MODULATION OF C-MYB-INDUCED TRANSCRIPTION ACTIVATION BY A PHOSPHORYLATION SITE NEAR THE NEGATIVE REGULATORY DOMAIN

The c-myb protooncogene encodes a highly conserved transcription factor that functions as both an activator and a repressor of transcription. The v-myb oncogenes of E26 leukemia virus and avian myeloblastosis virus encode proteins that are truncated at both the amino and the carboxyl terminus, deleting portions of the c-Myb DNA-binding and negative regulatory domains, This has led to speculation that the deleted regions contain important regulatory sequences, We previously reported that the 42-kDa mitogen-activated protein kinase (p42(mapk)) phosphorylates chicken and murine c-Myb at multiple sites in the negative regulatory domain in vitro, suggesting that phosphorylation might provide a mechanism to regulate c-Myb function, We now report that three tryptic phosphopeptides derived from in vitro phosphorylated c-Myb comigrate with three tryptic phosphopeptides derived from metabolically labeled c-Myb immunoprecipitated from murine erythroleukemia cells. At least two of these peptides are phosphorylated on serine-528, Replacement of serine-528 with alanine results in a 2- to 7-fold increase in the ability of c-Myb to transactivate a Myb-responsive promoter/reporter gene construct. These findings suggest that phosphorylation serves to regulate c-Myb activity and that loss of this phosphorylation site from the v-Myb proteins may contribute to their transforming potential.