Phosphorylation and activation of B-Myb by cyclin A-Cdk2.
BACKGROUND
Cyclins and their catalytic partners, the cyclin-dependent kinases (Cdks), function as key regulators of the eukaryotic cell cycle. Specific cyclin-Cdk complexes are active at successive stages during the cell cycle and control cell-cycle progression by phosphorylating specific target proteins, most of which have not yet been identified. B-Myb, a conserved member of the Myb oncoprotein family, is a sequence-specific DNA-binding protein expressed in virtually all proliferating mammalian cells. Increasing evidence suggests that B-Myb plays an important role during the late G1 and early S phases of the cell cycle. In this study, we have examined the regulation of B-Myb activity by cyclin-Cdks.


RESULTS
We found that the transcriptional transactivation potential of B-Myb was repressed by a regulatory domain located at the carboxyl terminus of the protein. Coexpression of B-Myb and cyclin A relieved this repression by phosphorylation of B-Myb in its carboxy-terminal region. Tryptic phosphopeptide mapping revealed that endogenous B-Myb was phosphorylated in cells undergoing S phase.


CONCLUSIONS
This work provides evidence for a link between the Myb oncoprotein family and the cell-cycle machinery. We have shown that the carboxyl terminus of B-Myb acts as a cell-cycle sensor that regulates the transactivation function of B-Myb. Moreover, our studies have identified B-Myb as a target of cyclin A-Cdk2 and have indicated that B-Myb activity is regulated by phosphorylation mediated by cyclin A-Cdk2.
Ziebold
U
U
Hans-Spemann-Laborataory, Max-Planck-Institute for Immunobiology, Stübeweg 51, D-79108, Fakultät für Biologie, Universität Freiburg, D-79104, Freiburg, Germany,
Bartsch
O
O
Marais
R
R
Ferrari
S
S
Klempnauer
K H
KH
D016475:3T3 Cells; D000818:Animals; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle; D018797:Cell Cycle Proteins; D002460:Cell Line; D003001:Cloning, Molecular; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000096:biosynthesis / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000378:metabolism*; D005656:Fungal Proteins / Q000096:biosynthesis; D016193:G1 Phase; D006801:Humans; D051379:Mice; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D016196:S Phase; D029701:Saccharomyces cerevisiae Proteins; D015534:Trans-Activators; D014157:Transcription Factors / Q000096:biosynthesis / Q000378:metabolism*; D015533:Transcriptional Activation
D018797:Cell Cycle Proteins; D016213:Cyclins; D004268:DNA-Binding Proteins; D005656:Fungal Proteins; C052130:GAL4 protein, S cerevisiae; C064352:MYBL2 protein, human; C499306:Mybl2 protein, mouse; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495893:Cdk2 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases