The family of Heterochromatin Protein 1 (HP1) ("Chromobox Homolog", CBX) are highly conserved adapter protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s, which have important functions in the cell nucleus

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

. These functions include gene repression by heterochromatin

Heterochromatin

Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...

 formation, transcriptional

Transcription (genetics)

Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

 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

Euchromatin

Euchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often under active transcription. Unlike heterochromatin, it is found in both cells with nuclei and cells without nuclei...

. HP1 protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s are fundamental units of heterochromatin

Heterochromatin

Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...

 packaging that are enriched at the centromere

Centromere

A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...

s and telomere

Telomere

A telomere is a region of repetitive DNA sequences at the end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos "end" and merοs "part"...

s of nearly all Eukaryotic

Eukaryote

A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s 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

Chromodomain

A chromodomain is a protein structural domain of about 40-50 amino acid residues commonly found in proteins associated with the remodeling and manipulation of chromatin. The domain is highly conserved among both plants and animals, and is represented in a large number of different proteins in many...

 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

Sarah Elgin

Sarah C.R. Elgin is an American biologist noted for her work in epigenetics, gene regulation, and heterochromatin and her contributions to science education....

 in 1986 as an important factor in the phenomenon known as position effect variegation in Drosophila melanogaster

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

.

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

Xenopus

Xenopus is a genus of highly aquatic frogs native to Sub-Saharan Africa. There are 19 species in the Xenopus genus...

 (Xhp1α and Xhp1γ) and Chicken

Chicken

The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

 (CHCB1, CHCB2 and CHCB3). In mammals, there are three paralogs: HP1α

CBX5 (gene)

Chromobox protein homolog 5 is a protein that in humans is encoded by the CBX5 gene.-Interactions:CBX5 has been shown to interact with HDAC9, Ku70, MIS12, Lamin B receptor, MBD1, TAF4, Histone deacetylase 5, TRIM28, HDAC4, CBX3, SMARCA4, DNMT3B, CHAF1A, CBX1 and SUV39H1....

, HP1β

CBX1

Chromobox protein homolog 1 is a protein that in humans is encoded by the CBX1 gene.-Interactions:CBX1 has been shown to interact with C11orf30, CBX5, CBX3 and SUV39H1.-Further reading:...

 and HP1γ

CBX3

Chromobox protein homolog 3 is a protein that in humans is encoded by the CBX3 gene.-Interactions:CBX3 has been shown to interact with PIM1, Ki-67, Lamin B receptor, CBX5 and CBX1....

.

HP1β in mammals

HP1β interacts with the Histone-Methyltransferase

Methyltransferase

A methyltransferase is a type of transferase enzyme that transfers a methyl group from a donor to an acceptor.Methylation often occurs on nucleic bases in DNA or amino acids in protein structures...

 (HMTase) Suv(3-9)h1 and is a component of both pericentric and telomeric heterochromatin

Heterochromatin

Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative 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

Chromodomain

A chromodomain is a protein structural domain of about 40-50 amino acid residues commonly found in proteins associated with the remodeling and manipulation of chromatin. The domain is highly conserved among both plants and animals, and is represented in a large number of different proteins in many...

 with the tri-methylated Histone H3

Histone H3

Histone H3 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a string' structure...

 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 H1

Histone H1 is one of the five main histone protein families which are components of chromatin in eukaryotic cells. Though highly conserved, it is nevertheless the most variable histone in sequence across species.- Structure :...

, Histone H3

Histone H3

Histone H3 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a string' structure...

, Methylated K9 Histone H3

Histone H3

Histone H3 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a string' structure...

, Histone H4

Histone H4

Histone H4 is one of the 5 main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N terminal tail, H4 is a structural component of the nucleosome, and is subject to covalent modification, including acetylation and methylation,...

, Histone methyltransferase

Histone methyltransferase

Histone methyltransferases are enzymes, histone-lysine N-methyltransferase and histone-arginine N-methyltransferase, that catalyze the transfer of one to three methyl groups from the cofactor S-Adenosyl methionine to lysine and arginine residues of histone proteins...

, DNA methyltransferase

DNA methyltransferase

In biochemistry, the DNA methyltransferase family of enzymescatalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions...

, Methyl CpG binding protein MeCP2

MECP2

MECP2 is a gene that provides instructions for making its protein product, MECP2, also referred to as MeCP2. MECP2 appears to be essential for the normal function of nerve cells. The protein seems to be particularly important for mature nerve cells, where it is present in high levels...

, and the origin recognition complex

Origin Recognition Complex

ORC or Origin Recognition Complex is a multi-subunit DNA binding complex that binds in all eukaryotes in an ATP-dependent manner to origins of replication....

protein ORC2