H3K9 methylation extends across natural boundaries of heterochromatin in the absence of an HP1 protein

EMBO J. 2015 Nov 12;34(22):2789-803. doi: 10.15252/embj.201591320. Epub 2015 Oct 5.


Proteins of the conserved HP1 family are elementary components of heterochromatin and are generally assumed to play a central role in the creation of a rigid, densely packed heterochromatic network that is inaccessible to the transcription machinery. Here, we demonstrate that the fission yeast HP1 protein Swi6 exists as a single highly dynamic population that rapidly exchanges in cis and in trans between different heterochromatic regions. Binding to methylated H3K9 or to heterochromatic RNA decelerates Swi6 mobility. We further show that Swi6 is largely dispensable to the maintenance of heterochromatin domains. In the absence of Swi6, H3K9 methylation levels are maintained by a mechanism that depends on polymeric self-association properties of Tas3, a subunit of the RNA-induced transcriptional silencing complex. Our results disclose a surprising role for Swi6 dimerization in demarcating constitutive heterochromatin from neighboring euchromatin. Thus, rather than promoting maintenance and spreading of heterochromatin, Swi6 appears to limit these processes and appropriately confine heterochromatin.

Keywords: H3K9 methylation; HP1 dynamics; Swi6; Tas3; heterochromatin.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism
  • Chromosomal Proteins, Non-Histone / genetics
  • Chromosomal Proteins, Non-Histone / metabolism*
  • Heterochromatin / genetics
  • Heterochromatin / metabolism*
  • Histones / genetics
  • Histones / metabolism*
  • Methylation
  • Protein Multimerization / physiology*
  • RNA, Fungal / genetics
  • RNA, Fungal / metabolism
  • Schizosaccharomyces / genetics
  • Schizosaccharomyces / metabolism*
  • Schizosaccharomyces pombe Proteins / genetics
  • Schizosaccharomyces pombe Proteins / metabolism*


  • Carrier Proteins
  • Chromosomal Proteins, Non-Histone
  • Heterochromatin
  • Histones
  • RNA, Fungal
  • Schizosaccharomyces pombe Proteins
  • Swi6 protein, S pombe
  • Tas3 protein, S pombe

Associated data

  • GEO/GSE42850
  • GEO/GSE61136
  • GEO/GSE70945
  • GEO/GSE70946