Whose end is destruction: cell division and the anaphase-promoting complex

The APC/C was found by searching for the apparatus responsible for destroying mitotic cyclins at the metaphase-to-anaphase transition. This apparatus is a key part of the regulatory network that generates oscillations in the activity of mitotic CDKs. Studies of how the various forms of APC/C are regulated promise to provide a deep understanding of how eukaryotic cells generate CDK waves. However, the APC/C turns out to have a much more fundamental role in the eukaryotic cell cycle than merely being a counterweight to cyclin B synthesis. It mediates the separation of sister chromatids and is the target of regulatory mechanisms whose role is to ensure that daughter cells inherit one (usually two) complete copies of the genome. The control of sister chromatid separation appears to have become more and more important as organisms became more complicated and as their genomes grew in size. Bacteria, for example, barely regulate sister chromatid separation. Indeed, they commence to separate sister chromatids almost as soon as replication has been initiated. At the other extreme are mammalian cells, in which a surveillance mechanism merely needed for high fidelity chromosome transmission in yeast has clearly become an integral part of mitosis. It was the discovery of the APC/C (and SCF) and the key roles that they have in eukaryotic cell reproduction that established once and for all the importance of ubiquitin mediated proteolysis in eukaryotic cell biology. Once perceived as a system exclusively involved in removing damaged proteins from the cell, ubiquitination is now perceived as a universal regulatory mechanism whose importance approaches that of protein phosphorylation. The irreversibility of proteolysis is utilized by cells to give the cell cycle directionality. Once CKIs have been destroyed by SCF, it is very difficult for cells to inactivate Cdks. Their activity drives the onset of DNA replication and entry into mitosis. Likewise, the destruction of cyclin B and Pds1 triggers the separation of sister chromatids and the inactivation of Cdk1. By utilizing the same apparatus to degrade mitotic cyclins and anaphase inhibitors, eukaryotic cells ensure that preparations for chromosome rereplication cannot normally precede the separation of sister chromatids generated by a previous round of DNA replication. The recent discovery that destruction of IκB and β-catenin are mediated by SCF suggests that this formula is not restricted to cell division but also has a key role in signal transduction. The persistence of APC/C in fully differentiated quiescent cells suggests that its powerful substrate recognition apparatus is not confined to proteins involved in cell cycle progression. That proteolysis should have a key role in cell cycle regulation is in retrospect possibly not too surprising. What is still less clear is why so much of the proteolysis that orders the cell cycle should be mediated by large ubiquitin ligase complexes as opposed to more conventional proteases. A key advantage of these ligases is that they can, with great specificity, promote the complete annihilation of a protein without the need for cleavage sites within functional domains of that protein. This then may be one of the key advantages of separating the marking process from proteolysis itself. It is interesting in this regard that the proteolytic program that mediates apoptosis in contrast utilizes proteolytic cleavage. Ubiquitination requires ATP, which is in short supply in dying cells. The cell cycle ends in an orgy of protein degradation that sets the scene for a new round of duplication. With the discovery of the apparatus responsible for this cleanup, we have embarked on a whole new area of cell biology.