Ubiquitin/proteasome-mediated degradation of p19(INK4d) determines its periodic expression during the cell cycle

Assembly and activity of the proto-oncogenic cyclin D/ CDK4(6) complexes, t he major driving force of G1 phase progression, is negatively regulated by a family of INK4 CDK inhibitors p16(INK4a), p15(INK4b), p18(INK4c), and p19 (INK4d). Expression of the INK4 family members is controlled at the transcr iptional level, through differential response to environmental and intracel lular signals such as cytokines, oncogenic overload, or cellular senescence . Here we show that the periodic oscillation of the p19(INK4d) protein duri ng the cell cycle is determined by the ubiquitin/proteasome-dependent mecha nism, allowing the protein abundance to follow the changes in its mRNA expr ession. Within the INK4 family, this regulatory mode appears restricted to p19(INK4d) whose ubiquitination was dependent on the integrity of lysine 62 , and binding to CDK4. These results highlight unexpected differences among the INK4 inhibitors, and suggest how p19(INK4d) may help regulate the rate of cyclin D/CDK4(6) complex formation, and thereby timely progression thro ugh the mammalian cell division cycle.