Basic helix-loop-helix
A basic helix-loop-helix (bHLH) is a protein structural motif that characterizes a family of transcription factors. It should not be confused with the helix-turn-helix domain.
Structure
The motif is characterized by two α-helices connected by a loop. In general, transcription factors including this domain are dimeric, each with one helix containing basic amino acid residues that facilitate DNA binding. In general, one helix is smaller, and, due to the flexibility of the loop, allows dimerization by folding and packing against another helix. The larger helix typically contains the DNA-binding regions. bHLH proteins typically bind to a consensus sequence called an E-box, CANNTG. The canonical E-box is CACGTG (palindromic), however some bHLH transcription factors, notably those of the bHLH-PAS family, bind to related non-palindromic sequences, which are similar to the E-box.
Examples
Examples of transcription factors containing a bHLH include:
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