Caspases from scleractinian coral show unique regulatory features

Abstract

Coral reefs are experiencing precipitous declines around the globe with coral diseases and temperature-induced bleaching being primary drivers of these declines. Regulation of apoptotic cell death is an important component in the coral stress response. Although cnidaria are known to contain complex apoptotic signaling pathways, similar to those in vertebrates, the mechanisms leading to cell death are largely unexplored. We identified and characterized two caspases each from Orbicella faveolata, a disease-sensitive reef-building coral, and Porites astreoides, a disease-resistant reef-building coral. The caspases are predicted homologs of the human executioner caspases-3 and -7, but OfCasp3a (Orbicella faveolata caspase-3a) and PaCasp7a (Porites astreoides caspase-7a), which we show to be DxxDases, contain an amino-terminal caspase activation/recruitment domain (CARD) similar to human initiator/inflammatory caspases. OfCasp3b (Orbicella faveolata caspase-3b) and PaCasp3 (Porites astreoides caspase-3), which we show to be VxxDases, have short pro-domains, like human executioner caspases. Our biochemical analyses suggest a mechanism in coral which differs from that of humans, where the CARD-containing DxxDase is activated on death platforms but the protease does not directly activate the VxxDase. The first X-ray crystal structure of a coral caspase, of PaCasp7a determined at 1.57Å resolution, reveals a conserved fold and an N-terminal peptide bound near the active site that may serve as a regulatory exosite. The binding pocket has been observed in initiator caspases of other species. These results suggest mechanisms for the evolution of substrate selection while maintaining common activation mechanisms of CARD-mediated dimerization.

Evolution of the folding landscape of effector caspases

“…Effector caspase-3, caspase-6, and caspase-7 have a significant role in apoptosis and serve overlapping but nonredundant functions ( 35 ), and multiple studies have examined enzyme specificity and regulation of extant caspases ( 21 , 33 , 36 , 37 ). In addition, we have previously examined evolutionary changes resulting in amino acid substitutions that affect enzyme specificity ( 15 , 38 ) and allosteric regulation ( 26 , 39 , 40 ), but there is a dearth of information regarding changes in the caspase folding landscape. To date, only human caspase-3 has been examined in detail ( 6 , 27 , 28 , 41 ), so it was not clear whether all effector caspases utilized the same folding landscape.…”

“…The oligomeric form of the zymogen and its activation mechanism is key to regulating apoptosis ( 13 , 15 ). While the amino acid sequence identity is low between caspase subfamilies (∼40%) ( 15 ), the caspase–hemoglobinase fold is well conserved ( 21 ) even from distantly related species of vertebrates and invertebrates, such as human, Danio rerio , Caenorhabditis elegans , Drosophila melanogaster , and Porites astreoides ( 22 , 23 , 24 , 25 , 26 ). The structure of the PCP monomer is characterized by a six-stranded β-sheet core with several α-helices on the surface ( 6 ).…”

Evolution of the folding landscape of effector caspases

“…Effector caspases-3, -6, and -7 have a significant role in apoptosis and serve overlapping but nonredundant functions (33), and multiple studies have examined enzyme specificity and regulation of extant caspases (20,31,34,35). In addition, we have previously examined evolutionary changes resulting in amino acid substitutions that affect enzyme specificity (15,36) and allosteric regulation (25,37,38), but there is a dearth of information regarding changes in the caspase folding landscape. To date, only human caspase-3 has been examined in detail (6,26,27,39), so it was not clear whether all effector caspases utilized the same folding landscape.…”

“…The oligomeric form of the zymogen and its activation mechanism is key to regulating apoptosis (13,15). While the amino acid sequence identity is low between caspase subfamilies (~40%) (15), the caspase-hemoglobinase fold is well-conserved (20) even from distantly related species of vertebrates and invertebrates, such as human, Danio rerio, Caenorhabditis elegans, Drosophila melanogaster, and Porites astreoides (21)(22)(23)(24)(25).The structure of the procaspase monomer is characterized by a 6stranded -sheet core with several -helices on the surface (6). Each monomer of the procaspase homodimer consists of approximately 300 amino acids organized into an Nterminal prodomain followed by a protease domain.…”

Promoted “Click” SERS Detection for Precise Intracellular Imaging of Caspase-3

“…Cell apoptosis is known as the common mode of programmed cell death for adjusting the physiological homeostasis, and unexpected apoptotic pathway could be activated by some diseases such as autoimmune disorders, chronic heart failure, cancers, and so forth. − Caspase-3 has been regarded as a frequently activated death protease, which plays a critical role in cell apoptosis, thus monitoring caspase-3 has aroused growing interest for medical applications. − Until now, various analytic methods including electrochemical sensing, , colorimetric assay, fluorescence, and bimodal fluorescence-magnetic resonance have been employed for monitoring caspase-3, but the complicated operation steps, long-time analytical process, high fluorescence background, and even photobleaching of fluorescence still limited their applications. Therefore, developing a novel method for precise in situ imaging of extracellular and intracellular caspase-3 is of great significance.…”