Heterochromatin protein 1

chromobox homolog 5
Identifiers
Symbol	CBX5
Alt. symbols	HP1-alpha
Entrez	23468
HUGO	1555
OMIM	604478
RefSeq	NM_012117
UniProt	P45973
Other data
Locus	Chr. 12 q13.13

chromobox homolog 1
Identifiers
Symbol	CBX1
Alt. symbols	HP1-beta
Entrez	10951
HUGO	1551
OMIM	604511
RefSeq	NM_006807
UniProt	P83916
Other data
Locus	Chr. 17 q21.32

chromobox homolog 3
Identifiers
Symbol	CBX3
Alt. symbols	HP1-gamma
Entrez	11335
HUGO	1553
OMIM	604477
RefSeq	NM_007276
UniProt	Q13185
Other data
Locus	Chr. 7 p21-15

The family of Heterochromatin Protein 1 (HP1) ("Chromobox Homolog", CBX) consists of highly conserved proteins, which have important functions in the cell nucleus. These functions include gene repression by heterochromatin formation, transcriptional activation, regulation of binding of cohesion complexes to centromeres, sequesteration of genes to nuclear periphery, transcriptional arrest, maintenance of heterochromatin integrity, gene repression at the single nucleosome level and gene repression by heterochromatization of euchromatin. HP1 proteins are fundamental units of heterochromatin packaging that are enriched at the centromeres and telomeres of nearly all Eukaryotic chromosomes with the notable exception of budding yeast, in which a yeast-specific silencing complex of SIR (silent information regulatory) proteins serve a similar function. Members of the HP1 family are characterized by an N-terminal chromodomain and a C-terminal chromoshadow domain, separated by a Hinge region. HP1 is also found at euchromatic sites, where its binding correlates with gene repression. HP1 was originally discovered by Tharappel C James and Sarah Elgin in 1986 as a factor in the phenomenon known as position effect variegation in Drosophila melanogaster.^[1]^[2]

Paralogs and Orthologs

Three different paralogs of HP1 are found in Drosophila melanogaster, HP1a, HP1b and HP1c. Subsequently orthologs of HP1 were also discovered in S. pombe (Swi6), Xenopus (Xhp1α and Xhp1γ) and Chicken (CHCB1, CHCB2 and CHCB3). In mammals,^[3] there are three paralogs: HP1α, HP1β and HP1γ. In Arabidopsis thaliana (a plant), there is one homolog: LIKE HETEROCHROMATIN PROTEIN 1 (LHP1), also known as TERMINAL FLOWER 2 (TFL2).^[4]

HP1β in mammals

HP1β interacts with the histone methyltransferase (HMTase) Suv(3-9)h1 and is a component of both pericentric and telomeric heterochromatin.^[5]^[6]^[7] HP1β is a dosage-dependent modifier of pericentric heterochromatin-induced silencing^[8] and silencing is thought to involve a dynamic association of the HP1β chromodomain with the tri-methylated Histone H3 Me(3)K9H3.

HP1 Interacting Proteins

HP1 seems to interact with numerous other proteins/molecules with different cellular functions in different organisms. Some of these HP1 interacting partners are: histone H1, histone H3, methylated K9 histone H3, histone H4, histone methyltransferase, DNA methyltransferase, methyl CpG binding protein MeCP2, and the origin recognition complex protein ORC2

HP1 binding affinity and cooperativity

HP1 binding affinity to nucleosomes containing histone H3 methylated at lysine K9 is higher than to those with unmethylated lysine K9. HP1 binds nucleosomes as a dimer and in principle can form multimeric complexes. Some studies have interpreted HP1 binding in terms of nearest-neighbor cooperative binding. This mode of chromatin interactions could potentially lead to spreading of HP1 along the nucleosome chain. However, the analysis of available data on HP1 binding to nucleosomal arrays in vitro shows that experimental HP1 binding isotherms can be explained by a simple model without cooperative interactions between neighboring HP1 dimers.^[9]

