title: Direct inhibition of G(1) cdk kinase activity by MyoD promotes myoblast cell cycle withdrawal and terminal differentiation.
abstract: MyoD has been proposed to facilitate terminal myoblast differentiation by binding to and inhibiting phosphorylation of the retinoblastoma protein (pRb). Here we show that MyoD can interact with cyclin-dependent kinase 4 (cdk4) through a conserved 15 amino acid (aa) domain in the C-terminus of MyoD. MyoD, its C-terminus lacking the basic helix-loop-helix (bHLH) domain, or the 15 aa cdk4-binding domain all inhibit the cdk4-dependent phosphorylation of pRb in vitro. Cellular expression of full-length MyoD or fusion proteins containing either the C-terminus or just the 15 aa cdk4-binding domain of MyoD inhibit cell growth and pRb phosphorylation in vivo. The minimal cdk4-binding domain of MyoD fused to GFP can also induce differentiation of C2C12 muscle cells in growth medium. The defective myogenic phenotype in MyoD-negative BC3H1 cells can be rescued completely only when MyoD contains the cdk4-binding domain. We propose that a regulatory checkpoint in the terminal cell cycle arrest of the myoblast during differentiation involves the modulation of the cyclin D cdk-dependent phosphorylation of pRb through the opposing effects of cyclin D1 and MyoD.
authors.affiliation: Laboratory of Biochemistry, NCI, National Institutes of Health, Building 37 Room 4A21, 9000 Rockville Pike, Bethesda, MD 20892, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Zhang
authors.forename: X
authors.initials: X
authors.lastname: Zhao
authors.forename: Q
authors.initials: Q
authors.lastname: Wei
authors.forename: B M
authors.initials: BM
authors.lastname: Paterson
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002453:Cell Cycle / Q000502:physiology*; D002460:Cell Line; D002467:Cell Nucleus / Q000502:physiology; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D016193:G1 Phase; D018257:Helix-Loop-Helix Motifs; D006801:Humans; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology; D017570:MyoD Protein / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014158:Transcription, Genetic; D014162:Transfection
chemical_list: D017570:MyoD Protein; D010446:Peptide Fragments; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D016160:Retinoblastoma Protein; C495900:CDK4 protein, human; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases