Multimeric structures influence the binding activity of bHLH muscle regulatory factors

Sucrose gradients and molecular sieve chromatography were used to determine the native molecular weight of the basic HLH proteins myogenin, MyoD and E12. The muscle bHLH proteins not only formed dimers but also associated in a variety of higher order complexes. Although homodimers bind to DNA sequences such as the MEF-1 site in the creatine kinase enhancer, homotetramers and larger forms do not recognize this DNA sequence. Little evidence for complexes larger than dimers was found for the ubiquitous bHLH protein E12. Most of the myogenin remains in large complexes when myogenin and E12 are mixed. The same result was obtained in nuclear extracts from differentiated myotubes, in which most of the myogenin was found to be present in large complexes that do not bind to the creatine kinase enhancer. A fusion protein that contains only the myogenin HLH region fused to glutathione-S-transferase also forms large homomeric complexes. A model to explain these results is that each helix of the HLH motif can associate with a different subunit to form chains or ring structures. The presence of myogenin in nuclear extracts as both dimers that recognize known DNA sequences as well as higher order complexes that do not raises significant issues concerning the regulation of skeletal muscle bHLH protein activity during myogenesis.