References

↑ James, T.C.; S.C Elgin (1986). "Identification of a nonhistone chromosomal protein associated with heterochromatin in Drosophila melanogaster and its gene". Mol. Cell. Biol. 6 (11): 3862–3872. PMC 367149 . PMID 3099166.
↑ Eissenberg, J.C., James, T.C., Foster-Hartnett, D.M., Hartnett, T., Ngan, V.K.W., Craig, C., Elgin, S.C.R. (1990). "Mutation in a heterochromatin-specific chromosomal protein is associated with suppression of position-effect variegation in Drosophila melanogaster". Proceedings of the National Academy of Sciences of the United States of America. 87 (24): 9923–7. PMC 55286 . PMID 2124708. doi:10.1073/pnas.87.24.9923.
↑ Singh PB; Miller JR; Pearce J; Kothary R; Burton RD; Paro R; James TC; Gaunt SJ. (February 1991). "A sequence motif found in a Drosophila heterochromatin protein is conserved in animals and plants". Nucleic Acids Res. 19 (4): 789–94. PMC 333712 . PMID 1708124. doi:10.1093/nar/19.4.789.
↑ Kotake, Toshihisa; Takada, Shinobu; Nakahigashi, Kenji; Ohto, Masaaki; Goto, Koji (2003-06-15). "Arabidopsis TERMINAL FLOWER 2 Gene Encodes a Heterochromatin Protein 1 Homolog and Represses both FLOWERING LOCUS T to Regulate Flowering Time and Several Floral Homeotic Genes". Plant and Cell Physiology. 44 (6): 555–564. ISSN 0032-0781. PMID 12826620. doi:10.1093/pcp/pcg091.
↑ Aagaard L, Laible G, Selenko P, Schmid M, Dorn R, Schotta G, Kuhfittig S, Wolf A, Lebersorger A, Singh PB, Reuter G, Jenuwein T (April 1999). "Functional mammalian homologues of the Drosophila PEV-modifier Su(var)3-9 encode centromere-associated proteins which complex with the heterochromatin component M31". EMBO J. 18 (7): 1923–38. PMC 1171278 . PMID 10202156. doi:10.1093/emboj/18.7.1923.
↑ Wreggett KA, Hill F, James PS, Hutchings A, Butcher GW, Singh PB (1994). "A mammalian homologue of Drosophila heterochromatin protein 1 (HP1) is a component of constitutive heterochromatin". Cytogenet. Cell Genet. 66 (2): 99–103. PMID 8287692. doi:10.1159/000133676.
↑ Sharma GG; Hwang KK; Pandita RK; Gupta A; Dhar S; Parenteau J; Agarwal M; Worman HJ; Wellinger RJ; Pandita TK. (Nov 2003). "Human Heterochromatin Protein 1 Isoforms HP1Hsα and HP1Hsβ Interfere with hTERT-Telomere Interactions and Correlate with Changes in Cell Growth and Response to Ionizing Radiation". Mol. Cell. Biol. 23 (22): 8363–76. PMC 262350 . PMID 14585993. doi:10.1128/MCB.23.22.8363-8376.2003.
↑ Festenstein R, Sharghi-Namini S, Fox M, Roderick K, Tolaini M, Norton T, Saveliev A, Kioussis D, Singh P (December 1999). "Heterochromatin protein 1 modifies mammalian PEV in a dose- and chromosomal-context-dependent manner". Nat. Genet. 23 (4): 457–61. PMID 10581035. doi:10.1038/70579.
↑ Teif V.B.; Kepper N.; Yserentant K; Wedemann G.; Rippe K. "Affinity, stoichiometry and cooperativity of heterochromatin protein 1 (HP1) binding to nucleosomal arrays". J. Phys.: Condens. Matter. arXiv:1408.6184 .

