MEF2 PROTEIN EXPRESSION, DNA-BINDING SPECIFICITY AND COMPLEX COMPOSITION, AND TRANSCRIPTIONAL ACTIVITY IN MUSCLE AND NONMUSCLE CELLS

Tissue-specific gene expression can be mediated by complex transcripti onal regulatory mechanisms. Based on the dichotomy of the ubiquitous d istribution of the myocyte enhancer factor 2 (MEF2) gene mRNAs compare d to their cell type-restricted activity, we investigated the basis fo r their tissue specificity. Electrophoretic mobility shift assays usin g the muscle creatine kinase MEF2 DNA binding site as a probe showed t hat HeLa, Schneider, L6E9 muscle, and C2C12 muscle cells have a functi onal MEF2 binding activity that is indistinguishable based on competit ion analysis. Interestingly, chloramphenicol acetyltransferase reporte r assays showed MEF2 site-dependent trans-activation in myogenic C2C12 cells but no trans-activation by the endogenous MEF2 proteins in HeLa cells. By immunofluorescence, we detected abundant nuclear localized MEF2A and MEF2D protein expression in HeLa cells and C2C12 muscle cell s. Using immuno-gel shift analysis and also co-immunoprecipitation stu dies, we show that the predominant MEF2 DNA binding complex bound to M EF2 sites from either the muscle creatine kinase or c-jun regulatory r egions in C2C12 muscle cells is comprised of a MEF2A homodimer, wherea s in HeLa cells, it is a MEF2A:MEF2D heterodimer. Thus, the presence o f MEF2 DNA binding complexes is not necessarily coupled with trans-act ivation of target genes. The ability of the MEF2 proteins to activate transcription in vivo correlates with the specific dimer composition o f the DNA binding complex and the cellular context.