A closed conformation of the Caenorhabditis elegans separase-securin complex

The protease separase plays a key role in sister chromatid disjunction and centriole disengagement. To maintain genomic stability, separase activity is strictly regulated by binding of an inhibitory protein, securin. Despite its central role in cell division, the separase and securin complex is poorly understood at the structural level. This is partly due to the difficulty of generating a sufficient quantity of homogenous, stable protein. Here we report the production of C. elegans separase-securin complex and its characterisation using biochemical methods and by negative staining electron microscopy. Single particle analysis generated a density map at a resolution of 21-24 Å that reveals a close, globular structure of complex connectivity harbouring two lobes. One lobe matches closely a homology model of the N-terminal HEAT repeat domain of separase while the second lobe readily accommodates homology models of the separase C-terminal death and caspase-like domains. The globular structure of the C. elegans separase-securin complex contrasts with the more elongated structure previously described for the H. sapiens complex, which could represent a different functional state of the complex, suggesting a mechanism for the regulation of separase activity through conformational change.