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Defining the substrate specificity of cdk4 kinase-cyclin D1 complex

R H Grafstrom1, W Pan, R H Hoess

  • 1Genetics and Cancer Group, Dupont Pharmaceutical Co., Wilmington, DE 19880-0336, USA. robert.h.grafstrom@dupontpharma.com

Carcinogenesis
|March 9, 1999
PubMed

Insights

The cyclin-dependent kinase 4 (CDK4) / cyclin D1 complex preferentially phosphorylates retinoblastoma protein (Rb) at Ser795. Specific amino acid sequences and structural context dictate this phosphorylation site preference.

Area of Science:

  • Molecular Biology
  • Cell Cycle Regulation
  • Protein Phosphorylation

Background:

  • The cyclin-dependent kinase 4 (CDK4) / cyclin D1 complex is a key regulator of cell cycle progression.
  • Retinoblastoma protein (Rb) is a critical tumor suppressor that controls cell cycle entry.
  • Rb is regulated by phosphorylation, with specific sites influencing its function.

Purpose of the Study:

  • To identify the preferred phosphorylation site of the CDK4/cyclin D1 complex on Rb.
  • To determine the sequence and structural determinants of this phosphorylation specificity.
  • To investigate the functional conservation of phosphorylation sites in related proteins.

Main Methods:

  • Site-directed mutagenesis of Rb phosphorylation sites.
  • In vitro kinase assays using CDK4/cyclin D1 complex.
  • Comparison of phosphorylation rates at different Rb and p107 sites.

Main Results:

  • Serine 795 (Ser795) in the Rb C-terminal domain is the preferred phosphorylation site for CDK4/D1.
  • Arginine at the third position C-terminal to the target serine is crucial for efficient phosphorylation.
  • Mutating less favorable sites in Rb to mimic Ser795 enhances CDK4/D1 phosphorylation.
  • The Rb Ser795 site can substitute for the p107 Ser842 site without altering phosphorylation rates.

Conclusions:

  • Phosphorylation site specificity by CDK4/D1 is determined by both surrounding amino acid sequences and the structural context of the site.
  • Understanding these determinants allows for the engineering of specific phosphorylation sites.
  • Conserved phosphorylation mechanisms exist between Rb and related proteins like p107.

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