Phosphorylation and alternative pre-mRNA splicing converge to regulate myocyte enhancer factor 2C activity

Myocyte enhancer factor 2 (MEF2) transcription factors play pivotal roles in cardiac, muscle, and neuron gene expression. All products of MEF2 genes have a common amino-terminal DNA binding and dimerization domain, but the four vertebrate MEF2 gene transcripts are alternatively spliced among coding exons to produce splicing isoforms. In MEF2C alone, alternative splice acceptors in the last exon give forms that include or exclude a short domain that we designate gamma. We show that MEF2C is expressed exclusively as gamma- isoforms in heart tissue and predominantly as gamma- in other adult tissues and in differentiating myocytes. MEF2C gamma- isoforms are much more robust than gamma+ forms in activating MEF2-responsive reporters in transfected fibroblasts despite indistinguishable expression levels, and they better synergize with MyoD in promoting myogenic conversion. One-hybrid transcription assays using Gal4-MEF2C fusions give similar distinctions between gamma- and gamma+ isoforms in all cell types tested, including myocytes. Cis effects of gamma on MEF2C DNA binding, dimerization, protein stability, or response to CaM or p38 mitogen-activated protein kinase signaling are not apparent, and the isolated gamma domain represses transcription when fused to Gal4. One phosphoserine residue is present within the gamma domain according to tandem mass spectrometry, and mutation of this residue abolishes gamma-mediated transrepression. A similar activity is present in the constitutive gamma domain and serine phosphoacceptor of MEF2A. Our findings indicate that gamma functions autonomously as a phosphoserine-dependent transrepressor to downregulate transactivation function of MEF2 factors and that alternative splicing and serine phosphorylation converge to provide complex combinatorial control of MEF2C activity.