Mitogen-activated protein (MAP) kinase-mediated signalling to the nucleus i
s an important event in the conversion of extracellular signals into a cell
ular response. However, the existence of multiple MAP kinases which phospho
rylate similar phosphoacceptor motifs poses a problem in maintaining substr
ate specificity and hence the correct biological response. Both the extrace
llular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) su
bfamilies of MAP kinases use a second specificity determinant and require d
ocking; to their transcription factor substrates to achieve maximal substra
te activation. In this study, we demonstrate that among the different MAP k
inases, the MADS-box transcription factors MEF2A and MEF2C are preferential
ly phosphorylated and activated by the p38 subfamily members p38 alpha and
p38 beta(2). The efficiency of phosphorylation in vitro and transcriptional
activation in vivo of MEF2A. and MEF2C by these p38 subtypes requires the
presence of a kinase docking domain (D-domain), Furthermore, the D-domain f
rom MEF2A is sufficient to confer p38 responsiveness on different transcrip
tion factors, and reciprocal effects are observed upon the introduction of
alternative D-domains into MEF2A, These results therefore contribute to our
understanding of signalling to MEF2 transcription factors and demonstrate
that the requirement far substrate binding by MAP kinases is an important f
acet of three different subclasses of MAP kinases (ERK, JNK, and p38).