Polycomb-group proteins (PcG proteins) are a family of protein complexes first discovered in fruit flies that can remodel

chromatin Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...

such that epigenetic silencing of

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

s takes place. Polycomb-group proteins are well known for silencing

Hox gene Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the cor ...

s through modulation of chromatin structure during embryonic development in fruit flies (''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...

''). They derive their name from the fact that the first sign of a decrease in PcG function is often a homeotic transformation of posterior legs towards anterior legs, which have a characteristic comb-like set of bristles.

In insects

In ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...

'', the Trithorax-group (trxG) and Polycomb-group (PcG) proteins (They derive their name from the fact that the first sign of a decrease in PcG function is often a homeotic transformation of posterior legs towards anterior legs, which have a characteristic comb-like set of bristles) act antagonistically and interact with chromosomal elements, termed Cellular Memory Modules (CMMs). Trithorax-group (trxG) proteins maintain the active state of gene expression while the Polycomb-group (PcG) proteins counteract this activation with a repressive function that is stable over many cell generations and can only be overcome by germline differentiation processes. Polycomb Gene complexes or PcG silencing consist of at least three kinds of multiprotein complex Polycomb Repressive Complex 1 (PRC1),

PRC2 PRC2 (polycomb repressive complex 2) is one of the two classes of polycomb-group proteins or (PcG). The other component of this group of proteins is PRC1 ( Polycomb Repressive Complex 1). This complex has histone methyltransferase activity and ...

and PhoRC. These complexes work together to carry out their repressive effect. PcGs proteins are evolutionarily conserved and exist in at least two separate protein complexes; the PcG repressive complex 1 (PRC1) and the PcG repressive complex 2–4 (PRC2/3/4). PRC2 catalyzes trimethylation of lysine 27 on histone H3 (H3K27me2/3), while PRC1 mono- ubiquitinates histone H2A on lysine 119 (H2AK119Ub1).

In mammals

In mammals Polycomb Group gene expression is important in many aspects of development like homeotic gene regulation and

X chromosome inactivation X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into a ...

, being recruited to the inactive X by Xist RNA, the master regulator of XCI or embryonic stem cell self-renewal. The

Bmi1 Polycomb complex protein BMI-1 also known as polycomb group RING finger protein 4 (PCGF4) or RING finger protein 51 (RNF51) is a protein that in humans is encoded by the ''BMI1'' gene (B cell-specific Moloney murine leukemia virus integration ...

polycomb

ring finger The ring finger, third finger, fourth finger, leech finger, or annulary is the fourth digit of the human hand, located between the middle finger and the little finger. Sometimes the term ring finger only refers to the fourth digit of a left-han ...

protein promotes neural stem cell self-renewal. Murine null mutants in PRC2 genes are embryonic lethals while most PRC1 mutants are live born homeotic mutants that die perinatally. In contrast overexpression of PcG proteins correlates with the severity and invasiveness of several

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

types. The mammalian PRC1 core complexes are very similar to Drosophila. Polycomb Bmi1 is known to regulate ink4 locus (p16^Ink4a, p19^Arf). Regulation of Polycomb-group proteins at

bivalent chromatin Bivalent chromatin are segments of DNA, bound to histone proteins, that have both repressing and activating epigenetic regulators in the same region. These regulators work to enhance or silence the expression of genes. Since these regulators work ...

sites is performed by

SWI/SNF In molecular biology, SWI/SNF (SWItch/Sucrose Non-Fermentable), is a subfamily of ATP-dependent chromatin remodeling complexes, which is found in eukaryotes. In other words, it is a group of proteins that associate to remodel the way DNA is pack ...

complexes, which oppose the accumulation of Polycomb complexes through ATP-dependent eviction.

In plants

In ''

Physcomitrella patens ''Physcomitrium patens'', (synonym: ''Physcomitrella patens'' ) the spreading earthmoss, is a moss (bryophyte) used as a model organism for studies on plant evolution, development, and physiology. Distribution and ecology ''Physcomitrella p ...

'' the PcG protein FIE is specifically expressed in stem cells such as the unfertilized egg cell. Soon after fertilisation the FIE gene is inactivated in the young

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

. The Polycomb gene FIE is expressed in unfertilised egg cells of the moss Physcomitrella patens and expression ceases after fertilisation in the developing diploid sporophyte. It has been shown that unlike in mammals the PcG are necessary to keep the cells in a differentiated state. Consequently, loss of PcG causes de-differentiation and promotes embryonic development. Polycomb-group proteins also intervene in the control of flowering by silencing the

Flowering Locus C Flowering Locus C (''FLC'') is a MADS-box gene that in late-flowering ecotypes of the plant '' Arabidopsis thaliana'' is responsible for vernalization. In a new seedling ''FLC'' is expressed, which prevents flowering. Upon exposure to cold, less ...

gene. This gene is a central part of the pathway that inhibits flowering in plants and its silencing during winter is suspected to be one of the main factors intervening in plant

vernalization Vernalization (from Latin ''vernus'', "of the spring") is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, ...

References

External links

*
The Polycomb and Trithorax page of the Cavalli lab
This page contains useful information on Polycomb and trithorax proteins, in the form of an introduction, links to published reviews, list of Polycomb and trithorax proteins, illustrative power point slides and a link to a genome browser showing the genome-wide distribution of these proteins in ''Drosophila melanogaster''.

in the Homeobox Genes DataBase

in The Interactive Fly * {{MeshName, polycomb+group+proteins Protein families Drosophila melanogaster genes Nuclear organization

In insects

In mammals

In plants

See also

References

Further reading

External links