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

Abstract

Abstract : Sequential activation and inactivation of cyclin dependent kinases (CDK) regulate eukaryotic cell cycle transitions. The periodicity of CDK activity provides a molecular basis for unidirectional cell cycle progression and is partly controlled by ubiquitin-mediated proteolysis of both cyclins and CDK inhibitors. Deregulated expression of G1 CDK inhibitors and G1 cyclins have been directly linked to breast cancer development. The mechanisms regulating the ubiquitination of these two protein families constitute essential components of G1 cell cycle control in mammalian cells, but they are poorly understood at present. Polyubiquitination of proteins is known to be catalyzed by a cascade of enzymes: El (ubiquitin activating), E2 (ubiquitin conjugating), and E3 (ubiquitin ligating). Two issues critical to our understanding of the regulation of protein turnover are how E3 ligases target proteins for ubiquitination and how they themselves are regulated. We have found that cullins partner with another multigene family of evolutionarily conserved proteins, the ROC/APC11 family, to form a complex that contains E3 ubiquitin ligase activity. We are currently investigating whether these complexes can ubiquitinate G1 cyclins and CDK inhibitors and how they are targeted to the ROC/cullin ligase. Understanding these mechanisms could potentially lead to improved diagnostic, prognostic and therapeutic strategies.