The 19S proteasome is directly involved in the regulation of heterochromatin spreading in fission yeast
Hogyu David Seo1, Yoonjung Choi1, Minhoo Kim1, Keunsoo Kang2, Takeshi Urano3 and Daeyoup Lee1*
- Author Affiliations
1 Korea Advanced Institute of Science and Technology, Korea, Republic of;
2 Dankook University, Korea, Republic of;
3 Shimane University, Japan
↵* Corresponding author; email: daeyoup@kaist.ac.kr
Author contributions: H.D.S. and D.L. conceived and designed the project; H.D.S. performed most of the experiments and data analyses with input from D.L.; Y.C. performed the sRNA-seq experiments and analyses; M.K took part in the random mutagenesis; K.K. contributed to the bioinformatic analysis; T.U. provided the anti-H3K9me2 antibody; H.D.S. and D.L. drafted the manuscript; and all authors contributed to revising the manuscript and gave final approval for its publication.
Abstract
Cumulative evidence suggests that non-proteolytic functions of the proteasome are involved in transcriptional regulation, mRNA export and ubiquitin-dependent histone modification, and thereby modulate the intracellular levels of regulatory proteins implicated in controlling key cellular functions. To date, the non-proteolytic roles of the proteasome have been mainly investigated in euchromatin; their effects on heterochromatin are largely unknown. Here, using fission yeast as a model, we randomly mutagenized the subunits of the 19S proteasome subcomplex, and sought to uncover a direct role of the proteasome in heterochromatin regulation. We identified a non-proteolytic allele, rpt4-1. Experiments performed using rpt4-1 cells revealed that the proteasome is involved in the regulation of heterochromatin spreading to prevent its uncontrolled invasion into neighboring euchromatin regions. Intriguingly, the phenotype of the non-proteolytic rpt4-1 mutant resembled that of epe1Δ cells, which lack the Epe1 protein that counteracts heterochromatin spreading. Both mutants exhibited variegated gene-silencing phenotypes across yeast colonies, spreading of heterochromatin, bypassing of the requirement for RNAi in heterochromatin formation at the outer repeat region (otr), and upregulation of RNA polymerase II. Further analysis revealed Mst2, another factor that antagonizes heterochromatin spreading, may function redundantly with Rpt4. These observations suggest that the 19S proteasome may be involved in modulating the activities of Epe1 and Mst2. In conclusion, our findings indicate that the proteasome appears to have a heterochromatin-regulating function that is independent of its canonical function in proteolysis.
chromatin regulation epigenetics heterochromatin proteasome RNA interference (RNAi) 19S RP Epe1 RNAi heterochromatin spreading non-proteolytic function
Received April 11, 2017. Accepted August 7, 2017.
Copyright © 2017, The American Society for Biochemistry and Molecular Biology
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