Solution structure of the CIDE-N domain of CIDE-B and a model for CIDE-N/CIDE-N interactions in the DNA fragmentation pathway of apoptosis

Apoptotic DNA fragmentation and chromatin condensation are mediated by the caspase-activated DFF40/ CAD nuclease, which is chaperoned and inhibited by DFF45/ICAD. CIDE proteins share a homologous regulatory CIDE-N domain with DFF40/CAD and DFF45/ ICAD. Here we report the solution structure of CIDE-N of human CIDE-B. We show that the CIDE-N of CIDE-B interacts with CIDE-N d omains of both DFF40 and DFF45. The binding epitopes are similar and map to a highly charged bipolar surface region of CIDE-B. Furthermore, we demonst rate that the CIDE-N of CIDE-B regulates enzymatic activity of the DFF40/ D FF45 complex in vitro. Based on these results and mutagenesis data, we prop ose a model for the CIDE-N/ CIDE-N complex and discuss the role of this nov el bipolar interaction in mediating downstream events of apoptosis.