Phosphorylation of B-Myb regulates its transactivation potential and DNA binding.

The transcription factor B-Myb is a cell cycle-regulated phosphoprotein and a potent regulator of cell cycle progression. Previous studies demonstrated that B-Myb was phosphorylated at the onset of S phase, suggesting that it could be due to cyclin-dependent kinases. We identified 10 B-Myb phosphorylation sites by automated peptide radiosequencing of ...
tryptic phosphopeptides derived from in vivo (32)P-labeled B-Myb. Each B-Myb phosphorylation site contained a phosphoserine or phosphothreonine followed by a proline, suggesting that this phosphorylation is due to a proline-directed kinase. Cyclin A-Cdk2 and cyclin E-Cdk2 complexes each phosphorylated B-Myb in a cell-free system on the same sites as in intact cells. Furthermore, the ability of B-Myb to activate a reporter plasmid was enhanced by the cotransfection of cyclin A, whereas mutagenesis of the 10 identified phosphorylation sites from B-Myb blocked the effect of cyclin A coexpression. Additional analysis revealed that the effect of phosphorylation on B-Myb transactivation potential was enhanced by phosphorylation sites in its carboxyl-terminal half. One phosphorylation site (Ser(581)) appeared to negatively regulate DNA binding, as mutation of this site enhanced the ability of B-Myb to bind a Myb-binding sequence. These data suggest that B-Myb is a target for phosphorylation by cyclin-Cdk2 and that phosphorylation of B-Myb regulates its transcriptional activity.
Mesh Terms:
Amino Acid Sequence, Animals, Cell Cycle Proteins, Cell Line, Cell-Free System, DNA, DNA-Binding Proteins, Molecular Sequence Data, Oncogene Proteins, Phosphorylation, Protein Binding, Trans-Activators, Transcriptional Activation, Transfection
J. Biol. Chem.
Date: Dec. 17, 1999
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