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A Cdk4/6-dependent phosphorylation gradient regulates the early to late G1 phase transition.

Scientific reports (2021-07-21)
Manuel Kaulich, Verena M Link, John D Lapek, Yeon J Lee, Christopher K Glass, David J Gonzalez, Steven F Dowdy
ABSTRACT

During early G1 phase, Rb is exclusively mono-phosphorylated by cyclin D:Cdk4/6, generating 14 different isoforms with specific binding patterns to E2Fs and other cellular protein targets. While mono-phosphorylated Rb is dispensable for early G1 phase progression, interfering with cyclin D:Cdk4/6 kinase activity prevents G1 phase progression, questioning the role of cyclin D:Cdk4/6 in Rb inactivation. To dissect the molecular functions of cyclin D:Cdk4/6 during cell cycle entry, we generated a single cell reporter for Cdk2 activation, RB inactivation and cell cycle entry by CRISPR/Cas9 tagging endogenous p27 with mCherry. Through single cell tracing of Cdk4i cells, we identified a time-sensitive early G1 phase specific Cdk4/6-dependent phosphorylation gradient that regulates cell cycle entry timing and resides between serum-sensing and cyclin E:Cdk2 activation. To reveal the substrate identity of the Cdk4/6 phosphorylation gradient, we performed whole proteomic and phospho-proteomic mass spectrometry, and identified 147 proteins and 82 phospho-peptides that significantly changed due to Cdk4 inhibition in early G1 phase. In summary, we identified novel (non-Rb) cyclin D:Cdk4/6 substrates that connects early G1 phase functions with cyclin E:Cdk2 activation and Rb inactivation by hyper-phosphorylation.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Protease Inhibitor Cocktail, for use with mammalian cell and tissue extracts, DMSO solution
Sigma-Aldrich
Phosphatase Inhibitor Cocktail 2, aqueous solution (dark coloration may develop upon storage, which does not affect the activity)