FIG. 7.

The p38-binding domain of MEF2A directs the phosphorylation of the key phosphoacceptor motifs in MEF2A–Elk-1 chimeras. (A) In vitro kinase assays of GST fusion proteins (5 pmol of each substrate) by p38β₂ MAP kinase were carried out for 15 min. Phosphorylation of Ser383 was detected by Western blotting with an anti-phospho–Elk-1(Ser383) antibody. (B) In vivo phosphorylation of MEF2A–Elk-1 chimeras. 293 cells were transfected with vectors encoding GAL–Elk-1 or GAL–MEF2A–Elk-1 and a GAL4-driven luciferase reporter plasmid and, where indicated, p38β₂ and MKK6(E). The phosphorylation of the Elk-1 moiety at Ser383 in cell extracts was detected by Western blotting as in panel A (top panel), and the total levels of each GAL4 fusion protein were detected with an anti-GAL4 antibody (bottom panel). The locations of the bands corresponding to nonphosphorylated and phosphorylated GAL4 fusion proteins are indicated.