Fifty C-terminal amino acid residues are necessary for the chaperone activity of DFF45 but not for the inhibition of DFF40.

Apoptotic DNA fragmentation, the hallmark of apoptosis, is mediated primarily by caspase-activated DFF40 (CAD) nuclease. DFF40 exists as a heterodimer with DFF45 (ICAD), which is a specific chaperone and inhibitor of DFF40 under normal conditions. To understand the mechanism through which the DFF40/DFF45 system is regulated, we analyzed the structural and biochemical properties of apoptotic DNA fragmentation mediated by DFF40/DFF45. Using limited proteolysis, we show that residues 1-281 of DFF45 form a rigid, crystallized domain, whereas the loop formed by residues 277-281 is accessible by trypsin. These results show that the C-terminal helix formed by residues 281-300 is dynamic and necessary for the chaperone activity of DFF45, but not for inhibition of DFF40.