Alpha-synuclein is a presynaptic protein of poorly understood function that is linked to both genetic and sporadic forms of Parkinson's disease. We have proposed that alpha-synuclein may function specifically at synaptic vesicles docked at the plasma membrane, and that the broken-helix state of the protein, comprising two antiparallel membrane-bound helices connected by a nonhelical linker, may target the protein to such docked vesicles by spanning between the vesicle and the plasma membrane. Here, we demonstrate that phosphorylation of alpha-synuclein at tyrosine 39, carried out by c-Abl in vivo, may facilitate interconversion of synuclein from the vesicle-bound extended-helix state to the broken-helix state. Specifically, in the presence of lipid vesicles, Y39 phosphorylation leads to decreased binding of a region corresponding to helix-2 of the broken helix state, potentially freeing this region of the protein to interact with other membrane surfaces. This effect is largely recapitulated by the phosphomimetic mutation Y39E, and expression of this mutant in yeast results in decreased membrane localization. Intriguingly, the effects of Y39 phosphorylation on membrane binding closely resemble those of the recently reported disease linked mutation G51D. These findings suggest that Y39 phosphorylation could modulate functional aspects of alpha-synuclein and perhaps influence pathological aggregation of the protein as well.