Helical filament structure of the DREP3 CIDE domain reveals a unified mechanism of CIDE-domain assembly

The CIDE domain was initially identified in apoptotic nucleases and now forms a highly conserved family with diverse functions ranging from cell death to lipid metabolism. Based on structural determination of the DREP3 domain, it is suggested that the head-to-tail helical filament structure might be a unified mechanism of CIDE-domain assembly and represents a critical higher-order scaffolding structure that is important for the function of CIDE-domain-containing proteins in DNA fragmentation and lipid-droplet fusion.
The cell-death-inducing DFF45-like effector (CIDE) domain is a protein-interaction module comprising ∼80 amino acids and was initially identified in several apoptotic nucleases and their regulators. CIDE-domain-containing proteins were subsequently identified among proteins involved in lipid metabolism. Given the involvement of CIDE-domain-containing proteins in cell death and lipid homeostasis, their structure and function have been intensively studied. Here, the head-to-tail helical filament structure of the CIDE domain of DNA fragmentation factor-related protein 3 (DREP3) is presented. The helical filament structure was formed by opposing positively and negatively charged interfaces of the domain and was assembled depending on protein and salt concentrations. Although conserved filament structures are observed in CIDE family members, the structure elucidated in this study and its comparison with previous structures indicated that the size and the number of molecules used in one turn vary. These findings suggest that this charged-surface-based head-to-tail helical filament structure represents a unified mechanism of CIDE-domain assembly and provides insight into the function of various forms of the filament structure of the CIDE domain in higher-order assembly for apoptotic DNA fragmentation and control of lipid-droplet size.