bhlh Proteins

Abstract As described in Chapter 3, the basic helix-loop-helix (bHLH) proteins are a large superfamily of dimeric transcription factors in which the HLH motif mediates protein dimerization and the basic domain mediates DNA binding. In some bHLH families, such as the myogenic bHLH family consisting of MYOD, MYOGENIN, MYFS, and MRF4, the HLH do main is both necessary and sufficient to mediate dimerization. In other families, an additional domain that is present immediately carboxyl terminal to the HLH domain is also required for dimerization. Thus, the bHLH-ZIP proteins contain a helix-loop-helix and an adjacent leucine zipper domain that are both required for proper protein dimerization. Because dimerization is a prerequisite for DNA binding, these domains are crucial for the biological activity of bHLH and bHLH-ZIP proteins.