Members of the MEF2 family of transcription factors bind as homo-and heterodimers to the MEF2 site found in the promoter regions of numerous muscle-specific, growth-or stress-induced genes. We showed previously that the transactivation activity of MEF2C is stimulated by p38 mitogen-activated protein (MAP) kinase. In this study, we examined the potential role of the p38 MAP kinase pathway in regulating the other MEF2 family members. We found that MEF2A, but not MEF2B or MEF2D, is a substrate for p38. Among the four p38 group members, p38 is the most potent kinase for MEF2A. Threonines 312 and 319 within the transcription activation domain of MEF2A are the regulatory sites phosphorylated by p38. Phosphorylation of MEF2A in a MEF2A-MEF2D heterodimer enhances MEF2-dependent gene expression. These results demonstrate that the MAP kinase signaling pathway can discriminate between different MEF2 isoforms and can regulate MEF2-dependent genes through posttranslational activation of preexisting MEF2 protein.The transactivation activity of many transcription factors is regulated by phosphorylation (2). The mitogen-activated protein (MAP) kinase family of serine/threonine kinases has been shown to play important roles in regulating gene expression via transcription factor phosphorylation (5,10,16,38,40,42). Unique structural features, specific activation pathways, and different substrate specificities provide evidence to support the contention that different MAP kinases are independently regulated and control different cellular responses to extracellular stimuli (7,38,40,44).p38 MAP kinase was first identified in studies designed to explore how bacterial endotoxin induces cytokine expression (11,13,23). Following the initial description of p38 (p38␣), three additional isoforms of this MAP kinase group have been cloned and characterized: p38 (18), p38␥ (also termed ERK6 or SAPK3) (22,24,30), and p38␦ (also termed SAPK4) (4, 17, 41). p38␣ and p38 are sensitive to pyridinyl imidazole derivatives, whereas p38␥ and p38␦ are not (4). In mammalian cells, these closely related p38 isoforms are activated coordinately by a broad panel of stimuli which include physical-chemical stresses and proinflammatory cytokines (17, 36). Two MAP kinase kinases (MKK), MKK3 and MKK6, are the upstream activators of the p38 group MAP kinases (6,12,14,37). Several proteins including transcription factors such as CHOP 10 (GADD153) (42), Sap1 (16), MEF2C (10), enzymes such as cPLA2 (20), and the protein kinases MAPKAPK2/3 (27, 29, 39), MNK1/2 (8, 45), and p38-regulated/activated protein kinase (33) have been shown by us and others to be substrates of p38.We showed that MEF2C, a member of the MEF2 family of transcription factors, is phosphorylated by p38 and that this event regulates the transactivation activity of MEF2C (10). Our studies showed that p38 specifically phosphorylates serine 387 and threonines 293 and 300 within the MEF2C transactivation domain (10). MEF2C phosphorylation by p38 was shown to play an important role in regulation of c-Jun ...
