CHIP, VCP, and Nucleolar Gatekeepers Remodel the Nucleolus into a Stress-Responsive Proteostasis Hub

SUMMARY The nucleolus, classically dedicated to ribosome biogenesis, also acts as a stress-responsive proteostasis hub. During heat shock, misfolded proteins accumulate in its granular component (GC), but whether this is passive or regulated has been unclear. We show that the ubiquitin ligase CHIP remodels the nucleolus into a reversible protein quality control (PQC) compartment by diverting HSP70 from refolding toward sequestration and transiently suppressing rRNA synthesis. This process requires the segregase VCP, which mediates ubiquitinated substrate flux and couples nucleolar PQC with nuclear and ER stress responses. Functional genomics identify NOL6 and WDR55 as intrinsic gatekeepers with opposing effects on sequestration. We further define three structural states - free-flow, peripheral, and sealed - that encode PQC capacity and recovery potential. Thus, nucleolar proteostasis is revealed as an actively regulated continuum, linking chaperones, ubiquitination, and ribosome biogenesis with global stress adaptation. Highlights CHIP remodels the nucleolus into a reversible protein quality control (PQC) compartment during heat stress VCP drives flux of ubiquitinated substrates and prevents premature resumption of rRNA synthesis NOL6 promotes, while WDR55 restricts, nucleolar storage of misfolded proteins Distinct nucleolar morphologies (free-flow, peripheral, sealed) encode PQC states Ubiquitination and deubiquitination act as switchable signals, not passive damage marks Nucleolar PQC is integrated with ISR, ER stress, and stress granule pathways Competing Interest Statement The authors have declared no competing interest. Footnotes ↵8 Lead contact