Heterochromatin protein 1

The family of Heterochromatin Protein 1 (HP1) ("Chromobox Homolog", CBX) are highly conserved adapter 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 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 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 Dr. Tharappel C James and Dr. Sarah Elgin in 1986 as an important factor in the phenomenon known as position effect variegation in Drosophila melanogaster.

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, there are three paralogs: HP1α, HP1β and HP1γ.

HP1β in mammals
HP1β interacts with the Histone-Methyltransferase (HMTase) Suv(3-9)h1 and is a component of both pericentric and telomeric heterochromatin. HP1β is a dosage-dependent modifier of pericentric heterochromatin-induced silencing 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