protein to identify critical determinants of human PLD2 inhibition in vitro. Deletion of residues 56-102 (exon 4) decreases PLD2 inhibition significantly; this activity of exon 4 may require adoption of an α-helical conformation, as mutations that disrupt α-helicity also abrogate inhibition. Deletion of C-terminal residues 130-140 (exon 6) completely abolishes inhibitory activity. In addition, PLD2 inhibition is blocked by phosphorylation at serine 129 or at tyrosine residues 125 and 136, or by mutations that mimic phosphorylation at these sites. We conclude that PLD2 inhibition by α-synuclein is mediated by a lipid-stabilized α-helical structure in exon 4 and also by residues within exon 6, and that this inhibition can be modulated by phosphorylation of specific residues in exons 5 and 6.