Cyclin D activates the Rb tumor suppressor by mono-phosphorylation

Elife. 2014 Jun 4;3:e02872. doi: 10.7554/eLife.02872.


The widely accepted model of G1 cell cycle progression proposes that cyclin D:Cdk4/6 inactivates the Rb tumor suppressor during early G1 phase by progressive multi-phosphorylation, termed hypo-phosphorylation, to release E2F transcription factors. However, this model remains unproven biochemically and the biologically active form(s) of Rb remains unknown. In this study, we find that Rb is exclusively mono-phosphorylated in early G1 phase by cyclin D:Cdk4/6. Mono-phosphorylated Rb is composed of 14 independent isoforms that are all targeted by the E1a oncoprotein, but show preferential E2F binding patterns. At the late G1 Restriction Point, cyclin E:Cdk2 inactivates Rb by quantum hyper-phosphorylation. Cells undergoing a DNA damage response activate cyclin D:Cdk4/6 to generate mono-phosphorylated Rb that regulates global transcription, whereas cells undergoing differentiation utilize un-phosphorylated Rb. These observations fundamentally change our understanding of G1 cell cycle progression and show that mono-phosphorylated Rb, generated by cyclin D:Cdk4/6, is the only Rb isoform in early G1 phase.

Keywords: G1 phase; Rb tumor suppressor; biochemistry; cell biology; cell cycle; cyclin D:Cdk4; human; mono-phosphorylated Rb; p16.

Publication types

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

MeSH terms

  • Animals
  • Cell Cycle
  • Cell Differentiation
  • Cell Line, Tumor
  • Cyclin D1 / metabolism*
  • DNA Damage
  • Fibroblasts / metabolism
  • Humans
  • Isoelectric Focusing
  • Mice
  • Mice, Knockout
  • Phosphorylation
  • Protein Isoforms
  • Retinoblastoma Protein / metabolism*


  • CCND1 protein, human
  • Protein Isoforms
  • Retinoblastoma Protein
  • Cyclin D1

Associated data

  • GEO/GSE56453