A molecular-level understanding of the events that occur during the process of myogenesis is crucial to our understanding a variety of muscle-related diseases and dysfunctions. Early events in myogenesis involve the activation of expression of muscle-specific genes. Genetic and molecular biological approaches showed that expression of a 68- residue fragment of a protein known as MyoD will convert non-muscle fibroblast cells to muscle, under conditions that promote differentiation. This same fragment has been shown to bind to DNA sequences that are found in the enhancer regions of several muscle- specific genes. Sequence analysis of the 68 residues that represent the myogenic unit of MyoD revealed that it is a member of a newly identified family of DNA-binding proteins, dubbed """"""""helix-loop-helix"""""""" (HLH) proteins, that includes other muscle-specific DNA-binding proteins such as myogenin and myf5, the immunoglobulin kappa-chain enhancer-binding proteins E12 and E47, and several proteins involved in Drosophila development. There is, however, no experimentally derived structural information available on these important proteins. Advances in multidimensional heteronuclear nuclear magnetic resonance (NMR) spectroscopy have made it possible to determine three-dimensional structures of small soluble proteins in solution. These techniques have been applied to a number of different DNA-binding proteins or domains (e.g., several prokaryotic repressor proteins, the homeodomain from Antennapedia, zinc finger domains, leucine zippers, etc.). We propose to apply these techniques to the """"""""helix-loop-helix"""""""" DNA-binding domain of a heterodimer of MyoD and E47, a non-tissue-specific transcription factor that purifies as a complex with MyoD, with an aim towards understanding the molecular details of its function. The structure of this novel """"""""helix-loop-helix"""""""" motif, both in the absence and presence of specific sequence DNA, will be determined.
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