Acquisition of myogenic specificity through replacement of one amino acid of MASH-1 and introduction of an additional alpha-helical turn.

The homologous transcription factors Myf-5, MyoD, myogenin, MRF-4, and MASH-1 bind with high affinity and modest sequence specificity to DNA containing an E-box (CANNTG). This similarity of the in vitro DNA binding specificity is in sharp contrast to the high physiological specificity displayed by these proteins. Myf-5, MyoD, myogenin, and MRF-4 induce cells to differentiate along a myogenic pathway, while MASH-1 promotes the differentiation of neuronal precursor cells. We show here that MASH-1 can be converted into a protein capable of inducing myogenesis in fibroblasts by replacing leucine (130) of MASH-1 with lysine and introducing an additional turn into its basic recognition helix. These changes do not significantly alter the DNA binding properties of the proteins in cell free conditions. Crystallographic data for the DNA complexes of MyoD and E12 suggest that Leu (130) points away from the DNA into the solvent. We postulate that the identity of the amino acid in position 130 is important for protein-protein interactions that might affect the DNA binding specificities displayed by BHLH-proteins in vivo and form the molecular basis of the different physiological properties of the myogenic and neurogenic BHLH-proteins.