Polycomb-group proteins
Encyclopedia
Polycomb-group proteins are a family of proteins
first discovered in fruit flies that can remodel chromatin
such that epigenetic silencing of gene
s takes place. Polycomb-group proteins are best known for silencing Hox genes through modulation of chromatin structure during embryonic development in fruit flies (Drosophila melanogaster
).
, the Trithorax-group
(trxG) and Polycomb-group (PcG) proteins 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 involves at least three kinds of multiprotein complex PRC1, PRC2
and PhoRC which work together to carry out their repressive effect.
The mammalian PRC1 core complexes are very similar to Drosophila.
Polycomb is known to regulate ink4 locus (P16, P19ARF).
In Physcomitrella patens
the PcG protein FIE is specifically expressed in stem cell
s such as the
unfertilized egg cell (see figure, right) as indicated by the blue colour after histochemical GUS staining
. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo
.
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
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
.
Protein family
A protein family is a group of evolutionarily-related proteins, and is often nearly synonymous with gene family. The term protein family should not be confused with family as it is used in taxonomy....
first discovered in fruit flies that can remodel chromatin
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
such that epigenetic silencing of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s takes place. Polycomb-group proteins are best known for silencing Hox genes through modulation of chromatin structure during embryonic development in fruit flies (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...
).
In insects
In DrosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, the Trithorax-group
Trithorax-group proteins
Trithorax-group proteins are chromatin regulatory proteins which generally act to maintain gene expression. This group of proteins is rather heterogeneous and contains members that form several multiprotein chromatin complexes...
(trxG) and Polycomb-group (PcG) proteins act antagonistically and interact with chromosomal elements, termed Cellular Memory Modules
Cellular Memory Modules
Cellular memory modules refers to the ability of cells to remember and propagate their gene expression programs throughout the entire development. This basic developmental function has been conserved during evolution, as related mechanisms were identified in other model organisms...
(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 involves at least three kinds of multiprotein complex PRC1, PRC2
PRC2
PRC2 is one of the two classes of Polycomb-group proteins or . The other component of this group of proteins is PRC1....
and PhoRC which work together to carry out their repressive effect.
In mammals
In humans Polycomb Group gene expression is important in many aspects of development. 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 types.The mammalian PRC1 core complexes are very similar to Drosophila.
Polycomb is known to regulate ink4 locus (P16, P19ARF).
In plants
Physcomitrella patens
Physcomitrella patens is a moss used as a model organism for studies on plant evolution, development and physiology.-Model organism:...
the PcG protein FIE is specifically expressed in stem cell
Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...
s such as the
unfertilized egg cell (see figure, right) as indicated by the blue colour after histochemical GUS staining
GUS reporter system
The GUS reporter system is a reporter gene system, particularly useful in plant molecular biology. Several kinds of GUS reporter gene assay are actually available, depending on the substrate used...
. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
.
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
Flowering Locus C 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 FLC is expressed , and flowering becomes possible...
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
Vernalization is the acquisition of a plant's ability to flower or germinate in the spring by exposure to the prolonged cold of winter...
.
Further reading
- Chromatin organization and the Polycomb and Trithorax groups in The Interactive Fly
- Drosophila Genes in Development: Polycomb-group in the Homeobox Genes DataBase
- polycomb group proteins on humpath.com
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.