Non-proteolytic role of the proteasome in heterochromatin regulation

Abstract

To date, the non-proteolytic roles of the proteasome have been mainly investigated in euchromatin

their ef-fects on heterochromatin are largely unknown. Here, using fission yeast as a model, I randomly mutagenized the subunits of the 19S proteasome subcomplex, and sought to uncover a direct role of the proteasome in het-erochromatin regulation. I 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 uncon-trolled 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. Further analysis revealed Mst2, another factor that antagonizes heterochromatin spreading, may function re-dundantly with Rpt4. These observations suggest that the 19S proteasome may be involved in modulating the activities of Epe1 and Mst2. Furthermore, I also observed a distinct pathway that regulates subtelomere silenc-ing and facultative heterochromatin formation through the Paf1 complex subunit, Leo1. My comparison of the two pathways revealed a new group of heterochromatin domains that are regulated exclusively by the pro-teasome pathway. In conclusion, my findings indicate that the proteasome appears to have a heterochromatin-regulating function that is independent of its canonical function in proteolysis.