Cyclin A1 directly interacts with B-myb and cyclin A1/cdk2 phosphorylate B-myb at functionally important serine and threonine residues:: tissue-specific regulation of B-myb function

Cyclin A1 is tissue-specifically expressed during spermatogenesis, but it is also highly expressed in acute myeloid leukemia (AML), Its pathogenetic role in AML and in the cell cycle of leukemic blasts is unknown. B-myb is essential for G1/S transition and has been shown to be phosphorylated by the cyclin A2/cdk2 complex. Here it is demonstrated that cyclin A1 interacts with the C-terminal portion of B-myb as shown by glutathione S-transferase (GST) precipitation. This interaction is confined to cyclin A1 because binding could not be detected between cyclin A2 and B-myb, Also, cdk2 was not pulled down by GST-8-myb from U937 lysates, In addition, co-immunoprecipitation of cyclin Al and B-myb in leukemic cells evidenced protein interaction in vivo. Baculovirus-expressed cyclin All cdk2 complexes were able to phosphorylate human as well as murine B-myb in vitro. Tryptic phosphopeptide mapping revealed that cyclin A1/cdk2 complexes phosphorylated the C-terminal part of B-myb at several sites including threonine 447, 490, and 497 and serine 581, These phosphorylation sites have been demonstrated to be important for the enhancement of B-myb transcriptional activity. Further studies showed that cyclin A1 cooperated with B-myb to transactivate myb binding site containing promoters including the promoter of the human cyclin A1 gene. Taken together, the data suggest that cyclin A1 is a tissue-specific regulator of B-myb function and activates B-myb in leukemic blasts.