The Purkinje cell degeneration (pcd) phenotype is characterized by adult onset neurodegeneration resulting from mutations in Nna1, a gene encoding an intracellular protein with a putative metallocarboxypeptidase domain. As Nna1 is also induced in axotomized motor neurons, the elucidation of its function can shed light on previously unsuspected mechanisms common to degenerative and regenerative responses. Structural modeling revealed that Nna1 and three related gene products constitute a new subfamily of metallocarboxypeptidases with a distinctive substrate-binding site. To test whether the metallocarboxypeptidase domain is functionally essential, transgenic mice were generated that expressed Nna1 or a substrate-binding site mutant of Nna1 selectively in Purkinje cells using the L7/pcp2 promoter. When bred onto a homozygous pcd(3J) background, wild type but not mutant Nna1 rescued ataxic behavior and Purkinje cell loss. Therefore, loss of Nna1 in Purkinje cells leads directly to their degeneration and Nna1's carboxypeptidase domain is essential for survival of these neurons.