Basic-helix-loop-helix: Information from Answers.com

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.^[5] 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.^[6] The canonical E-box is CACGTG (palindromic), however some bHLH transcription factors bind to non-palindromic sequences, which are often similar to the E-box.

Examples

Examples of transcription factors containing a bHLH include:

AhR
Beta2/NeuroD1
BMAL-1-CLOCK
C-Myc, N-Myc
MyoD
Pho4
HIF
NPAS1, NPAS3, MOP5
Scl, also known as Tal1
proneural bHLH genes like p-CaMKII, and pSer(336)NeuroD.
Scleraxis
Neurogenins
MAX

bHLH transcription factors are often important in development or cell activity. BMAL1-Clock 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.

Regulation

Since many bHLH transcription factors are heterodimeric, their activity is often highly regulated by the dimerization of the subunits. One subunit's expression or availability is often controlled, whereas the other subunit is constituitively expressed. Many of the known regulatory proteins, such as the Drosophila extramacrochaetae protein, have the helix-loop-helix structure but lack the basic region, making them unable to bind to DNA on their own. They are, however, able to form heterodimers with proteins that have the bHLH structure, and inactivate their abilities as transcription factors.^[7]

History

1989: Murre et al. could show that dimers of various bHLH proteins bind to a short DNA motif (later called E-Box).^[8] This E-box consists of the DNA sequence CANNTG, where N can be any nucleotide.^[6]

Human proteins with helix-loop-helix DNA-binding domain

AHR; AHRR; ARNT; ARNT2; ARNTL; ARNTL2; ASCL1; ASCL2; ASCL3; ASCL4; ATOH1; ATOH7; ATOH8; BHLHB2; BHLHB3; BHLHB4; BHLHB5; BHLHB8; CLOCK; EPAS1; FERD3L; FIGLA; HAND1; HAND2; HES1; HES2; HES3; HES4; HES5; HES6; HES7; HEY1; HEY2; HIF1A; ID1; ID2; ID3; ID4; KIAA2018; LYL1; MASH1; MATH2; MAX; MESP1; MESP2; MIST1; MITF; MLX; MLXIP; MLXIPL; MNT; MSC; MSGN1; MXD1; MXD3; MXD4; MXI1; MYC; MYCL1; MYCL2; MYCN; MYF5; MYF6; MYOD1; MYOG; NCOA1; NCOA3; NEUROD1; NEUROD2; NEUROD4; NEUROD6; NEUROG1; NEUROG2; NEUROG3; NHLH1; NHLH2; NPAS1; NPAS2; NPAS3; OAF1; OLIG1; OLIG2; OLIG3; PTF1A; SCL; SCXB; SIM1; SIM2; SOHLH1; SOHLH2; SREBF1; SREBF2; TAL1; TAL2; TCF12; TCF15; TCF21; TCF3; TCF4; TCFL5; TFAP4; TFE3; TFEB; TFEC; TWIST1; TWIST2; USF1; USF2;

References

^ PDB 1x0o; Card PB, Erbel PJ, Gardner KH (October 2005). "Structural basis of ARNT PAS-B dimerization: use of a common beta-sheet interface for hetero- and homodimerization". J. Mol. Biol. 353 (3): 664–77. doi:10.1016/j.jmb.2005.08.043. PMID 16181639.
^ Murre C, Bain G, van Dijk MA, Engel I, Furnari BA, Massari ME, Matthews JR, Quong MW, Rivera RR, Stuiver MH (June 1994). "Structure and function of helix-loop-helix proteins". Biochim. Biophys. Acta 1218 (2): 129–35. PMID 8018712. http://linkinghub.elsevier.com/retrieve/pii/0167-4781(94)90001-9.
^ Littlewood TD, Evan GI (1995). "Transcription factors 2: helix-loop-helix". Protein Profile 2 (6): 621–702. PMID 7553065.
^ Massari ME, Murre C (January 2000). "Helix-loop-helix proteins: regulators of transcription in eucaryotic organisms". Mol. Cell. Biol. 20 (2): 429–40. doi:10.1128/MCB.20.2.429-440.2000. PMID 10611221. PMC 85097. http://mcb.asm.org/cgi/pmidlookup?view=long&pmid=10611221.
^ Lawrence Zipursky; Arnold Berk; Monty Krieger; Darnell, James E.; Lodish, Harvey F.; Kaiser, Chris; Matthew P Scott; Matsudaira, Paul T.. McGill Lodish 5E Package - Molecular Cell Biology & McGill Activation Code. San Francisco: W. H. Freeman. ISBN 0-7167-8635-4.
^ ^a ^b Chaudhary J, Skinner MK (1999). "Basic helix-loop-helix proteins can act at the E-box within the serum response element of the c-fos promoter to influence hormone-induced promoter activation in Sertoli cells". Mol. Endocrinol. 13 (5): 774–86. doi:10.1210/me.13.5.774. PMID 10319327.
^ Cabrera CV, Alonso MC, Huikeshoven H (1994). "Regulation of scute function by extramacrochaete in vitro and in vivo". Development 120 (12): 3595–603. PMID 7821225.
^ Murre C, McCaw PS, Vaessin H, et al. (1989). "Interactions between heterologous helix-loop-helix proteins generate complexes that bind specifically to a common DNA sequence". Cell 58 (3): 537–44. doi:10.1016/0092-8674(89)90434-0. PMID 2503252.

