The Distribution of Polycomb-Group Proteins During Cell Division and Development in Drosophila Embryos: Impact on Models for Silencing

Abstract

The subcellular three-dimensional distribution of three polycomb-group (PcG) proteins—polycomb, polyhomeotic and posterior sex combs—in fixed whole-mount Drosophila embryos was analyzed by multicolor confocal fluorescence microscopy. All three proteins are localized in complex patterns of 100 or more loci throughout most of the interphase nuclear volume. The rather narrow distribution of the protein intensities in the vast majority of loci argues against a PcG-mediated sequestration of repressed target genes by aggregation into subnuclear domains. In contrast to the case for PEV repression (Csink, A.K., and S. Henikoff. 1996. Nature. 381:529–531), there is a lack of correlation between the occurrence of PcG proteins and high concentrations of DNA, demonstrating that the silenced genes are not targeted to heterochromatic regions within the nucleus. There is a clear distinction between sites of transcription in the nucleus and sites of PcG binding, supporting the assumption that most PcG binding loci are sites of repressive complexes. Although the PcG proteins maintain tissue-specific repression for up to 14 cell generations, the proteins studied here visibly dissociate from the chromatin during mitosis, and disperse into the cytoplasm in a differential manner. Quantitation of the fluorescence intensities in the whole mount embryos demonstrate that the dissociated proteins are present in the cytoplasm. We determined that <2% of PH remains attached to late metaphase and anaphase chromosomes. Each of the three proteins that were studied has a different rate and extent of dissociation at prophase and reassociation at telophase. These observations have important implications for models of the mechanism and maintenance of PcG- mediated gene repression.

Maintenance of Integrated Proviral Gene Expression Requires Brm, a Catalytic Subunit of SWI/SNF Complex

“…However, this gene family does not usually overlap with Pc-G or trx-G. Moreover, Pc-G proteins, unlike the HP-1 family proteins, do not appear to favor areas in the nucleus where there are visible compactions of chromatin (37). These observations suggest that the molecular mechanisms involving HP-1 family proteins and Pc-G proteins are different.…”

Functional Characterization of the dRYBP Gene in Drosophila

“…Although dRYBP mutations alone do not result in homeotic-like phenotypes, they do when in combination with mutations in some PcG and trxG genes. We have found that the dRYBP protein is localized in a nuclear pattern and colocalizes with some of the Polycomb nuclear bodies (Buchenau et al 1998;Saurin et al 1998;Netter et al 2001;Ficz et al 2005;Grimaud et al 2006a). Moreover, the protein is dynamically distributed during the mitotic cycle in the syncytial embryo.…”

A human homolog of Drosophila lethal(3)malignant brain tumor (l(3)mbt) protein associates with condensed mitotic chromosomes

“…However, our double-labeling experiment with anti-hl(3)mbt antibody and the anti-Bmi1 antibody showed that h-l(3)mbt protein does not colocalize with Bmi1 protein. In addition, h-l(3)mbt protein associates strongly with the condensed chromosome during mitosis, whereas PcG proteins including Bmi1 are known to dissociate from metaphase chromosomes and disperse into the cytoplasm (Buchenau et al, 1998). These lines of evidence suggest that h-l(3)mbt protein is not a member of the family of previously identi®ed PcG proteins and potentially compose another chromatin-binding protein complex.…”