GPN2

GPN2 encodes a conserved GPN-loop GTPase that acts as an assembly chaperone in the biogenesis of RNA polymerases II and III (PMID:23267056). It operates within a network of GPN proteins, binding both Gpn3 and Npa3/Gpn1, and temperature-sensitive loss of Gpn2 function causes specific defects in the nuclear localization of RNAPII and RNAPIII, placing GPN proteins upstream of the dedicated import factor Iwr1 (PMID:23267056). Mechanistically, Gpn2 directly contacts the RNAPII subunit Rpb12 and cooperates with the assembly factor Rba50 to drive ordered subunit assembly: the Gpn2-Rba50 pair is required for formation of the Rpb3 subcomplex and then jointly recruits Rpb2 to the nascent polymerase, with Gpn2 facilitating the Rba50-Rpb2 association (PMID:29661922, PMID:35176321). A genome-wide localization screen established that this function is exquisitely specific, as RNAPII and RNAPIII subunits are the dominant proteins mislocalized upon Gpn2 inactivation, with CTD modification status linked to RNAPII nuclear distribution downstream of Gpn2 (PMID:34180355). When Gpn2 (or its partners Gpn1 and Gpn3) is inactivated, unassembled RNAPII subunits accumulate in the cytoplasm as reversible, nucleic-acid-free, Hsp82-containing condensates in a redox-sensitive stress response termed the RNAPII Assembly Stress Response (RASR) (PMID:35314265, PMID:41500282).

1. 2012 High

   Established that Gpn2 is part of a GPN protein interaction network and is functionally required for nuclear localization of RNA polymerases II and III, acting upstream of the import factor Iwr1.

   Evidence Temperature-sensitive allele genetics, polymerase localization microscopy, and epistasis with iwr1Δ NLS suppression in yeast

   PMID:23267056

   Open questions at the time

   • Did not define the biochemical step (assembly vs. import) at which Gpn2 acts
   • No direct RNAPII subunit contact identified
   • GTPase activity of Gpn2 not linked to function
2. 2018 High

   Identified the direct molecular target of Gpn2 in polymerase assembly, showing it contacts Rpb12 and cooperates with Rba50 to build the Rpb3 subcomplex.

   Evidence Co-immunoprecipitation, pulldown, and temperature-sensitive mutant analysis of RNAPII subcomplex assembly in yeast

   PMID:29661922

   Open questions at the time

   • Order of subunit addition relative to Rpb2 not yet resolved
   • No structural model of the Gpn2-Rba50-Rpb3 intermediate
   • Role of nucleotide binding/hydrolysis in assembly unaddressed
3. 2021 Medium

   Demonstrated the remarkable substrate specificity of Gpn2, with RNAPII/III subunits as the dominant mislocalized proteins, and connected CTD phosphorylation to RNAPII nuclear distribution.

   Evidence High-content GFP-tagged nuclear proteome localization screen plus CTD kinase/phosphatase mutant analysis in gpn2 mutant yeast

   PMID:34180355

   Open questions at the time

   • Mechanistic link between Ess1/CTD status and Gpn2 function not established
   • Screen reports localization, not direct binding
   • Specificity determinants on Gpn2 not mapped
4. 2022 Medium

   Resolved a later assembly step, showing Gpn2 and Rba50 cooperate to recruit Rpb2 after Rpb3 subcomplex formation, extending the role to RNAPIII assembly.

   Evidence Extragenic and multicopy suppressor mapping, Rba50 depletion with Rpb3-Rpb2 Co-IP, and genetic epistasis in yeast

   PMID:35176321

   Open questions at the time

   • Single lab; reciprocal in vitro reconstitution of Rpb2 recruitment absent
   • How suppressor variants restore assembly mechanistically unclear
   • Quantitative kinetics of assembly steps undefined
5. 2022 Medium

   Revealed the cellular consequence of failed assembly: Gpn2 inactivation drives reversible cytoplasmic condensation of unassembled RNAPII subunits (RASR) chaperoned by Hsp82.

   Evidence Fluorescence microscopy, FRAP, 1,6-hexanediol treatment, and biochemical fractionation of GFP-tagged subunits in gpn2 mutant yeast

   PMID:35314265

   Open questions at the time

   • Whether RASR is protective or pathological is not resolved
   • Mechanism of Hsp82 recruitment to foci unknown
   • Reversibility pathway / disaggregation machinery not identified
6. 2024 Low

   Mapped specific Gpn2 residues (Phe105Tyr, Leu164Pro) required for RNAPII assembly and surveyed genetic modifiers of gpn2 defects.

   Evidence Site-specific mutagenesis and large-scale multicopy suppressor screen (>30,000 colonies) in yeast

   PMID:39642114

   Open questions at the time

   • Suppressors (PAB1, CDC5, RGS2) are genetic interactions without established direct mechanism
   • Structural consequences of the residue substitutions not determined
   • Functional connection of identified suppressors to assembly untested
7. 2026 Medium

   Extended the RASR model by showing oxidative stress modulates foci formation and that RASR coordinates transcriptional reprogramming of ribosome biogenesis and metabolic genes.

   Evidence Fluorescence microscopy, FRAP, condensate biochemistry, oxidative stress treatment, and RNA-seq transcriptomics in GPN-inactivated yeast

   PMID:41500282

   Open questions at the time

   • Redox sensor governing foci formation not identified
   • Direct causal link between RASR and transcriptomic changes not established
   • Whether redox sensitivity reflects a normal Gpn2 regulatory mechanism is unknown

• How nucleotide binding/hydrolysis by the GPN-loop GTPase drives the ordered assembly cycle, and how the Gpn2-Gpn1-Gpn3 complex is structurally organized on nascent polymerase, remain unresolved.
• No structure of the GPN complex bound to RNAPII assembly intermediates
• Catalytic cycle of Gpn2 not linked to specific assembly transitions
• Function in organisms beyond yeast not characterized in the corpus