Post-translational control of the MEF2A transcriptional regulatory protein

Myocyte enhancer factor 2 (MEF2) transcriptional regulatory proteins are key regulators of muscle-specific gene expression and also play a general role in the cellular response to growth factors, cytokines and environmental stressors, To identify signaling pathway components that might mediate these events, the potential role of MAP kinase and PKC signaling in the modulation of MEF2A phosphorylation and transcriptional activity were therefore studied. In transient transfection reporter assays, activated p38 MAP kinase potently increased MEF2A trans-activating potential, PKC delta and E isotypes enhanced MEF2A transactivation to a lesser extent, while the ERK1/2 and JNK/SAPK pathways were without effect. A GAL4-based assay system showed that p38 MAP kinase and PKC delta target the MEF2A transactivation domain. We also observed an increase in p38 MAP kinase activity in congruence with the increase in MEF2A expression in differentiating primary muscle cells. COS cells overexpressing MEF2A alone or with one of the kinases were metabolically labeled with [P-32]orthophosphate and MEF2A was immunoprecipitated using specific anti-MEF2A antibodies, MEF2A from cells co-transfected with activated p38 MAP kinase showed a decreased electrophoretic mobility due to phosphorylation. Subsequent phosphopeptide mapping and phosphoamino acid analysis indicated the appearance of several phoshopeptides due to p38 MAP kinase activation of MEF2A which were due to phosphorylation on serine and threonine residues. These studies position MEF2A as a nuclear target for the p38 MAP kinase signaling pathway.