Paroxysmal nonkinesigenic dyskinesia (PNKD) is an autosomal dominant episodic movement disorder precipitated by coffee, alcohol, and stress. We previously identified the causative gene but the function of the encoded protein remains unknown. We also generated a PNKD mouse model that revealed dysregulated dopamine signaling in vivo. Here, we show that PNKD interacts with synaptic active zone proteins Rab3-interacting molecule (RIM)1 and RIM2, localizes to synapses, and modulates neurotransmitter release. Overexpressed PNKD protein suppresses release, and mutant PNKD protein is less effective than wild-type at inhibiting exocytosis. In PNKD KO mice, RIM1/2 protein levels are reduced and synaptic strength is impaired. Thus, PNKD is a novel synaptic protein with a regulatory role in neurotransmitter release.paroxysmal dyskinesia | exocytosis | neurological disease P aroxysmal nonkinesigenic dyskinesia (PNKD)* is a rare dominantly inherited episodic movement disorder. First reported in 1940 (1), PNKD is characterized by childhood onset with involuntary movements in the limbs, trunk, and face manifesting as dystonia, chorea, and athetosis (2). PNKD shows nearly complete penetrance and attacks are precipitated by fatigue, stress, hunger, and consumption of coffee or alcohol. Patients are completely normal between attacks.Hereditary forms of many episodic disorders are recognized and include movement disorders, muscle diseases, cardiac arrhythmias, epilepsy, and headache. A majority of the causative genes that have been identified encode ion channels (3). Studies in several spontaneous mouse mutants with a paroxysmal dyskinesia phenotype have provided intriguing insights into these otherwise complicated diseases (4-6). The tottering and lethargic mice display motor abnormalities mimicking paroxysmal dyskinesia and harbor mutations in the genes encoding the α1A and β4 subunits of the P/Q-type voltage-gated Ca 2+ -channel, respectively (7,8). Like PNKD, the dyskinesia phenotype in tottering mice can also be triggered by caffeine and stress. PNKD is interesting in that the gene encodes a novel protein with homology to human glyoxalase II, an enzyme in a stress-response pathway. Although the normal role of PNKD is unknown, we previously identified the causative gene of PNKD (9) and a mouse model of the human mutations recapitulates the phenotype and shows dopamine signaling dysregulation (10).At synapses, vesicle priming, docking, and fusion at synaptic terminals are complex and coordinately regulated by proteins from the active zone, presynaptic membrane, and vesicles (11). Rab3-interacting molecules (RIMs) are a family of active zone proteins encoded by genes, Rims 1 to 4 (12). Through their interactions with vesicle proteins, active zone proteins, and presynaptic membrane proteins, RIMs are centrally involved in basic parameters of neurotransmitter release, and they contribute to both long-term and short-term synaptic plasticity (13-18).Given that PNKD is a novel protein whose function is unknown, we set out to identify ...