rDNA Condensation Promotes rDNA Separation from Nucleolar Proteins Degraded for Nucleophagy after TORC1 Inactivation

Abstract

Summary: Nutrient starvation and inactivation of target of rapamycin complex 1 (TORC1) protein kinase induce nucleophagy preferentially degrading only nucleolar components in budding yeast. Nucleolar proteins are relocated to sites proximal to the nucleus-vacuole junction (NVJ), where micronucleophagy occurs, whereas rDNA, which is embedded in the nucleolus under normal conditions, moves to NVJ-distal regions, causing rDNA dissociation from nucleolar proteins after TORC1 inactivation. This repositioning is mediated via chromosome linkage INM protein (CLIP)-cohibin complexes that tether rDNA to the inner nuclear membrane. Here, we show that TORC1 inactivation-induced rDNA condensation promotes the repositioning of rDNA and nucleolar proteins. Defects in condensin, Rpd3–Sin3 histone deacetylase (HDAC), and high-mobility group protein 1 (Hmo1), which are involved in TORC1 inactivation-induced rDNA condensation, compromised the repositioning and nucleophagic degradation of nucleolar proteins, although rDNA still escaped from nucleophagic degradation in these mutants. We propose a model in which rDNA condensation after TORC1 inactivation generates a motive force for the repositioning of rDNA and nucleolar proteins. : Nutrient starvation and TORC1 inactivation induce the repositioning of rDNA and nucleolar proteins and nucleophagic degradation of nucleolar proteins in budding yeast. Mostofa et al. report that condensin, Rpd3 HDAC, and Hmo1 promote rDNA condensation, as well as the repositioning and nucleophagic degradation of nucleolar proteins. Keywords: condensin, Hmo1, nucleolus, nucleophagy, rDNA, Rpd3, target of rapamycin complex 1, TORC1