Structure of the CAD domain of caspase-activated DNase and interaction with the CAD domain of its inhibitor

Caspase-activated DNase (CAD), which causes a genome fragmentation at the f inal stage of apoptosis, is a protein of about 40 kDa and exists as a compl ex form with the inhibitor ICAD in Living cells. There is sequence homology of about 80 amino acid residues at the N termini of CAD and ICAD (called t he CAD domain). Here, we report the three-dimensional structure of the CAD domain of CAD determined by multi-dimensional NMR spectroscopy and the prop erty of CAD domains investigated by a surface plasmon resonance experiment. The CAD domain of CAD is an independently folded domain composed of one al pha-helix and five beta-strands forming a single sheet. The overall structu re is categorized in the ubiquitin superfold. This domain can bind strongly to the isolated CAD domain of ICAD (dissociation constant: 5.48(+/-0.003)x 10(-8) M). It suggests the function of the CAD domains in the CAD-ICAD sys tem, that the protein-protein interaction through the CAD domains plays an important role in the inhibition of CAD DNase activity and in the correct f olding of CAD. On the basis of structural comparison with other protein com plexes containing the ubiquitin superfold, the interaction mode of the CAD domains is proposed. (C) 2000 Academic Press.