A novel p21Cip1 phosphorylation/degradation pathway playing a role in the ERK‐mediated cell proliferation signal

Abstract

p21Cip1 is an inhibitor of cell cycle progression that promotes G1 phase arrest by direct binding to cyclin‐dependent kinase and proliferating cell nuclear antigen. Here we demonstrate that mitogenic stimuli, such as epidermal growth factor treatment and oncogenic Ras transformation, induce p21Cip1 downregulation at the post‐translational level. We show that ERK2 interacts with and phosphorylates p21Cip1, promoting p21Cip1 nucleocytoplasmic translocation and ubiquitin‐dependent degradation. Phosphopeptide analysis of in vitro ERK2‐phosphorylated p21Cip1 revealed two phosphorylation sites, Thr57 and Ser130. Both the phosphorylation‐deficient T57A/S130A p21Cip1 mutant and the NES mutant, that are resistant to ERK2‐mediated downregulation, retain the ability to potently inhibit the G1/S transition. These results indicate that ERK2 activation transduces mitogenic signals, at least in part, by phosphorylation, translocation, and degradation of the cell cycle inhibitory protein, p21Cip1, suggesting an alternative mechanism of Ras‐ERK signaling‐mediated cell proliferation. This work was supported by grants M10503010002, M10601000164, and M10642040003.