Transcriptional activator which binds specifically to the MEF2 element, 5'-YTA[AT](4)TAR-3', found in numerous muscle-specific, growth factor- and stress-induced genes. Mediates cellular functions not only in skeletal and cardiac muscle development, but also in neuronal differentiation and survival. Plays diverse roles in the control of cell growth, survival and apoptosis via p38 MAPK signaling in muscle-specific and/or growth factor-related transcription. Plays a critical role in the regulation of neuronal apoptosis. Forms a complex with class II HDACs in undifferentiating cells. On myogenic differentiation, HDACs are released into the cytoplasm allowing MEF2s to interact with other proteins for activation. Interacts with HDAC4 (in undifferentiating cells); the interaction translocates MEF2D to nuclear dots. Forms a heterodimer with MEF2A (By similarity). Interacts with MAPK7; the interaction phosphorylates but does not activate MEF2D (By similarity). Interacts with MYOG. Interacts with CCAR2 and HDAC3 (By similarity). Translocated by HDAC4 to nuclear dots. Widely expressed though mainly restricted to skeletal and cardiac muscle, brain, neurons and lymphocytes. Differentially expressed depending on if isoforms contain the beta domain or not, with the total expression of the beta domain-lacking isoforms vastly exceding that of the beta domain-containing isoforms. Isoforms containing the beta domain are expressed primarily in skeletal and cardiac muscle and in brain. Also present in lung and testis. Splicing to include the beta domain is induced in differentiating myocytes. Isoforms lacking the beta domain are expressed less abundantly in skeletal muscle, brain and lymphocytes, and are uniquely found in ovary, liver, spleen and kidney. In embryos, the beta domain-containing and beta domain-lacking isoforms are equally expressed. Also expressed cerebellar granule neurons and other regions of the CNS. Highest levels in the olfactory bulb, cortex, hippocampus, thalamus and cerebellum. In the developing cerebellum, increasing levels after birth. The majority of this increase occurs around postnataL day 9 reaching a peak at postnatal day 15-18 which is maintained in adults. Phosphorylated on Ser-437 is which is required for Lys-432 sumoylation and inhibits transcriptional activity. Phosphorylation on this residue by CDK5 is dependent on p35 and calpains. Phosphorylated by PKA at Ser-121 and Ser-190 represses transcriptional activity in embryonic and postnatal skeletal muscle, and stabilizes protein levels. No in vitro phosphorylation by PKA on Thr-20. Phosphorylated and activated by CaMK4 (By similarity). Acetylated on Lys-432 by CREBBP. Acetylated by EP300. Deacetylated by SIRT1 and HDAC3 (By similarity). Sumoylated on Lys-432 with SUMO2 but not SUMO1; which inhibits transcriptional activity and myogenic activity. Desumoylated by SENP3 (By similarity). Proteolytically cleaved in cerebellar granule neurons by caspase 7 following neurotoxicity. Preferentially cleaves the CDK5-mediated hyperphosphorylated form which leads to neuron apoptosis and transcriptional inactivation. Belongs to the MEF2 family.