HLH forced dimers: tethering MyoD to E47 generates a dominant positive myogenic factor insulated from negative regulation by Id.

Basic-helix-loop-helix (bHLH) class transcription factors bind DNA as hetero- and homodimers. In murine myogenic cells the HLH network includes multiple members of the E protein, MyoD, and Id families; changes in the network characterize muscle determination and differentiation and have been proposed as causal for these developmental transitions. To test the importance of HLH partner choice in these cellular decisions, we have designed a strategy in which the identity of a bHLH dimer is specified by joining two monomers via a flexible polypeptide linker. A MyoD-E47 polyprotein avidly bound the same DNA targets as its unlinked counterpart, but, unlike intermolecular dimers that are very sensitive to inhibition by Id, MyoD-E47 was resistant to Id challenge. In cells MyoD-E47 acted as a dominant positive myogenic factor, capable of initiating myogenic determination and also substantially bypassing negative regulation of differentiation by serum growth factors.