HETERONUCLEAR (H-1,C-13,N-15) NMR ASSIGNMENTS AND SECONDARY STRUCTUREOF THE BASIC REGION-HELIX-LOOP-HELIX DOMAIN OF E47

E47 is an immunoglobulin enhancer DNA-binding protein that contains a basic region-helix-loop-helix (b/HLH) domain. This structural motif de fines a class of transcription factors that are central to the develop mental regulation of many tissues. Its function is to provide a dimeri zation interface through the formation of a parallel four-helix bundle , resulting in the juxtaposition of two basic DNA-recognition alpha-he lices that control sequence-specific DNA-binding. In order to gain ins ight into the biophysical nature of b/HLH domains, we have initiated s tructural studies of the E47 homodimer by NMR. Sequence-specific reson ance assignments have been obtained using a combination of heteronucle ar double- and triple-resonance NMR experiments. The secondary structu re was deduced from characteristic patterns of NOEs, C-13(alpha/beta) chemical shifts, and measurements of (3)J(HNH alpha), scalar couplings . Except for the basic region recognition helix, the secondary structu ral elements of the E47 homodimer are preserved in the absence DNA whe n compared with the co-crystal structure of E47 bound to DNA (Ellenber ger T, Fass D, Arnaud M, Harrison SC, 1994, Genes & Dev 8:970-980). As expected, the DNA-binding helix is largely unstructured, but does sho w evidence of nascent helix formation. The HLH region of E47 is struct ured, but highly dynamic as judged by the rapid exchange of backbone h ydrogen atoms and the relatively weak intensities of many of the NOEs defining the dimerization helices. This dynamic nature may be relevant to the ability of E47 both to homodimerize and to heterodimerize with MyoD, Id, and Tall.