Intramolecular regulation of MyoD activation domain conformation and function

The MyoD family of basic helix-loop-helix (bHLH) proteins is required for myogenic determination and differentiation. The basic region carries the myogenic code and DNA binding specificity, while the N terminus contains a potent transcriptional activation domain. Myogenic activation is abolished when the basic region, bound to a myogenic E box, carries a mutation of Ala-114. It has been proposed that DNA binding of the MyoD basic region leads to recruitment of a recognition factor that unmasks the activation domain. Here we dem-onstrate that an A114N mutant exhibits an altered conformation in the basic region and that this local conformational difference can lead to a more global change affecting the conformation of the activation domain. This suggests that the deleterious effects of this class of mutations may result directly from defective confor-mation. Thus, the activation domain is unmasked only upon DNA binding by the correct basic region. Such a coupled conformational relationship may have evolved to restrict myogenic specificity to a small number of bHLH proteins among many with diverse functions yet with DNA binding specificities known to be similar. The skeletal muscle-specific basic helix-loop-helix (bHLH) transcription factors (including MyoD, Myf-5, myogenin, and MRF-4) are required for myogenic determination and differ-entiation (34). Previous work has identified Ala-114 and Thr