H3K9 methylation (H3K9me) specifies the establishment and maintenance of transcriptionally silent epigenetic states or heterochromatin. The enzymatic erasure of histone modifications is widely assumed to be the primary mechanism that reverses epigenetic silencing. Here, we reveal an inversion of this paradigm where a putative histone demethylase Epe1 in fission yeast, has a non-enzymatic function that opposes heterochromatin assembly. Mutations within the putative catalytic JmjC domain of Epe1 disrupt its interaction with Swi6HP1 suggesting that this domain might have other functions besides enzymatic activity. The C-terminus of Epe1 directly interacts with Swi6HP1, and H3K9 methylation stimulates this protein-protein interaction in vitro and in vivo. Expressing the Epe1 C-terminus is sufficient to disrupt heterochromatin by outcompeting the histone deacetylase, Clr3 from sites of heterochromatin formation. Our results underscore how histone modifying proteins that resemble enzymes have non-catalytic functions that regulate the assembly of epigenetic complexes in cells.

In the last decade, Hsp90 has emerged as a major regulator of cancer cell growth and proliferation. In cancer cells, it assists in giving maturation to oncogenic proteins including several kinases and transcription factors (TF). Recent studies have shown that apart from its chaperone activity, it also imparts regulation of transcription machinery and thereby alters the cellular physiology. Hsp90 and its co-chaperones modulate transcription at least at three different levels. In the first place, they alter the steady-state levels of certain TFs in response to various physiological cues. Second, they modulate the activity of certain epigenetic modifiers, such as histone deacetylases or DNA methyl transferases, and thereby respond to the change in the environment. Third, they participate in the eviction of histones from the promoter region of certain genes and thereby turn on gene expression. In this review, we discuss the role of Hsp90 in all the three aforementioned mechanisms of transcriptional control, taking examples from various model organisms with a special emphasis on cancer progression.

The molecular mechanism behind hyperthermia coupled to radiation-induced DNA damage sensitivity is not known. The model organism Saccharomyces cerevisiae is used to establish that a transient heat shock and particularly the concomitant induction of Hsp90 lead to increased genomic instability via transcriptional regulation of the major checkpoint kinase Rad53.

Recently, Hsp90 functional loss has been linked to aneuploidy; however, until now none of the components of sister chromatid cohesion (SCC) have been demonstrated as the putative clients of Hsp90. In this study, we have established that Chl1, the protein which is involved in maintaining sister chromatid cohesion as well as in preventing chromosome loss, is a direct client of Hsp90. Thus, with understanding of the molecular mechanism, how Hsp90 controls the cohesion machinery might reveal new insights which can be exploited further for attenuation of tumorigenesis.

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H3K9 methylation (H3K9me) specifies the establishment and maintenance of transcriptionally silent epigenetic states or heterochromatin. The enzymatic erasure of histone modifications is widely assumed to be the primary mechanism that reverses epigenetic silencing. Here, we reveal an inversion of this paradigm where a putative histone demethylase Epe1 in fission yeast, has a non-enzymatic function that opposes heterochromatin assembly. An auto-inhibitory conformation regulates the non-enzymatic properties of Epe1 and licenses its interaction with Swi6 HP1 . Mutations that map to the putative catalytic JmjC domain preserve Epe1 in an auto-inhibited state which disrupts its localization to sites of heterochromatin formation and interaction with Swi6 HP1 . H3K9me relieves Epe1 from auto-inhibition and stimulates its ability to form an inhibitory complex with Swi6 HP1 in vitro and in cells. Our work reveals that histone modifications, rather than being passive scaffolds, actively participate in regulating the assembly of specific epigenetic complexes in cells.

Heterochromatin is characterized by histone hypoacetylation and an enrichment of methylation marks on histones which are associated with epigenetic silencing. In fission yeast, a single H3 methyltransferase Clr4 (Suv39h) directs all of the methylation of H3K9 i.e., H3K9me2/3 which enforces the establishment of transcriptionally silent regions in the genome referred to as heterochromatin. A putative histone demethylase, Epe1, is recruited to sites of heterochromatin via interaction with a chromo‐domain containing protein Swi6, homolog of HP1. The loss of Epe1 has been associated with the propagation of silent epigenetic states in a DNA or RNA sequence independent manner. How Epe1 carries out its function as an anti‐silencing factor remains unknown. Despite having sequence homology with known histone demethylases, Epe1 exhibits no detectable demethylase activity in vitro. Here, we demonstrate that mutations within the catalytic JmjC domain of Epe1 disrupt its interaction with Swi6. Using a combination of genetic and biochemical assays, we show that the interaction between Epe1 and Swi6 is direct, depends on the active conformation of the JmjC domain and is sensitive to mutations that affect co‐factor binding. Surprisingly, in cells the interaction between Epe1 and Swi6 requires functional heterochromatin. We reconstituted this dependence in vitro and demonstrate that the presence of an H3K9 tri‐methyl peptide enhances the interaction between the two proteins. We attribute this stimulatory effect of H3K9 methylation to an auto‐inhibitory role for the C‐terminal region of Epe1. Deleting this segment resulted in a dramatic increase in Swi6 binding and no further stimulation in the interaction between Epe1 and Swi6 upon addition of an H3K9me3 peptide. Finally, we demonstrate that this non‐enzymatic role of Epe1 regulates the binding of a histone deacetylase. Our work demonstrates that Epe1 selectively binds and inhibits heterochromatin bound Swi6 molecules which in turn impedes downstream heterochromatin assembly.Support or Funding InformationUniversity of MichiganThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.