Lifting the ban on nuclear import activates Gdown1-mediated modulation of global transcription and facilitates adaptation to cellular stresses

Abstract

Dynamic regulation of transcription is crucial for cellular response to various environmental or developmental cues. Gdown1 is a ubiquitously expressed, RNA polymerase II (Pol II) interacting protein, essential for embryonic development. It tightly binds Pol II in vitro and competitively blocks binding of TFIIF and other transcriptional regulatory factors, yet its cellular functions and regulatory circuits remain unclear. Here, we show that Gdown1 strictly localizes in the cytoplasm of mammalian somatic cells and exhibits potent resistance to the imposed driving force for nuclear localization. Combined with genetic and microscope-based approaches, two types of functionally coupled and evolutionally conserved localization regulatory motifs are identified, including the CRM1-dependent nucleus export signal (NES) and a novel Cytoplasm Anchoring Signal (CAS) which mediates nuclear pore retention. Mutagenesis of CAS alleviates the cytoplasmic retention activity thus unlocks its nucleocytoplasmic shuttling properties, and increased nuclear import of Gdown1 causes drastic reduction of Pol II levels and global transcription. Importantly, nuclear translocation of Gdown1 occurs in a stress-responsive manner and ablation of GDOWN1 significantly weakens cellular tolerance. Collectively, our work uncovers the molecular basis of the localization of Gdown1 and highlights that its controlled nuclear translocation serves as a key strategy in modulating global transcription and stress-adaptation.