Structural basis for SMAC-mediated antagonism of caspase inhibition by the giant ubiquitin ligase BIRC6

• Published in: Science (American Association for the Advancement of Science) 2023-03, Vol.379 (6637), p.1112-1117
• Main Authors: Dietz, Larissa, Ellison, Cara J, Riechmann, Carlos, Cassidy, C Keith, Felfoldi, F Daniel, Pinto-Fernández, Adán, Kessler, Benedikt M, Elliott, Paul R
• Format: Article
• Language: English
• Subjects: Antagonism; Apoptosis; Autophagy; Caspase; Caspase 3 - metabolism; Caspase 7 - metabolism; Caspase 9 - metabolism; Catalytic Domain; Cell death; Cryoelectron Microscopy; DIABLO protein; Humans; Inhibitor of Apoptosis Proteins - chemistry; Inhibitor of Apoptosis Proteins - metabolism; Molecular modelling; Protein Multimerization; Proteins; Substrates; Ubiquitin; Ubiquitin-protein ligase; Ubiquitin-Protein Ligases - metabolism; X-Linked Inhibitor of Apoptosis Protein - metabolism
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.ade8840

Certain inhibitor of apoptosis (IAP) family members are sentinel proteins that prevent untimely cell death by inhibiting caspases. Antagonists, including second mitochondria-derived activator of caspases (SMAC), regulate IAPs and drive cell death. Baculoviral IAP repeat-containing protein 6 (BIRC6), a giant IAP with dual E2 and E3 ubiquitin ligase activity, regulates programmed cell death through unknown mechanisms. We show that BIRC6 directly restricts executioner caspase-3 and -7 and ubiquitinates caspase-3, -7, and -9, working exclusively with noncanonical E1, UBA6. Notably, we show that SMAC suppresses both mechanisms. Cryo-electron microscopy structures of BIRC6 alone and in complex with SMAC reveal that BIRC6 is an antiparallel dimer juxtaposing the substrate-binding module against the catalytic domain. Furthermore, we discover that SMAC multisite binding to BIRC6 results in a subnanomolar affinity interaction, enabling SMAC to competitively displace caspases, thus antagonizing BIRC6 anticaspase function.