Reversible cytoplasmic foci of RNA polymerase II subunits serve as proteostatic hubs orchestrating transcriptional...

Prolonged defects in RNA polymerase II (RNAPII) assembly lead to the accumulation of its subunits in cytoplasmic foci, but the cellular consequences of this phenomenon remain unclear. Building on our previous discovery that GPN3 dysfunction induces reversible formation of Rpb1 foci, a process termed the RNAPII Assembly Stress Response (RASR), we examined whether RASR represents a general response to RNAPII assembly defects. Here, we show that inactivation of all three GPN proteins (Npa3/Gpn1, Gpn2, and Gpn3) results in reversible accumulation of Rpb1, Rpb2, and Rpb3 in cytoplasmic foci, establishing RASR as a conserved pathway triggered by impaired polymerase assembly. We also identify the molecular chaperone Hsp82 as a component that accumulates in these foci and partially colocalizes with them. Biochemical analyses indicate that the condensates are protein based and nucleic acid free, resist dissolution by 1,6-hexanediol, and show dynamic behavior in fluorescence recovery after photobleaching (FRAP) experiments. Transcriptomic profiling reveals coordinated regulation of ribosome biogenesis genes and metabolic pathways, including amino acid metabolism, the TCA cycle, and purine biosynthesis. Oxidative stress induced by H₂O₂ further increases foci formation, highlighting the redox sensitivity of this process. Together, these findings support a model in which RASR-induced cytoplasmic foci function as a proteostatic quality-control hub that integrates molecular chaperoning and metabolic adaptation during transcriptional stress, thereby helping to maintain the fidelity of eukaryotic gene expression.