(1.1) Basic leucine zipper (bZIP)	Activating transcription factor AATF 1 2 3 4 5 6 7 AP-1 c-Fos FOSB FOSL1 FOSL2 JDP2 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 GCN4 HLF MAF B F G K NFE 2 L1 L2 L3 NFIL3 NRL NRF 1 2 3 XBP1
(1.2) Basic helix-loop-helix (bHLH)	ATOH1 AhR AHRR ARNT ASCL1 BHLH 2 3 9 ARNTL ARNTL ARNTL2 CLOCK EPAS1 FIGLA HAND 1 2 HES 5 6 HEY 1 2 L HES1 HIF 1A 3A ID 1 2 3 4 LYL1 MESP2 MXD4 MYCL1 MYCN Myogenic regulatory factors MyoD Myogenin MYF5 MYF6 Neurogenins 1 2 3 NeuroD 1 2 NPAS 1 2 3 OLIG 1 2 Pho4 Scleraxis SIM 1 2 TAL 1 2 Twist USF1
(1.3) bHLH-ZIP	AP-4 MAX MXD3 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 6 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
  - 1
  - 2
- TFIIF
- 1
- 2
  - TFIIH
  - 1
  - 2
  - 4
  - 2I
  - 3A
  - 3C1
  - 3C2

ATBF1
BCL
- 6
- 11A
- 11B
CTCF
E4F1
EGR
- 1
- 2
- 3
- 4
ERV3
GFI1
GLI-Krüppel family
- 1
- 2
- 3
- REST
- S1
- S2
- YY1
HIC
- 1
- 2
HIVEP
- 1
- 2
- 3
IKZF
- 1
- 2
- 3
ILF
- 2
- 3
KLF
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10
- 11
- 12
- 13
- 14
- 15
- 17
MTF1
MYT1
OSR1
PRDM9
SALL
- 1
- 2
- 3
- 4
SP
- 1
- 2
- 4
- 7
- 8
TSHZ3
WT1
Zbtb7
- 7A
- 7B
ZBTB
- 11
- 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
- 300
- 318
- 330
- 346
- 350
- 365
- 366
- 384
- 423
- 451
- 452
- 471
- 593
- 638
- 644
- 649
- 655

(2.4) Cys₆

HIVEP1

(2.5) Alternating composition

AIRE
DIDO1
GRLF1
ING
- 1
- 2
- 4
JARID
- 1A
- 1B
- 1C
- 1D
- 2
JMJD1B

(2.6) WRKY

WRKY

(3) Helix-turn-helix domains

(3.1) Homeodomain	ARX CDX 1 2 CRX CUTL1 DBX 1 2 DLX 3 4 5 EMX 1 2 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 C12 C13 D1 D3 D4 D8 D9 D10 D11 D12 D13 HOPX IRX 1 2 3 4 5 6 MKX 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 6-1 6-2 NOBOX PBX 1 2 3 PHF 1 3 6 8 10 16 17 20 21A PHOX 2A 2B 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 SATB2 SHOX2 SIX 1 2 3 4 5 VAX1 ZEB 1 2
(3.2) Paired box	PAX 1 2 3 4 5 6 7 8 9 PRRX 1 2 RAX
(3.3) Fork head / winged helix	E2F 1 2 3 4 5 FOX proteins A1 A2 A3 C1 C2 D3 D4 E1 E3 F1 G1 H1 I1 J1 J2 K1 K2 L2 M1 N1 N3 O1 O3 O4 P1 P2 P3 P4
(3.4) Heat shock factors	HSF 1 2 4
(3.5) Tryptophan clusters	ELF 2 4 5 EGF ELK 1 3 4 ERF ETS 1 2 ERG 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 TBR2 TFT MYRF TP63
(4.4) MADS box	Mef2 A B C D SRF
(4.6) TATA-binding proteins	TBP TBPL1
(4.7) High-mobility group	BBX HMGB 1 2 3 4 HMGN 1 2 3 4 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
(4.9) Grainyhead	TFCP2
(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

see also transcription factor/coregulator deficiencies

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