Nuclear clusterin (nCLU) is an ionizing radiation (IR)-inducible protein that binds Ku70, and triggers apoptosis when overexpressed in MCF-7 cells. We demonstrate that endogenous nCLU synthesis is a product of alternative splicing. Reverse transcriptase-PCR analyses revealed that exon II, containing the first AUG and encoding the endoplasmic reticulum-targeting peptide, was omitted. Exons I and III are spliced together placing a downstream AUG in exon III as the first available translation start site. This shorter mRNA produces the 49-kDa precursor nCLU protein. Ku70 binding activity was localized to the C-terminal coiled-coil domain of nCLU. Leucine residues 357, 358, and 361 of nCLU were necessary for Ku70-nCLU interaction. The N- and C-terminal coiled-coil domains of nCLU interacted with each other, suggesting that the protein could dimerize or fold. Mutation analyses indicate that the C-terminal NLS was functional in nCLU with the same contribution from N-terminal NLS. The C-terminal coiled-coil domain of nCLU was the minimal region required for Ku binding and apoptosis. MCF-7 cells show nuclear as well as cytoplasmic expression of GFP-nCLU in apoptotic cells. Cytosolic aggregation of GFP-nCLU was found in viable cells. These results indicate that an inactive precursor of nCLU exists in the cytoplasm of non-irradiated MCF-7 cells, translocates into the nucleus following IR, and induces apoptosis.