TI - Regulation of muscle gene expression by TGF-beta signalling . AB - Skeletal muscle cells are known to be highly responsive to TGF-beta signalling . In these studies we show that the endogenous MEF2 proteins are co-localised with the Smad proteins in the nuclei of differentiated mt . These data suggest a possible role for TGF-beta signalling in the control of gene expression in differentiated muscle cells . Conversely , in mb , prior to the initiation of differentiation , the Smads are cytoplasmic and MEF2 proteins are not expressed at high levels ( ZAQuinn , unpublished data ) . It is known that if mb are stimulated with exogenous TGF-beta this inhibits the differentiation programme , which leads to the nuclear accumulation of Smads ( present study ) . The mechanism of mb inhibition seems to be mediated by transcriptional down-regulation of the MyoD gene and constitutive MyoD expression can rescue myogenesis in the face of TGF-beta signalling ( ZAQuinn , unpublished data ) . Taken together , these observations suggest that once the differentiation programme is initiated and MEF2 , functioning as a Smad co-regulator , is expressed , the inhibitory effect of TGF-beta is lost and the association of the Smads with the MEF2 proteins promotes rather than inhibits the differentiation programme . One property of the MEF2-Smad interaction is that the Smads and MEF2 are spatially separated binding partners in which juxtaposition by regulated re-location allows the interaction to occur ; the cooperative association thus being formed between the nuclear localised Smads and the PHOSphorylated MEF2 protein . A recent study has provided evidence that the subcellular localisation of MEF2C can be modified by TGF-beta signalling and that a possible cytoplasmic tethering factor might be involved ( 86 ) . In our studies we have no evidence to suggest that MEF2A is regulated in this manner . First , TGF-beta treatment does not change the nuclear localisation of MEF2A and second , a Smad2 dominant negative that is permanently retained in the cytoplasm does not change this pattern . Therefore , the expression of MEF2 proteins may be a key switch in converting Smad -dependent TGF-beta signalling in muscle cells from an inhibitory to a stimulatory differentiation signal . In summary , investigation of transcriptional activation by the Smads has revealed a high degree of complexity involving multiple proteins and diverse promoter elements . In this report we identify a novel interaction between the Smad proteins and the MEF2 transcriptional regulatory proteins that could mediate transcriptional responses to TGF-beta in a variety of cell types .