Myocyte enhancer factor 2 (MEF2) transcriptional regulatory proteins are ke
y regulators of muscle-specific gene expression and also play a general rol
e in the cellular response to growth factors, cytokines and environmental s
tressors, To identify signaling pathway components that might mediate these
events, the potential role of MAP kinase and PKC signaling in the modulati
on of MEF2A phosphorylation and transcriptional activity were therefore stu
died. In transient transfection reporter assays, activated p38 MAP kinase p
otently increased MEF2A trans-activating potential, PKC delta and E isotype
s enhanced MEF2A transactivation to a lesser extent, while the ERK1/2 and J
NK/SAPK pathways were without effect. A GAL4-based assay system showed that
p38 MAP kinase and PKC delta target the MEF2A transactivation domain. We a
lso observed an increase in p38 MAP kinase activity in congruence with the
increase in MEF2A expression in differentiating primary muscle cells. COS c
ells overexpressing MEF2A alone or with one of the kinases were metabolical
ly labeled with [P-32]orthophosphate and MEF2A was immunoprecipitated using
specific anti-MEF2A antibodies, MEF2A from cells co-transfected with activ
ated p38 MAP kinase showed a decreased electrophoretic mobility due to phos
phorylation. Subsequent phosphopeptide mapping and phosphoamino acid analys
is indicated the appearance of several phoshopeptides due to p38 MAP kinase
activation of MEF2A which were due to phosphorylation on serine and threon
ine residues. These studies position MEF2A as a nuclear target for the p38
MAP kinase signaling pathway.