Ccq1-Raf2 interaction mediates CLRC recruitment to establish heterochromatin at telomeres.

Shaohua Shi,Hong Sun,Jing Xue,Zhenfang Wu,Yuanze Zhou,Ming Lei,Yanjia Lu

doi:10.26508/lsa.202101106

Abstract

Telomeres, highly ordered DNA-protein complexes at eukaryotic linear chromosome ends, are specialized heterochromatin loci conserved among eukaryotes. In Schizosaccharomyces pombe, the shelterin complex is important for subtelomeric heterochromatin establishment. Despite shelterin has been demonstrated to mediate the recruitment of the Snf2/histone deacetylase-containing repressor complex (SHREC) and the Clr4 methyltransferase complex (CLRC) to telomeres, the mechanism involved in telomeric heterochromatin assembly remains elusive due to the multiple functions of the shelterin complex. Here, we found that CLRC plays a dominant role in heterochromatin establishment at telomeres. In addition, we identified a series of amino acids in the shelterin subunit Ccq1 that are important for the specific interaction between Ccq1 and the CLRC subunit Raf2. Finally, we demonstrated that the Ccq1-Raf2 interaction is essential for the recruitment of CLRC to telomeres, that contributes to histone H3 lysine 9 methylation, nucleosome stability and the shelterin-chromatin association, promoting a positive feedback mechanism for the nucleation and spreading of heterochromatin at subtelomeres. Together, our findings provide a mechanistic understanding of subtelomeric heterochromatin assembly by shelterin-dependent CLRC recruitment to chromosomal ends.

Highlights

Eukaryotic genomes are organized into structurally and functionally distinct domains, with euchromatin and heterochromatin transcriptionally active and repressed, respectively (Jenuwein & Allis, 2001)
The Clr4 methyltransferase complex (CLRC) complex plays a dominant role in transcriptional silencing at telomeres To investigate the specific role of Ccq1 in telomeric heterochromatin establishment, we used two Ccq1 truncation mutants, Ccq11–500 and Ccq11–436
Because Ccq11–436 is sufficient for the interaction with Tpz1 (Jun et al, 2013; Harland et al, 2014; Armstrong et al, 2018), the failure of Ccq11–500 and Ccq11–436 truncation mutants in heterochromatin formation at telomeres might be due to defect in recruitment of the Snf2/histone deacetylase–containing repressor complex (SHREC) and/or CLRC complexes to telomeres

Summary

Introduction

Eukaryotic genomes are organized into structurally and functionally distinct domains, with euchromatin and heterochromatin transcriptionally active and repressed, respectively (Jenuwein & Allis, 2001). In fission yeast Schizosaccharomyces pombe, heterochromatin is mainly present at three regions, centromeres, silent mating-type loci and telomeres (Grewal, 2000). Extensive studies on heterochromatin assembly at centromeres and silent mating-type loci have revealed RNAi- and DNA-mediated pathways of heterochromatin nucleation in S. pombe (Hall et al, 2002; Volpe et al, 2002; Jia et al, 2004). Heterochromatin nucleation by DNA-binding factors was revealed by the observation that Atf1/Pcr functions in parallel with RNAi pathway to nucleate heterochromatin at silent mating-type loci (Jia et al, 2004). In contrast to the well-characterized mechanisms of heterochromatin assembly at centromeres and silent mating-type loci, the nucleation and maintenance of heterochromatin at telomeres are still not fully understood

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