The activity and stability of the intrinsically disordered Cip/Kip protein family are regulated by non-receptor tyrosine kinases

J Mol Biol. 2015 Jan 30;427(2):371-386. doi: 10.1016/j.jmb.2014.11.011. Epub 2014 Nov 20.


The Cip/Kip family of cyclin-dependent kinase (Cdk) inhibitors includes p21(Cip1), p27(Kip1) and p57(Kip2). Their kinase inhibitory activities are mediated by a homologous N-terminal kinase inhibitory domain. The Cdk inhibitory activity and stability of p27 have been shown to be regulated by a two-step phosphorylation mechanism involving a tyrosine residue within the kinase inhibitory domain and a threonine residue within the flexible C-terminus. We show that these residues are conserved in p21 and p57, suggesting that a similar phosphorylation cascade regulates these Cdk inhibitors. However, the presence of a cyclin binding motif within its C-terminus alters the regulatory interplay between p21 and Cdk2/cyclin A, as well as its responses to tyrosine phosphorylation and altered p21:Cdk2/cyclin A stoichiometry. We also show that the Cip/Kip proteins can be phosphorylated in vitro by representatives of many non-receptor tyrosine kinase (NRTK) sub-families, suggesting that NRTKs may generally regulate the activity and stability of these Cdk inhibitors. Our results further suggest that the Cip/Kip proteins integrate signals from various NRTK pathways and cell cycle regulation.

Keywords: cell cycle; cyclin-dependent kinase; intrinsically disordered protein; non-receptor tyrosine kinase; protein phosphorylation.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Binding Sites / genetics
  • Calcium-Binding Proteins / genetics
  • Calcium-Binding Proteins / metabolism*
  • Cell Cycle
  • Cyclin-Dependent Kinase 2 / genetics
  • Cyclin-Dependent Kinase 2 / metabolism
  • Cyclin-Dependent Kinase Inhibitor p21 / genetics
  • Cyclin-Dependent Kinase Inhibitor p21 / metabolism*
  • Humans
  • Magnetic Resonance Spectroscopy
  • Molecular Sequence Data
  • Phosphorylation
  • Protein Conformation
  • Receptor Protein-Tyrosine Kinases / genetics
  • Receptor Protein-Tyrosine Kinases / metabolism*
  • Signal Transduction


  • CDKN1A protein, human
  • CIB1 protein, human
  • Calcium-Binding Proteins
  • Cyclin-Dependent Kinase Inhibitor p21
  • Receptor Protein-Tyrosine Kinases
  • CDK2 protein, human
  • Cyclin-Dependent Kinase 2