Basic helix–loop–helix
transcription regulator protein
A basic helix–loop–helix (bHLH) is a protein structural motif that characterizes one of the largest families of dimerizing transcription factors. The word "basic" does not refer to complexity but to the chemistry of the motif because transcription factors in general contain basic amino acid residues in order to facilitate DNA binding.
bHLH transcription factors are often important in development or cell activity. For one, BMAL1-Clock (also called ARNTL) is a core transcription complex in the molecular circadian clock. Other genes, like c-Myc and HIF-1, have been linked to cancer due to their effects on cell growth and metabolism.
Structure
The motif is characterized by two α-helices connected by a loop. In general, transcription factors (including this type) 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 this 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.
“Basic helix–loop–helix” enters the record as transcription regulator protein. Crown Archives preserves that source wording while asking what Basic, helix and loop can confirm, complicate or overturn.
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