Repression of myogenin function by TGF-β1 is targeted at the basic helix- loop-helix motif and is independent of E2A products

The muscle-specific helix-loop-helix (HLH) proteins myogenin, MyoD, myf5, and MRF4 form hetero-oligomers with ubiquitous HLH proteins encoded by the E2A gene and activate muscle transcription by binding to a DNA sequence known as an E-box (CANNTG). Transforming growth factor-β (TGF-β) can inhibit muscle differentiation by silencing the transcription-activating potential of myogenic HLH proteins without affecting their ability to bind DNA. We show that repression by TGF-β is directed at the basic-HLH motif of myogenin and is independent of E2A products. Using a series of reporter genes as targets for trans-activation by myogenin, transcriptional repression by TGF-β is also shown to map to the E-box motif and to not require heterologous DNA sequence elements. These results demonstrate that TGF-β represses muscle- specific transcription through a post-translational mechanism that renders the basic-HLH regions of the myogenic regulators nonfunctional. The selective repression of myogenic HLH proteins by TGF-β indicates that the TGF-β signaling system can discriminate between different classes of HLH proteins and implies that myogenic HLH proteins activate muscle-specific transcription through a unique mechanism.