TI - Smad proteins function as co-modulators for MEF2 transcriptional regulatory proteins . AB - An emerging theme in transforming growth factor-beta ( TGF-beta ) signalling is the association of the Smad proteins with diverse groups of transcriptional regulatory proteins . Several Smad cofactors have been identified to date but the diversity of TGF-beta effects on gene transcription suggests that interactions with other co-regulators must occur . In these studies we addressed the possible interaction of Smad proteins with the myocyte enhancer-binding factor 2 ( MEF2 ) transcriptional regulators . Our studies indicate that Smad2 and 4 ( Smad2/4 ) complexes cooperate with MEF2 regulatory proteins in a GAL4-based one hybrid reporter gene assay . We have also observed in vivo interactions between Smad2 and MEF2A using co-immunoprecipitation assays . This interaction is confirmed by glutathione S-transferase pull-down analysis . Immunofluorescence studies in C2C12 myotubes show that Smad2 and MEF2A co-localise in the nucleus of multinuclear myotubes during differentiation . Interestingly , phosphorylaTED acceptor site mutations of MEF2 that render it unresponsive to p38 MAP kinase signalling abrogate the cooperativity with the Smads suggesting that p38 MAP Kinase -catalysed PHOSphorylation of MEF2 is a prerequisite for the Smad-MEF2 interaction . Thus , the association between Smad2 and MEF2A may subserve a physical link between TGF-beta signalling and a diverse array of genes controlled by the MEF2 cis element .