APCINTERACTIONS · Molecular basis for securin and cyclin ubiquitylation by the anaphase-promoting complex (APC/C)

Molecular basis for securin and cyclin ubiquitylation by the anaphase-promoting complex (APC/C)

Equal distribution of the genetic information (each sister chromatid is distributed into the two emerging daughter cells) is the primary goal during cell cycle progression. High accuracy of this process is crucial to maintain the integrity of the genetic information and is achieved by a number of highly specialized proteins and protein complexes. At the very centre of this process is a protein complex termed the anaphase-promoting complex (APC/C). This complex marks a number of regulatory proteins (e. g. securin) for timely degradation via a specialized pathway. Destruction of securin, in turn, leads to the activation of an evolutionary conserved protease named separase. Proteases are molecular scissors that can cleave and therefore destruct protein targets. Separases promote sister chromatid separation in mitosis through cleavage of a protein complex called cohesin, which forms a ring like structure that physically entraps sister chromatids, and thus cleavage of this ring allows for timely sister chromatid separation (when equal distribution of the genetic information is desired). In summary, all projects carried out during the time of the fellowship aimed at understanding the molecular details of how cells duplicate, a process that is impaired in cancer and hence these studies might lead to the development of new anti-cancer drugs. Securin is an inhibitor of separase. Previous studies suggested - albeit lacking structural evidence and therefore the atomic details - that securin is likely to bind as a pseudo-substrate to the cleavage site of the separase, thereby inhibiting its activity (Nagao et al., 2006). Separase has previously been proposed as a promising and clinically applicable proliferation marker (Gurvits et al., BJC 2017). Elevated levels of separase, for instance, has been shown to be linked to aneuploidy (the unequal distribution of sister chromatids) and mammary tumorigenesis (Zhang et al. PNAS 2008). Consequently, separase expression levels could be utilized as a biomarker for treatment decisions in e.g. breast carcinoma treatments. In this work, we aimed at understanding the molecular mechanisms of securin-mediated separase inhibition in atomic detail. We employed a mixture of biochemical and structural approaches to further characterize the formation of the separase-securin complex, with a main focus on cryo-electron microscopy (cryo-EM) studies. A second project aims at understanding the molecular details of the interaction of the human separase-securin complex with the APC/C. Such a structure would not only provide critical insights into substrate recognition of the APC/C but would also represent the first structure of the APC/C bound to a natural, full-length substrate determined. Lastly, we have also directed our efforts towards obtaining a higher resolution structure of the human APC/C complex. The APC/C might represent an interesting drug target in cancer treatment and as such high-resolution structure information will be of invaluable interest.

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