Connell-Crowley L, Harper JW, Goodrich DWCyclin D1/Cdk4 regulates retinoblastoma protein-mediated cell cycle arrest by site-specific phosphorylation. Mol Biol Cell 8:287-301

Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA.
Molecular Biology of the Cell (Impact Factor: 4.47). 03/1997; 8(2):287-301. DOI: 10.1091/mbc.8.2.287
Source: PubMed


The retinoblastoma protein (pRb) inhibits progression through the cell cycle. Although pRb is phosphorylated when G1 cyclin-dependent kinases (Cdks) are active, the mechanisms underlying pRb regulation are unknown. In vitro phosphorylation by cyclin D1/Cdk4 leads to inactivation of pRb in a microinjection-based in vivo cell cycle assay. In contrast, phosphorylation of pRb by Cdk2 or Cdk3 in complexes with A- or E-type cyclins is not sufficient to inactivate pRb function in this assay, despite extensive phosphorylation and conversion to a slowly migrating "hyperphosphorylated form." The differential effects of phosphorylation on pRb function coincide with modification of distinct sets of sites. Serine 795 is phosphorylated efficiently by Cdk4, even in the absence of an intact LXCXE motif in cyclin D, but not by Cdk2 or Cdk3. Mutation of serine 795 to alanine prevents pRb inactivation by Cdk4 phosphorylation in the microinjection assay. This study identifies a residue whose phosphorylation is critical for inactivation of pRb-mediated growth suppression, and it indicates that hyperphosphorylation and inactivation of pRb are not necessarily synonymous.

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Available from: David W Goodrich, Feb 04, 2014
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    • "During the G1-S phase transition, pRb is successively phosphorylated by cyclin D1-CDK4/6 and cyclin E/A-CDK2 complexes, resulting in E2F1 activation and the expression of E2F target genes that promote cell cycle progression [34]. Analysis of the phosphorylation status of pRb at serine 795, a site targeted by CDK2 and CDK4 [35], revealed that there was a sustained decrease in pRb phosphorylation in MYC-dependent cells after MYC knockdown. Accordingly, a significant decrease of E2F1 expression was observed after MYC siRNA treatment. "
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    • "Hypophosphorylated pRb on S780 then releases E2F from an inhibitory complex and enables it to promote the transcription necessary for progression into late G1-phase and S-phase [24]–[26]. It has been reported that Cyclin D1 and cyclin-depended-kinase 4 (CDK4) phosphorylated pRb and that pRb lost its ability to bind to E2F [27]. Thus, when Cyclin D1 is up-regulated by STAT3 it can phosphorylate pRb and promote cell growth by releasing E2F. "
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