The structure of the CARD8 caspase-recruitment domain suggests its association with the FIIND domain and procaspases through adjacent surfaces

CARD8 plays crucial roles in regulating apoptotic and inflammatory signaling pathways through the association of its caspase-recruitment domain (CARD) with those of procaspase-9 and procaspase-1. The CARD8 CARD has also been predicted to form an intramolecular complex with its FIIND domain. Here, the first crystal structure of the CARD8 CARD is reported; it adopts a six-helix bundle fold with a unique conformation of the alpha 6 helix that is described here for the first time. The surface of the CARD8 CARD displays a prominent acidic patch at its alpha 2, alpha 3 and alpha 5 helices that may interact with the procaspase-9 CARD, whereas an adjacent charged surface at its alpha 3 and alpha 4 helices may associate with the CARD8 FIIND domain without interfering with the CARD-CARD interaction. This suggests that the function of CARD8 may be regulated by both intramolecular and intermolecular interactions involving electrostatic attractions.