A variety of mutations in the gene encoding DJ-1 protein cause autosomal recessive early-onset parkinsonism. Recently, a novel pathogenic homozygous DJ-1 missense mutation resulting in the L10P amino acid substitution was reported. In a separate study, a novel homozygous mutation resulting in the deletion of DJ-1 residue P158 was also reported to be causative of disease. The specific effects of the novel L10P and P158DEL mutations on protein function have not been studied. Here, L10P and P158DEL DJ-1 proteins were assessed for protein stability, dimerization, solubility, subcellular localization, and protective function in comparison with WT and the L166P DJ-1 pathogenic variant. It was discovered that, compared with WT protein, L10P, L166P, and P158DEL DJ-1 variants exhibited dramatically reduced protein stabilities. Degradation of each of the pathogenic mutants appeared to be mediated in part by the proteasome. Interestingly, unlike L166P DJ-1, the L10P and P158DEL DJ-1 variants retained the ability to dimerize with WT DJ-1 protein; however, neither of these mutants was able to form homodimers. Additionally, the L10P, L166P, and P158DEL DJ-1 variants exhibited altered profiles on size-exclusion chromatography and demonstrated reduced solubilities in comparison with WT protein, and the latter aberration could be exacerbated in the presence of MG-132. Furthermore, cells stably expressing L10P DJ-1 were more vulnerable to treatments with proteasome inhibitors, suggesting that L10P DJ-1 may be toxic to cells under conditions of proteasome stress. Taken together, these findings suggest that diverse aberrant mechanisms, including alterations in protein stability and protein folding, are associated with the pathogenicity of the L10P and P158DEL DJ-1 variants. © 2010 Wiley-Liss, Inc.