Balance between Distinct HP1 Family Proteins Controls Heterochromatin Assembly in Fission Yeast

Abstract

Heterochromatin protein 1 (HP1) is a conserved chromosomal protein with important roles in chromatin packaging and gene silencing. In fission yeast, two HP1 family proteins, Swi6 and Chp2, are involved in transcriptional silencing at heterochromatic regions, but how they function and whether they act cooperatively or differentially in heterochromatin assembly remain elusive. Here, we show that both Swi6 and Chp2 are required for the assembly of fully repressive heterochromatin, in which they play distinct, nonoverlapping roles. Swi6 is expressed abundantly and plays a dose-dependent role in forming a repressive structure through its self-association property. In contrast, Chp2, expressed at a lower level, does not show a simple dose-dependent repressive activity. However, it contributes to the recruitment of chromatin-modulating factors Clr3 and Epe1 and possesses a novel ability to bind the chromatin-enriched nuclear subfraction that is closely linked with its silencing function. Finally, we demonstrate that a proper balance between Swi6 and Chp2 is critical for heterochromatin assembly. Our findings provide novel insight into the distinct and cooperative functions of multiple HP1 family proteins in the formation of higher-order chromatin structure.

Elimination of shelterin components bypasses RNAi for pericentric heterochromatin assembly

“…We suspect that the ability of pot1D and tpz1D to bypass dcr1D is due to the loss of telomere and subtelomere sequences, which consequently affects silencing at telomeres. Such a model is consistent with recent imaging and biochemical studies showing that HP1/ Swi6 proteins are highly mobile on chromatin (Cheutin et al 2003(Cheutin et al , 2004Festenstein et al 2003;Sadaie et al 2008).…”

“…We obtained similar results with cells containing one additional copy of Swi6 inserted at the ars1 locus driven under the endogenous swi6 promoter, which is expected to increase Swi6 levels by twofold (Supplemental Fig. S8B; Sadaie et al 2008).…”

“…Chp2 is another HP1 homolog and is enriched at all heterochromatin domains (Thon and Verhein-Hansen 2000;Sadaie et al 2004;Motamedi et al 2008;Fischer et al 2009). However, incorporating an additional copy of chp2 + under the control of its endogenous promoter into the ars1 locus (Sadaie et al 2008) could not rescue dcr1D (Supplemental Fig. S8B).…”

“…H3K9me allows for the binding of heterochromatin protein 1 (HP1), which compacts chromatin through its self-association and the recruitment of other chromatin-modifying activities (Grewal and Jia 2007). Although heterochromatin has traditionally been considered silent, stable, and static, recent biochemical and imaging studies show that the binding of HP1 proteins to heterochromatin regions is surprisingly dynamic (Cheutin et al 2003(Cheutin et al , 2004Festenstein et al 2003;Sadaie et al 2008). Moreover, although heterochromatin normally excludes the transcriptional machinery, the DNA repeats within heterochromatin are transcribed, and the transcripts are processed by the RNAi pathway into siRNAs, which target histone-modifying complexes to repetitive DNA elements (Moazed, 2009;Lejeune and Allshire 2011;Castel and Martienssen 2013).…”

Diverse roles of HP1 proteins in heterochromatin assembly and functions in fission yeast

“…H3K9me has an important role in targeting HP1 family proteins that bind to methylated H3 tail via their amino-terminal chromodomain (7)(8)(9)(10). HP1 proteins also contain a carboxy-terminal chromoshadow domain that is critical for their dimerization and interactions with diverse factors (11)(12)(13). Indeed, HP1 proteins contribute to diverse chromosomal processes, including silencing of target loci, proper segregation of chromosomes, and developmentally controlled long-range chromatin interactions (1).…”