Heterochromatin protein 1

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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) are highly conserved adapter molecules, which have important functions in the cell nucleus. These functions include gene repression by heterochromatin formation, transcriptional activation, regulation of binding of cohesin complexes to centromere, sequesteration of genes to nuclear periphery, transcriptional arrest, maintenance of heterochromatin integrity, gene repression at 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 are in action. Members of the HP1 family are characterized by two conserved domains: an N-terminal chromodomain and a C-terminal chromoshadow domain, which are separated by an Hinge Region in between. HP1 is also found at euchromatic sites, where its binding correlates with the repression of an increasing number of genes. HP1 was originally discovered in 1986 as an important factor in the phenomenon known as position effect variegation in Drosophila melanogaster.^[1][1]

Isoforms

Three different isoforms of HP1 are found in Drosophila melanogaster, HP1a, HP1b and HP1c. Subsequently homologues of HP1 were also discovered in S. pombe (Swi6), Xenopus (Xhp1α and Xhp1γ) and Chicken (CHCB1, CHCB2 and CHCB3). In mammals,^[1] there are three isoforms termed HP1α, HP1β and HP1γ each encoded by different genes.

HP1β

HP1β interacts with the Histon-Methyltransferase (HMTase) Suv(3)9h1 and is a component of both pericentric and telomeric heterochromatin.^[1][1][1] HP1β is a dosage-dependent modifier of pericentric heterochromatin-induced silencing^[1] 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.

See also

• Epigenetics
• Heterochromatin

References

Singh PB, Georgatos SD. HP1: facts, open questions, and speculation. J Struct Biol. 2002 Oct-Dec;140(1-3):10-6. Reviewde:Heterochromatin Protein 1