Solution structure of Apaf-1 CARD and its interaction with caspase-9 CARD:A structural basis for specific adaptor/caspase interaction

Direct recruitment and activation of caspase-9 by Apaf-1 through the hemoph ilic CARD/CARD (Caspase Recruitment Domain) interaction is critical for the activation of caspases downstream of mitochondrial damage in apoptosis. He re we report the solution structure of the Apaf-1 CARD domain and its surfa ce of interaction with caspase-9 CARD. Apaf-1 CARD consists of six tightly packed amphipathic cu-helices and is topologically similar to the RAIDD CAR D, with the exception of a kink observed in the middle of the N-terminal he lix. By using chemical shift perturbation data, the hemophilic interaction was mapped to the acidic surface of Apaf-1 CARD centered around helices 2 a nd 3. Interestingly, a significant portion of the chemically perturbed resi dues are hydrophobic, indicating that in addition to the electrostatic inte ractions predicted previously, hydrophobic interaction is also an important driving force underlying the CARD/CARD interaction. On the basis of the id entified functional residues of Apaf-1 CARD and the surface charge compleme ntarity, we propose a model of CARD/CARD interaction between Apaf-1 and cas pase-9.