One hypothesis for the etiology of Parkinson's disease (PD) is that subsets of neurons are vulnerable to a failure in proteasome-mediated protein turnover. Here we show that overexpression of mutant α-synuclein increases sensitivity to proteasome inhibitors by decreasing proteasome function. Overexpression of parkin decreases sensitivity to proteasome inhibitors in a manner dependent on parkin's ubiquitin-protein E3 ligase activity, and antisense knockdown of parkin increases sensitivity to proteasome inhibitors. Mutant α-synuclein also causes selective toxicity to catecholaminergic neurons in primary midbrain cultures, an effect that can be mimicked by the application of proteasome inhibitors. Parkin is capable of rescuing the toxic effects of mutant α-synuclein or proteasome inhibition in these cells. Therefore, parkin and α-synuclein are linked by common effects on a pathway associated with selective cell death in catecholaminergic neurons.