Identification of DNA-binding protein(s) in the developing heart.

An antiserum (anti-H2) directed at the second helix of the helix-loop-helix (HLH) protein MyoD1 reacts with a protein expressed during avian cardiac myocyte differentiation. Indirect immunohistochemical whole mount staining with anti-H2 detected a protein expressed in stage 11 hearts, but not in hearts of older embryos. At the cellular level, this staining is confined to the nucleus of cardiac cells suggesting that these proteins may have DNA-binding abilities. Several proteins were immunoprecipitated by anti-H2 from stage 11 heart tissue. Protein extracts from similarly staged hearts, when incubated with the muscle-specific enhancer sequence of muscle creatinine kinase (MCK), gave a stage-specific band shift in electromobility shift assays (EMSA), and these protein-DNA complexes were recognized and supershifted by anti-H2. Incubation with a MCK sequence containing a mutated E box did not produce a shift. The specific shift was present as early as stage 6, remained through stage 13, and disappeared by stage 17. These data suggest the presence of at least one protein that is transiently expressed in the differentiating cardiac myocyte, that is immunochemically reactive with an antiserum raised against the second helix of MyoD1, and that binds to a muscle-specific DNA enhancer sequence.