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Alpha-synuclein is a synaptic modulatory protein implicated in the pathogenesis of Parkinson disease. The precise functions of this small cytosolic protein are still under investigation. alpha-Synuclein has been proposed to regulate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins involved in vesicle fusion. Interestingly, alpha-synuclein fails to interact with SNARE proteins in conventional protein-binding assays, thus suggesting an indirect mode of action. As the structural and functional properties of both alpha-synuclein and the SNARE proteins can be modified by arachidonic acid, a common lipid regulator, we analysed this possible tripartite link in detail. Here, we show that the ability of arachidonic acid to stimulate SNARE complex formation and exocytosis can be controlled by alpha-synuclein, both in vitro and in vivo. Alpha-synuclein sequesters arachidonic acid and thereby blocks the activation of SNAREs. Our data provide mechanistic insights into the action of alpha-synuclein in the modulation of neurotransmission.