α-Synuclein Membrane Association Is Regulated by the Rab3a Recycling Machinery and Presynaptic Activity

alpha-Synuclein is an abundant presynaptic protein and a primary component of Lewy bodies in Parkinson disease. Although its pathogenic role remains unclear, in healthy nerve terminals alpha-synuclein undergoes a cycle of membrane binding and dissociation. An alpha-synuclein binding assay was used to screen for vesicle proteins involved in alpha-synuclein membrane interactions and showed that antibodies directed to the Ras-related GTPase Rab3a and its chaperone RabGDI abrogated alpha-synuclein membrane binding. Biochemical analyses, including density gradient sedimentation and co-immunoprecipitation, suggested that alpha-synuclein interacts with membrane-associated GTP-bound Rab3a but not to cytosolic GDP-Rab3a. Accumulation of membrane-bound alpha-synuclein was induced by the expression of a GTPase-deficient Rab3a mutant, by a dominant-negative GDP-dissociation inhibitor mutant unable to recycle Rab3a off membranes, and by Hsp90 inhibitors, radicicol and geldanamycin, which are known to inhibit Rab3a dissociation from membranes. Thus, all treatments that inhibited Rab3a recycling also increased alpha-synuclein sequestration on intracellular membranes. Our results suggest that membrane-bound GTP-Rab3a stabilizes alpha-synuclein on synaptic vesicles and that the GDP dissociation inhibitor.Hsp90 complex that controls Rab3 a membrane dissociation also regulates alpha-synuclein dissociation during synaptic activity.