ICAD/DFF regulator of apoptotic nuclease is nuclear.

In nonapoptotic cells, the caspase-activated DNase CAD/CPAN is associated with a regulatory subunit, ICAD/DFF, that binds to it and blocks its enzymatic activity. It has been proposed that a major function of ICAD is to restrain CAD in the cytoplasm in healthy cells. The experiments presented here demonstrate that rather than being cytoplasmic, a GFP-ICAD fusion protein is nuclear in healthy human, pig, and chicken cells. Furthermore, immunoblots using antibodies to murine ICAD confirm the presence of endogenous ICAD and the marker protein DNA topoisomerase I in human nuclei, while tubulin was found solely in the cytosolic fraction. Since ICAD is located in cell nuclei, it is unlikely that the protein functions primarily in the cytoplasm either as an anchor for CAD/CPAN or as a factor that enters the nucleus following caspase cleavage in order to activate resident endonucleases.