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).
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. Not to be confused with the Basic helix-loop-helix domain

Its discovery was based on similarities between the genes for Cro, CAP, and λ lambda 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.

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Protein secondary structure
Helices: α-helix | 310 helix | π-helix | β-helix | Polyproline helix | Collagen helix
Extended: β-strand | Turn | Beta hairpin | Beta bulge | α-strand
Supersecondary: Coiled coil | Helix-turn-helix | EF hand
Secondary structure propensities of amino acids
Helix-favoring: Methionine | Alanine | Leucine | Glutamic acid | Glutamine | Lysine
Extended-favoring: Threonine | Isoleucine | Valine | Phenylalanine | Tyrosine | Tryptophan
Disorder-favoring: Glycine | Serine | Proline | Asparagine | Aspartic acid
No preference: Cysteine | Histidine | Arginine
←Primary structure Tertiary structure→
v • d • e
Transcription factors and intracellular receptors
(1) Basic domains
(1.1) Basic leucine zipper (bZIP)
Activating transcription factor (AATF, 1, 2, 3, 4, 5, 6, 7) • AP-1 (c-Fos, FOSB, FOSL1, FOSL2, c-Jun, JUNB, JUND) • BACH (1, 2) • BATF • BLZF1 • C/EBP (α, β, γ, δ, ε, ζ) • CREB (1, 3, L1) • CREM • DBP • DDIT3 • GABPA • HLF • MAF (B, F, G, K) • NFE (2, L1, L2) • NRL • NRF1 • XBP1
(1.2) Basic helix-loop-helix (bHLH)
ATOH1 • AhR • AHRR • ARNT • ASCL1 • BHLHB2 • BMAL (ARNTL, ARNTL2) • CLOCK • EPAS1 • HAND (1, 2) • HES (5, 6) • HEY (1, 2, L) • HES1 • HIF (1A, 3A) • ID (1, 2, 3, 4) • LYL1 • MXD4 • MYCL1 • MYCN • Myogenic regulatory factors (MyoD, Myogenin, MYF5, MYF6) • Neurogenins • NeuroD (1, 2) • NPAS (1, 2, 3) • OLIG (1, 2) • Scleraxis • TAL1 • Twist • USF1
(1.3) bHLH-ZIP
MAX • MITF • MNT • MLX • MXI1 • Myc • SREBP (1, 2)
(1.6) Basic helix-span-helix (bHSH)
(2) Zinc finger
DNA-binding domains
(2.1) Nuclear receptor (Cys4)
subfamily 1 (Thyroid hormone (α, β), CAR, FXR, LXR (α, β), PPAR (α, β/δ, γ), PXR, RAR (α, β, γ), ROR (α, β, γ), Rev-ErbA (α, β), VDR) • subfamily 2 (COUP-TF (I, II), Ear-2, HNF4 (α, γ), PNR, RXR (α, β, γ), Testicular receptor (2, 4), TLX) • subfamily 3 (Steroid hormone (Estrogen (α, β), Estrogen related (α, β, γ), Androgen, Glucocorticoid, Mineralocorticoid, Progesterone)) • subfamily 4 NUR (NGFIB, NOR1, NURR1) • subfamily 5 (LRH-1, SF1) • subfamily 6 (GCNF) • subfamily 0 (DAX1, SHP)
(2.2) Other Cys4
GATA (1, 2, 3, 4, 5, 6) • MTA (1, 2, 3)
(2.3) Cys2His2
ATBF1 • BCL(6, 11A, 11B) • CTCF • E4F1 • EGR (2, 3) • ERV3 • General transcription factors (TFIIA, TFIIB, TFIID, TFIIE, TFIIF: 1, 2, TFIIH: 1, 2, 4, 2I, 3A, 3C1, 3C2) • GFI1 • GLI-Krüppel family (1, 2, 3, YY1) • HIC (1, 2) • HIVEP (1, 2, 3) • IKZF (1, 2, 3) • ILF (2, 3) • KLF (2, 4, 5, 6, 9, 10, 11, 12, 13) • MTF1 • MYT1 • OSR1 • Sp1 • zinc finger ((3, 7, 9, 10, 19, 22, 24, 33B, 34, 35, 41, 43, 44, 51, 74, 143, 146, 148, 165, 202, 217, 219, 238, 239, 259, 267, 268, 281, 295, 318, 330, 346, 350, 365, 366, 384, 451, 452, 471, 593, 638, 649, 655) • Zbtb7 (7A) • ZBT (16, 17, 33)
(2.4) Cys6
(2.5) Alternating composition
AIRE • DIDO1 • GRLF1 • ING (1, 2, 4) • JARID (1A, 1B, 1C, 1D, 2) • JMJD1B
(3) Helix-turn-helix
domains
ARX • CDX (1, 2) • CRX • CUTL1 • DLX (3, 4, 5) • EMX2 • EN (1, 2) • FHL (1, 2, 3) • HESX1 • HHEX • HLX • Homeobox (A1, A3, A4, A5, A7, A9, A10, A11, A13, B1, B2, B3, B4, B5, B6, B7, B8, B9, B13, C4, C5, C6, C8, C9, C10, C11, C13, D1, D3, D4, D8, D9, D10, D11, D12, D13) • HOPX • MEIS (1, 2) • MEOX2 • MNX1 • MSX (1, 2) • NANOG • NKX (2-1, 2-5, 3-1) • PHF (3, 6, 10, 17) • POU domain (PIT-1, BRN-3: 1, 2, Octamer transcription factor: 1, 2, 3/4, 6, 7) • OTX (1, 2) • PDX1
(3.2) Paired box
PAX (1, 2, 3, 4, 5, 6, 7, 8, 9)
E2F (1, 2, 3, 4, 5) • FOX proteins (C1, C2, E1, G1, H1, K2, L2, M1, N3, O1A, , O3A, O4, P1, P2, P3)
HSF (1, 2, 4)
(3.5) Tryptophan clusters
ELF (4, 5) • EGF • ELK (1, 3, 4) • ERF • ERG • ETS (1, 2) • ETV (1, 4, 5, 6) • FLI1 • Interferon regulatory factors (1, 2, 3, 4, 5, 6, 7, 8) • MYB • MYBL2
(3.6) TEA domain
transcriptional enhancer factor 1, 2
(4) β-Scaffold factors with
minor groove contacts
(4.1) Rel homology region
NF-κB (NFKB1, NFKB2, REL, RELA, RELB) • NFAT (C1, C2, C3, C4, 5)
(4.2) STAT
STAT (1, 2, 3, 4, 5, 6)
(4.3) p53
(4.4) MADS box
Mef2 (A, B, C, D) • SRF
HNF (1A, 1B) • LEF1 • SOX (2, 3, 4, 5, 6, 8, 9, 10, 11, 13, 15, 18) • SRY • SSRP1
(4.10) Cold-shock domain
(4.11) Runt
(0) Other
transcription factors
(0.2) HMGI(Y)
HMGA (1, 2) • HBP1
Rb • RBL1 • RBL2
(0.5) AP-2/EREBP-related factors
Apetala 2 • EREBP • B3
(0.6) Miscellaneous
ARID (1A, 1B, 2, 3A, 3B, 4A) • CAP • IFI (16, 35) • MLL (2, 3, T1) • MNDA • NFI (A, B, C, X) • NFY (A, B, C) • Rho/Sigma • R-SMAD
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