CRYSTAL-STRUCTURE OF MYOD BHLH DOMAIN-DNA COMPLEX - PERSPECTIVES ON DNA RECOGNITION AND IMPLICATIONS FOR TRANSCRIPTIONAL ACTIVATION

The crystal structure of a MyoD basic-helix-loop-helix (bHLH) domain-D NA complex has been solved and refined at 2.8 Angstrom resolution. Thi s structure proves that bHLH and bHLH-leucine zipper (bHLH-ZIP) protei ns are remarkably similar; it helps us understand subtle differences i n binding preferences for these proteins; and it has surprising implic ations for our understanding of transcription. Specifically, Ala-114 a nd Thr-115, which are required for positive control in the myogenic pr oteins, are buried at the protein-DNA interface. These residues are no t available for direct protein-protein contacts, but they may determin e the conformation of Arg-111. Comparisons with Max suggest that the c onformation of this arginine, which is different in the two structures , may play an important role in myogenic transcription.