External links

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) · NFIL3 · 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) · Pho4 · Scleraxis · TAL (1, 2) · Twist · USF1

(1.3) bHLH-ZIP	AP-4 · MAX · MITF · MNT · MLX · MXI1 · Myc · SREBP (1, 2)

(1.4) NF-1	NFI (A, B, C, X) · SMAD (R-SMAD (1, 2, 3, 5, 9) - I-SMAD (6, 7) - 4)

(1.5) RF-X	RFX (1, 2, 3, 4, 5, ANK)

(1.6) Basic helix-span-helix (bHSH)	AP-2 (α, β, γ, δ, ε)

(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 (Androgen, Estrogen (α, β), Glucocorticoid, Mineralocorticoid, Progesterone), Estrogen related (α, β, γ)) 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) · TRPS1

(2.3) Cys₂His₂	General transcription factors (TFIIA, TFIIB, TFIID, TFIIE, TFIIF (1, 2), TFIIH (1, 2, 4, 2I, 3A, 3C1, 3C2)) ATBF1 · BCL (6, 11A, 11B) · CTCF · E4F1 · EGR (2, 3) · ERV3 · GFI1 · GLI-Krüppel family (1, 2, 3, REST, S2, YY1) · HIC (1, 2) · HIVEP (1, 2, 3) · IKZF (1, 2, 3) · ILF (2, 3) · KLF (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 17) · MTF1 · MYT1 · OSR1 · SP (1, 2, 4, 7) · Zbtb7 (7A, 7B) · ZBTB (16, 17, 20, 32, 33, 40) · 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, 423, 451, 452, 471, 593, 638, 649, 655)

(2.4) Cys₆	HIVEP1

(2.5) Alternating composition	AIRE · DIDO1 · GRLF1 · ING (1, 2, 4) · JARID (1A, 1B, 1C, 1D, 2) · JMJD1B

(3) Helix-turn-helix domains

(3.1) Homeodomain	ARX · CDX (1, 2) · CRX · CUTL1 · DLX (3, 4, 5) · EMX2 · EN (1, 2) · FHL (1, 2, 3) · HESX1 · HHEX · HLX · Homeobox (A1, A2, 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 · LMX (1A, 1B) · MEIS (1, 2) · MEOX2 · MNX1 · MSX (1, 2) · NANOG · NKX (2-1, 2-2, 2-3, 2-5, 3-1, 3-2) · PBX (1, 2, 3) · PHF (1, 3, 6, 8, 10, 16, 17, 20, 21A) · PITX (1, 2, 3) · POU domain (PIT-1, BRN-3: A, B, C, Octamer transcription factor: 1, 2, 3/4, 6, 7, 11) · OTX (1, 2) · PDX1 · ZEB (1, 2)

(3.2) Paired box	PAX (1, 2, 3, 4, 5, 6, 7, 8, 9)

(3.3) Fork head / winged helix	E2F (1, 2, 3, 4, 5) · FOX proteins (C1, C2, D3, E1, G1, H1, K2, L2, M1, N3, O1, O3, O4, P1, P2, P3)

(3.4) Heat Shock Factors	HSF (1, 2, 4)

(3.5) Tryptophan clusters	ELF (2, 4, 5) · EGF · ELK (1, 3, 4) · ERF · ERG · ETS (1, 2, SPIB) · 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, 3, 4)

(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	p53 · TBX (1, 2, 3, 5, 19, 21, 22, TBR1)

(4.4) MADS box	Mef2 (A, B, C, D) · SRF

(4.6) TATA binding proteins	TBP · TBPL1

(4.7) High mobility group	HMGB (1, 2, 3) · HNF (1A, 1B) · LEF1 · SOX (1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 18, 21) · SRY · SSRP1 · TCF (3, 4) · TOX (1, 2, 3, 4)

(4.10) Cold-shock domain	CSDA, YBX1

(4.11) Runt	CBF (CBFA2T2, CBFA2T3, RUNX1, RUNX2, RUNX3, RUNX1T1)

(0) Other transcription factors

(0.2) HMGI(Y)	HMGA (1, 2) · HBP1

(0.3) Pocket domain	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 · NFY (A, B, C) · Rho/Sigma

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Basic-helix-loop-helix

Contents

Structure