title: Cyclin D activates the Rb tumor suppressor by mono-phosphorylation.
abstract: 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.
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego School of Medicine, La Jolla, United States.
authors.forename: Anil M
authors.initials: AM
authors.lastname: Narasimha
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego School of Medicine, La Jolla, United States.
authors.forename: Manuel
authors.initials: M
authors.lastname: Kaulich
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego School of Medicine, La Jolla, United States.
authors.forename: Gary S
authors.initials: GS
authors.lastname: Shapiro
authors.affiliation: Department of Genetics, Harvard Medical School, Boston, United States.
authors.forename: Yoon J
authors.initials: YJ
authors.lastname: Choi
authors.affiliation: Department of Genetics, Harvard Medical School, Boston, United States.
authors.forename: Piotr
authors.initials: P
authors.lastname: Sicinski
authors.affiliation: Department of Cellular and Molecular Medicine, University of California, San Diego School of Medicine, La Jolla, United States.
authors.forename: Steven F
authors.initials: SF
authors.lastname: Dowdy
mesh_terms: D000818:Animals; D002453:Cell Cycle; D002454:Cell Differentiation; D045744:Cell Line, Tumor; D019938:Cyclin D1 / Q000378:metabolism*; D004249:DNA Damage; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D007525:Isoelectric Focusing; D051379:Mice; D018345:Mice, Knockout; D010766:Phosphorylation; D020033:Protein Isoforms; D016160:Retinoblastoma Protein / Q000378:metabolism*
chemical_list: C530104:CCND1 protein, human; D020033:Protein Isoforms; D016160:Retinoblastoma Protein; D019938:Cyclin D1
keywords: G1 phase; Rb tumor suppressor; biochemistry; cell biology; cell cycle; cyclin D:Cdk4; human; mono-phosphorylated Rb; p16