Cyclin E-CDK2 is a regulator of p27Kip1.

Abstract

CDK inhibitors are thought to prevent cell proliferation by negatively regulating cyclin-CDK complexes. We propose that the opposite is also true, that cyclin-CDK complexes in mammmalian cells can promote cell cycle progression by directly down-regulating CDK inhibitors. We show that expression of cyclin E-CDK2 in murine fibroblasts causes phosphorylation of the CDK inhibitor p27 Kip1 on T187, and that cyclin E-CDK2 can directly phosphorylate p27 T187 in vitro. We further show that cyclin E-CDK2-dependent phosphorylation of p27 results in elimination of p27 from the cell, allowing cells to transit from G1 to S phase. Moreover, mutation of T187 in p27 to alanine creates a p27 protein that causes a G1 block resistant to cyclin E and whose level of expression is not modulated by cyclin E. A kinetic analysis of the interaction between p27 and cyclin E-CDK2 explains how p27 can be regulated by the same enzyme it targets for inhibition. We show that p27 interacts with cyclin E-CDK2 in at least two distinct ways: one resulting in p27 phosphorylation and release, the other in tight binding and cyclin E-CDK2 inhibition. The binding of ATP to the CDK governs which state predominates. At low ATP (< 50 ~M) p27 is primarily a CDK inhibitor, but at ATP concentrations approaching physiological levels (> 1 mM) p27 is more likely to be a substrate. Thus, we have identified p27 as a biologically relevant cyclin E-CDK2 substrate, demonstrated the physiological consequences of p27 phosphorylation, and developed a kinetic model to explain how p27 can be both an inhibitor and a substrate of cyclin E-CDK2.

Limiting amounts of p27Kip1 correlates with constitutive activation of cyclin E-CDK2 complex in HTLV-I-transformed T-cells

“…Northern blot analysis of the p27 Kip1 transcripts revealed no signi®cant di erences in all cell lines regardless of their IL-2 status (Figure 1b and c). Thus, the maintenance of a low steady state level of p27 Kip1 in HTLV-I-transformed (IL-2 independent) cells appears to be regulated at a post-transcriptional level, consistent with the notion that p27 Kip1 cellular level is regulated by phosphorylation and ubiquitination (Pagano et al, 1995;Shea et al, 1997;Vlach et al, 1997).…”

JunB is a gatekeeper for B-lymphoid leukemia

“…Both proteins are regulated at a post-transcriptional level. Among the kinases that regulate p27 degradation is cdk2 itself (Sheaff et al, 1997). Phosphorylation of p27 by cdk2 induces ubiquitylation of the protein, and this may partially account for the reduced p27 protein levels in junB D/D cells that indeed show elevated cdk2 kinase activity.…”

Evidence for a p23 caspase-cleaved form of p27[KIP1] involved in G1 growth arrest

“…Nevertheless, this hypothesis is consistent with the results of U È ren et al (1997) which have shown that N-terminal and C-terminal domains of p27 can be endowed of opposite e ects. Recent works (Morisaki et al, 1997;Shea et al, 1997;Vlach et al, 1997) have demonstrated that the phosphorylation of the Threonine 187 by cyclin E/CDK2 triggers the degradation of p27 following the ubiquitin/proteasome pathway. Mutagenesis experiments have shown that the mutation of this residue into Alanine or Valine prevents p27 degradation.…”