Abstract
Human Polycomb group (PcG) proteins are involved in cell-type-dependent epigenetic gene silencing in an evolutionarily conserved manner. We have analysed the subnuclear localisation of these regulatory proteins in two different human cell lines and in rat liver tissue by means of light and electron immunomicroscopy using specific antibodies. We find that the PcG proteins HPC2, HPH1, BMI1 and RING1 are highly concentrated in the perichromatin compartment, situated at the surface of condensed chromatin domains. This compartment was demonstrated earlier to be the nuclear site where most pre-mRNA synthesis takes place. Interestingly, these PcG proteins are virtually absent from the interior of condensed chromatin areas. The present observations therefore show that transcriptionally active and PcG-silenced loci occur within the same spatially limited nuclear domain. Our novel high-resolution data strongly support the idea that epigenetic PcG-mediated gene silencing is a local event, rather than affecting large chromatin domains. In addition to being associated with the perichromatin region, PcG proteins also occur in the interchromatin space. Implications of these observations for higher order chromatin structure and for the mechanisms of PcG-mediated gene silencing are discussed.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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Animals
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Carrier Proteins*
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Cell Compartmentation / genetics*
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Cell Differentiation / genetics
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Cell Nucleus / genetics
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Cell Nucleus / metabolism*
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Cell Nucleus / ultrastructure
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Chromatin / genetics
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Chromatin / metabolism*
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Chromatin / ultrastructure
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DNA-Binding Proteins / genetics
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DNA-Binding Proteins / metabolism
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Eukaryotic Cells / metabolism*
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Eukaryotic Cells / ultrastructure
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Gene Silencing / physiology*
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Homeodomain Proteins / genetics
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Homeodomain Proteins / metabolism
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Humans
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Immunohistochemistry
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Ligases
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Microscopy, Electron
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Nuclear Proteins / genetics
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Nuclear Proteins / metabolism
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Polycomb Repressive Complex 1
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Polycomb-Group Proteins
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Proto-Oncogene Proteins / genetics
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Proto-Oncogene Proteins / metabolism
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Rats
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Repressor Proteins / genetics
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Repressor Proteins / metabolism*
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Transcription, Genetic / genetics
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Tumor Cells, Cultured
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Ubiquitin-Protein Ligases
Substances
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BMI1 protein, human
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Bmi1 protein, mouse
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Bmi1 protein, rat
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Carrier Proteins
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Chromatin
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DNA-Binding Proteins
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Homeodomain Proteins
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Nuclear Proteins
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PHC1 protein, human
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Polycomb-Group Proteins
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Proto-Oncogene Proteins
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Repressor Proteins
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Polycomb Repressive Complex 1
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RING1 protein, human
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Ring1 protein, mouse
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Ubiquitin-Protein Ligases
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Ligases
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CBX4 protein, human