We showed early on that SCS is regulated by ubiquitin-mediated proteolysis. Ubiquitin-tagged proteins are then recognized by a large multicatalytic protease called the proteasome and degraded. Scc1p is localized to chromatin, so its removal by cleavage is thought to release the glue that holds sister chromatids together. This protein normally unlinks DNA molecules that are linked together, suggesting that DNA linkages also hold sister chromatids together. This in vitro system is based on cytoplasmic extracts of frog eggs.
Spindles formed in extracts containing CSF are stable for hours.
The CSF that normally holds unfertilized eggs in metaphase is inactivated by a wave of calcium that moves through the egg upon fertilization. We have used our in vitro SCS assay to produce biochemical quantities of metaphase and anaphase extracts. This protein is being microsequenced to determine its nature. Clearly, ubiquitin-mediated proteolysis plays an essential role in the regulation of cell growth. However, it also plays an important role in human diseases.