Hornig, N.C.D.; (2004) Regulation of cohesin cleavage by separase in Saccharomyces cerevisiae. Doctoral thesis , University of London.

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Abstract

The final irreversible step in the duplication and dissemination of genomes to daughter cells takes place at the metaphase to anaphase transition. Sister chromatid pairs that are aligned in metaphase split and separate in anaphase. This splitting is triggered by a specific protease called "separase", conserved in all eukaryotes. Separase cleaves Scc1, the central subunit of the "cohesin" complex that is required for holding together the sister chromatids in metaphase. In budding yeast, separase' proteolytic activity is regulated in two ways: by its binding partner securin and through modification of the substrate Scc1. This thesis shows that securin supports separase localisation in the nucleus and ensures that separase gains full proteolytic activity by acting like a chaperone. It also shows that securin is a direct inhibitor of separase' proteolytic activity. Securin prevents binding of separase to its substrates. It also hinders the separase N-terminus from interacting with and possibly inducing an activating conformational change at the protease active site at the proteins C-terminus. During chromosome condensation in prophase, part of cohesin leaves chromatin and it is unclear how separase specifically finds and cleaves the fraction of the cohesin that is left on chromosomes in metaphase. An assay was established to compare the cleavage of chromatin-bound versus soluble cohesin. This revealed that Scc1 in chromatin-bound cohesin is significantly preferred by separase over Scc1 in soluble cohesin. This difference was not due to the chromatin environment and can be explained by preferential phosphorylation of chromatin-bound Scc1. Site directed mutagenesis of ten Polo kinase phosphorylation sites in Scc1 reduced the rate of cleavage of chromatin-bound cohesin, and hyperphosphorylation of soluble Scc1 by overexpression of Polo kinase accelerated its cleavage in vitro. Phosphorylation site mutant Scc1 slowed anaphase progression in vivo leading to an increase in chromosome loss.

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