Nuclear import of PKCδ is required for apoptosis: identification of a novel nuclear import sequence

We have shown previously that protein kinase Cδ (PKCδ) is required for mitochondrial-dependent apoptosis. Here we show that PKCδ is imported into the nucleus of etoposide-treated cells, that nuclear import is required for apoptosis and that it is mediated by a nuclear localization signal (NLS) in the C-terminus of PKCδ. Mutation of the caspase cleavage site of PKCδ inhibits nuclear accumulation in apoptotic cells, indicating that caspase cleavage facilitates this process. Expression of the PKCδ catalytic fragment (CFδ) in transfected cells results in nuclear localization and apoptosis. We show that the PKCδ NLS is required for nuclear import of both full-length PKCδ and CFδ, and drives nuclear localization of a multimeric green fluorescent protein. Mutations within the NLS of CFδ prevent nuclear accumulation and block apoptosis. Conversely, nuclear expression of a kinase-negative catalytic fragment (KN-CFδ) protects cells from etoposide-induced apoptosis. Mutation of the NLS blocks the ability of KN-CFδ to protect against etoposide-induced apoptosis. These results indicate that PKCδ regulates an essential nuclear event(s) that is required for initiation of the apoptotic pathway.