Regulation of Ubiquitin Mediated Proteolysis of G1 Cyclins and the CDK Inhibitor p27 by the Cullin Gene Family in Normal Tumorigenic Human Breast Cells

摘要

Ubiquitin-mediated proteolysis of cyclins, cyclin-dependent kinase inhibitors and certain other substrates, including proteins whose degradation is necessary for progression through mitosis, provide a molecular basis for unidirectional progression through the eukaryotic cell cycle. Aberrations in the ubiquitin-mediated destruction pathways, and particularly in the destruction of the Dl cyclin, have been implicated in human breast cancer. A cascade of enzymes leads to ubiquitination of substrate: El enzyme (ubiquitin activating); E2 enzyme (ubiquitin conjugating), and E3 enzyme (ubiquitin ligase). This last class of enzyme is large, and it performs the functions of targeting substrate and catalyzing formation of the polvubiquitin chains that mark substrate for destruction. We have found that all three members of the yeast cullin family are capable of performing the ubiquitination activity of E3 enzyme. All three yeast cullins undergo covalent modification with Rub1 protein, presumably to enhance ubiqutination activity. Cul-C protein contains sufficient homology to human cullin-l to identify the Rod-binding domain of the yeast protein and its site of modification with Rubl. Mutation of these regions in Cul-C produces an aberrant phenotype in yeast cells. Understanding the ubiquitination pathways in yeast is expected to illuminate the more complicated mammalian pathways.