Cdc14 protein phosphatase and topoisomerase II mediate rDNA dynamics and nucleophagic degradation of nucleolar proteins after TORC1 inactivation

Abstract

Nutrient starvation and inactivation of target of rapamycin complex 1 (TORC1) protein kinase elicits nucleophagy degrading nucleolar proteins in budding yeast. After TORC1 inactivation, nucleolar proteins are relocated to sites proximal to the nucleus–vacuole junction (NVJ), where micronucleophagy occurs, whereas ribosomal DNA (rDNA encoding rRNA) escapes from the NVJ. Condensin-mediated rDNA condensation promotes the repositioning and nucleophagic degradation of nucleolar proteins. However, the molecular mechanism of TORC1 inactivation-induced chromosome condensation is still unknown. Here, we show that Cdc14 protein phosphatase and topoisomerase II (Topo II), which are engaged in rDNA condensation in mitosis, facilitate rDNA condensation after TORC1 inactivation. rDNA condensation after rapamycin treatment was compromised in cdc14-1 and top2-4 mutants. In addition, the repositioning of rDNA and nucleolar proteins and nucleophagic degradation of nucleolar proteins were impeded in these mutants. Furthermore, Cdc14 and Topo II were required for the survival of quiescent cells in prolonged nutrient-starved conditions. This study reveals that these factors are critical for starvation responses.