Polycomb-group proteins

Polycomb-group proteins are a family of proteins first discovered in fruit flies that can remodel chromatin such that epigenetic silencing of genes 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).

In insects
In Drosophila, 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.

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
In Physcomitrella patens the PcG protein FIE is specifically expressed in stem cells 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.