Meeting Abstract
Coral undergo bleaching when polyps expel endosymbionts as a response to stress, which is usually followed by cell death. Cnidaria contain complex apoptotic signaling pathways, but the mechanisms leading to cell death are largely unexplored. We examined two caspases each from Orbicella faveolata (OfCasp3a and OfCasp3b), a disease-sensitive stony coral, and from Porites astreoides (PaCasp7a and PaCasp3), a disease-resistance stony coral. The four caspases are predicted homologs of human caspases-3 and -7, but OfCasp3a and PaCasp7a also contain an N-terminal caspase activation and recruitment domain (CARD) similar to those of human initiator caspases. Using substrate-phage display assays, we show that OfCasp3a and PaCasp7a are DxxDases, like human caspases-3 and -7. In contrast, OfCasp3b and PaCasp3 are more similar to human caspase-6, with VxxDase activity. Our biochemical analyses suggest a unique mechanism in coral in which the CARD-caspase-7 may be activated on death platforms in response to stress, and that the protease then directly activates the VxxDase. We also report the first X-ray crystal structure of a coral caspase that of PaCasp7a determined at 1.57Å resolution. The structure reveals the overall conservation of the caspase-hemoglobinase fold in coral as well as an N-terminal peptide bound near the active site that may serve as a regulatory exosite. The exosite has been observed in initiator caspases of other species, suggesting mechanisms for the evolution of substrate selection while maintaining common activation mechanisms of CARD-mediated dimerization.
