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First published online 27 November 2002
doi: 10.1242/jcs.00225
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Research Article |
1 Centre of Electron Microscopy, University of Lausanne, 27 Bugnon, CH-1005
Lausanne, Switzerland
2 Swammerdam Institute for Life Sciences, BioCentrum Amsterdam, University of
Amsterdam, P.O. Box 94062, 1090 GB Amsterdam, The Netherlands
* Author for correspondence (e-mail: sfakan{at}cme.unil.ch)
Accepted 15 October 2002
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.
Key words: PcG proteins, Gene silencing, Chromatin, Immunoelectron microscopy, Transcription
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