How mutant prion protein (PrP) leads to neurological dysfunction in genetic prion diseases is unknown. Tg(PG14) mice synthesize a misfolded mutant PrP which is partially retained in the neuronal endoplasmic reticulum (ER). As these mice age, they develop ataxia and massive degeneration of cerebellar granule neurons. We report that motor behavioral deficits in Tg(PG14) mice emerge before neurodegeneration and are associated with defective glutamate exocytosis from granule neurons due to impaired calcium dynamics. We found that PrP interacts with the voltage-gated calcium channel α2δ-1 subunit which promotes the anterograde trafficking of the channel. Owing to ER retention of mutant PrP, α2δ-1 accumulates intracellularly, impairing delivery of the channel complex to the cell surface. Thus mutant PrP disrupts cerebellar glutamatergic neurotransmission by reducing the number of functional channels in cerebellar granule neurons. These results link intracellular PrP retention to synaptic dysfunction, indicating new modalities of neurotoxicity and potential therapeutic strategies.