RNA interference (RNAi) is critical for the assembly of heterochromatin at Schizosaccharomyces pombe centromeres. Central to this process is the RNA-induced initiation of transcriptional gene silencing (RITS) complex, which physically anchors small noncoding RNAs to chromatin. RITS includes Ago1, the chromodomain protein Chp1, and Tas3, which forms a bridge between Chp1 and Ago1. Chp1 is a large protein with no recognizable domains, apart from its chromodomain. Here we describe how the structured C-terminal half of Chp1 binds the Tas3 N-terminal domain, revealing the tight association of Chp1 and Tas3. The structure also shows a PIN domain at the C-terminal tip of Chp1 that controls subtelomeric transcripts through a post-transcriptional mechanism. We suggest that the Chp1–Tas3 complex provides a solid and versatile platform to recruit both RNAi-dependent and RNAi-independent gene-silencing pathways for locus-specific regulation of heterochromatin.
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We thank C. Ashar and A. Kosinski for technical support and O. George and B. Lowe for assistance, A. Héroux for help at the National Synchrotron Light Source, which is supported by Department of Energy, Office of Basic Energy Sciences. T.S. was supported by a Human Frontiers in Research Program fellowship. This work was supported by US National Institutes of Health grant R01GM084045, NCI Cancer Center support grant P30CA021765 and the American Lebanese Syrian Associated Charities (to J.F.P.), and the Louis Morin Charitable Trust (to L.J.). L.J. is an investigator of the Howard Hughes Medical Institute. We thank G. Thon, T. Nakamura, R. Martienssen and M. Zaratiegui for helpful discussions, I. Berger (European Molecular Biology Laboratory) and T.J. Richmond (Eidgenössische Technische Hochschule Zürich) for the MultiBac system, G. Thon (University of Copenhagen) and R. Allshire (University of Edinburgh) for strains, and K. Gull (University of Manchester) for the anti-TAT1 antibody.
The authors declare no competing financial interests.
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Schalch, T., Job, G., Shanker, S. et al. The Chp1–Tas3 core is a multifunctional platform critical for gene silencing by RITS. Nat Struct Mol Biol 18, 1351–1357 (2011). https://doi.org/10.1038/nsmb.2151
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