B-Myb function can be markedly enhanced by cyclin A-dependent kinase and protein truncation.
Transcription of the B-Myb gene is cell cycle regulated by an E2F-dependent mechanism and its product, B-Myb, is itself a transcription factor required for S-phase entry. Previously, we have shown that B-Myb is specifically phosphorylated during S-phase and that similar modification to a less electrophoretically mobile form could be induced by baculovirus-expressed cyclin A/Cdk2 kinase. We report here that cyclin A-mediated phosphorylation of B-Myb is associated with a marked increase in transactivation function in U-2 OS cells. In contrast to previous studies, transactivation of the luciferase reporter was dependent upon Myb binding sites located upstream of the promoter. Enhancement of B-Myb activation function was also obtained by truncation of the C-terminus just downstream of a domain conserved in evolution. Potentiation of B-Myb activity by phosphorylation was not simply a consequence of overcoming the negative effect of the C-terminus, however, as the truncated protein was to a lesser extent also activated by cyclin A/Cdk2. Whereas wild-type B-Myb transactivation activity could not be potentiated by cyclin A/Cdk2 in NIH3T3 cells, the truncated protein was hyperactive. Finally, we showed that B-Myb synergises with cyclin A to promote U-2 OS cells into S-phase.
Lane
S
S
Ludwig Institute for Cancer Research, Imperial College School of Medicine at St Mary's, Norfolk Place, London, UK.
Farlie
P
P
Watson
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R
D016475:3T3 Cells / Q000473:pathology; D000818:Animals; D002453:Cell Cycle; D018797:Cell Cycle Proteins; D017124:Conserved Sequence; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D051379:Mice; D010766:Phosphorylation; D011494:Protein Kinases / Q000502:physiology*; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation / Q000235:genetics*; D014162:Transfection; D014407:Tumor Cells, Cultured / Q000473:pathology
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; C064352:MYBL2 protein, human; C499306:Mybl2 protein, mouse; D015534:Trans-Activators; D014157:Transcription Factors; D011494:Protein Kinases; C102333:histone H1 kinase