Helix-turn-helix

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The λ repressor of bacteriophage lambda employs a helix-turn-helix (top; green) to bind DNA (bottom; blue and red).

In proteins, the helix-turn-helix (HTH) is a major structural motif capable of binding DNA. It is composed of two α helices joined by a short strand of amino acids and is found in many proteins that regulate gene expression. It should not to be confused with the helix-loop-helix domain.

Its discovery was based on similarities between the genes for Cro, CAP, and λ repressor, which share a common 20-25 amino acid sequence that facilitates DNA recognition. In particular, recognition and binding to DNA is done by the two α helices, one occupying the N-terminal end of the motif, the other at the C-terminus. In most cases, such as in the Cro repressor, the second helix contributes most to DNA recognition, and hence it is often called the "recognition helix". It binds to the major groove of DNA through a series of hydrogen bonds and various Van der Waals interactions with exposed bases. The other α helix stabilizes the interaction between protein and DNA, but does not play a particularly strong role in its recognition.

[edit] See also

• Winged helix turn helix
• DNA binding domain
• Secondary structure
• Zinc finger
• DNA-binding protein

[edit] References

• Brennan R, Matthews B (5 February 1989). "The helix-turn-helix DNA binding motif". J. Biol. Chem. 264 (4): 1903–6. PMID 2644244. http://www.jbc.org/cgi/reprint/264/4/1903. 
• Matthews B, Ohlendorf D, Anderson W, Takeda Y (1982). "Structure of the DNA-binding region of lac repressor inferred from its homology with cro repressor". Proc. Natl. Acad. Sci. U.S.A. 79 (5): 1428–32. doi:10.1073/pnas.79.5.1428. PMID 6951187. http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=345986&action=stream&blobtype=pdf. 

[edit] External links

• Helix-turn-helix motif, lambda-like repressor, from EMBL
• Full PDB entry for PDB ID 1LMB
• Cro/C1-type HTH domain, more HTHs in PROSITE