PHOSPHORYLATION OF THE MADS-BOX TRANSCRIPTION FACTOR MEF2C ENHANCES ITS DNA-BINDING ACTIVITY

Members of the myocyte enhancer factor-2 (MEF2) family of transcriptio n factors activate muscle gene expression by binding an A/T-rich DNA s equence in the control regions of muscle-specific genes. There are fou r MEF2 factors in vertebrates, MEF2A-D, which share homology in an ami no-terminal MADS domain and an adjacent region known as the MEF2 domai n, that together mediate DNA binding and dimerization. We show that se rine 59 located between the MADS and MEF2 domains of MEF2C is phosphor ylated in vivo and can be phosphorylated in vitro by casein kinase-II (CKII). Phosphorylation of this site enhanced the DNA binding and tran scriptional activity of MEF2C by increasing its DNA binding activity 5 -fold. In vivo P-32 labeling experiments showed that serine 59 is the only phosphorylation site in the MADS and MEF2 domains, Mutagenesis of this serine to an aspartic acid resulted in an increase in DNA bindin g and transcriptional activity of MEF2 comparable to that observed whe n this site was phosphorylated, suggesting that phosphorylation augmen ts DNA binding activity by introducing negative charge, This phosphory lation site, which corresponds to a CKII recognition site, is conserve d in all known MEF2 factors in organisms ranging from flies to humans, consistent with its importance for the functions of MEF2C.