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

Abstract

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.

Keywords: Hmo1; Rpd3; TORC1; condensin; nucleolus; nucleophagy; rDNA; target of rapamycin complex 1.