Stress-induced Phosphorylation of Thr486 in c-Myb by p38 Mitogen-activated Protein Kinases Attenuates Conjugation of SUMO-2/3

c-Myb plays an essential role in regulation of properly balanced hematopoiesis through transcriptional regulation of genes directly controlling cellular processes such as proliferation, differentiation, and apoptosis. The transcriptional activity and protein levels of c-Myb are strictly controlled through post-translational modifications such as phosphorylation, acetylation, ubiquitination, and SUMOylation. Conjugation of small ubiquitin-like modifier (SUMO) proteins has been shown to suppress the transcriptional activity of c-Myb. SUMO-1 modifies c-Myb under physiological conditions, whereas SUMO-2/3 conjugation was reported in cells under stress. Because stress also activates several cellular protein kinases, we investigated whether phosphorylation of c-Myb changes in stressed cells and whether a mutual interplay exists between phosphorylation and SUMOylation of c-Myb. Here we show that several types of environmental stress induce a rapid change in c-Myb phosphorylation. Interestingly, the phosphorylation of Thr486, located in close proximity to SUMOylation site Lys499 of c-Myb, is detected preferentially in nonSUMOylated protein and has a negative effect on stress-induced SUMOylation of c-Myb. Stress-activated p38 MAPKs phosphorylate Thr486 in c-Myb, attenuate its SUMOylation, and increase its proteolytic turnover. Stressed cells expressing a phosphorylation-deficient T486A mutant demonstrate decreased expression of c-Myb target genes Bcl-2 and Bcl-xL and accelerated apoptosis because of increased SUMOylation of the mutant protein. These results suggest that phosphorylation-dependent modulation of c-Myb SUMOylation may be important for proper response of cells to stress. In summary, we have identified a novel regulatory interplay between phosphorylation and SUMOylation of c-Myb that regulates its activity in stressed cells. Background: Regulation of c-Myb transcriptional activity and protein level is essential for hematopoietic homeostasis. Results: p38 MAPKs phosphorylate c-Myb on Thr486, resulting in repression of SUMOylation in stressed cells. Conclusion: There is negative cross-talk between the p38 MAPK phosphorylation pathway and the SUMO-2/3 conjugation pathway that regulates c-Myb activity in stressed cells. Significance: The novel mechanism that modulates activity of c-Myb in stressed cells was identified.