EXOSC7

EXOSC7 (Rrp42) is a structural subunit of the nine-subunit human RNA exosome core, where it contributes to the barrel architecture that scaffolds processive ribonucleases in both the nucleus and cytoplasm. X-ray crystallography of the reconstituted human exosome core at 3.35 Å demonstrated that EXOSC7 is integrated into the hexameric ring, and the assembled nine-subunit core is catalytically inert, instead serving as a platform for the processive RNases hDIS3 (nuclear) and hDIS3L (cytoplasmic) (PMID:17174896, PMID:20531386). Through this scaffolding role the exosome core participates in AU-rich element (ARE)-mediated mRNA decay, ZAP-directed degradation of HIV-1 transcripts, and AID-dependent DNA deamination on immunoglobulin switch regions during class switch recombination (PMID:11719186, PMID:21876179, PMID:21255825). EXOSC7 is one of three subunits (with EXOSC2 and EXOSC4) that nucleate hierarchical exosome assembly, with orphan unincorporated subunits cleared by the ubiquitin–proteasome system [PMID:bio_10.1101_2025.03.14.643291].

1. 2001 High

   Identification of EXOSC7 as a subunit of the human RNA exosome and demonstration that the complex is required for ARE-mRNA decay established the gene's involvement in regulated mRNA turnover.

   Evidence Mass spectrometry purification of the human exosome; cell-free ARE-mRNA decay assay with immunodepletion

   PMID:11719186

   Open questions at the time

   • Whether EXOSC7 contributes catalytic activity or is purely structural was unresolved
   • Stoichiometry and architecture of the human exosome were unknown
2. 2004 Medium

   Yeast two-hybrid mapping of intra-exosome contacts placed EXOSC7 adjacent to specific ring subunits, providing the first prediction of the internal organization of the barrel.

   Evidence Yeast two-hybrid interaction mapping among 38 human mRNA decay proteins

   PMID:15231747

   Open questions at the time

   • Binary yeast two-hybrid contacts lacked structural validation at atomic resolution
   • Direct versus bridged interactions could not be distinguished
3. 2006 High

   Reconstitution and X-ray crystallography of the nine-subunit human exosome at 3.35 Å resolved EXOSC7's position in the barrel ring and showed that the full core displays processive phosphorolytic activity, directly answering the question of subunit architecture.

   Evidence Recombinant reconstitution of nine-subunit human exosome; X-ray crystallography (3.35 Å); exoribonuclease activity assays

   PMID:17174896

   Open questions at the time

   • Whether the in vivo exosome core retains catalytic activity or is inert awaited resolution
   • Association with processive catalytic subunits (Dis3 family) was not yet characterized in human cells
4. 2010 High

   Demonstration that the human exosome core is catalytically inert and serves as a scaffold for compartment-specific ribonucleases (hDIS3 nuclear, hDIS3L cytoplasmic) redefined EXOSC7's role as purely structural.

   Evidence Co-immunoprecipitation, subcellular fractionation, RNase activity assays, immunofluorescence in human cells

   PMID:20531386

   Open questions at the time

   • How RNA is channeled through the barrel to the catalytic subunits remained structurally unclear
   • Contribution of individual barrel subunits to substrate specificity was unknown
5. 2011 High

   Two studies expanded the functional repertoire of the exosome core beyond mRNA decay: the core was shown to associate with AID to direct DNA deamination on both strands during class switch recombination, and to mediate ZAP-directed degradation of HIV-1 transcripts.

   Evidence ChIP, co-immunoprecipitation in activated B cells, in vitro transcription-coupled DNA deamination assay with recombinant exosome core (CSR study); siRNA knockdown of exosome subunits with HIV-1 infection assays (ZAP study)

   PMID:21255825 PMID:21876179

   Open questions at the time

   • Whether EXOSC7 specifically contacts AID or is engaged indirectly through other core subunits was not determined
   • The mechanism by which the structural core promotes strand-specific AID activity is incompletely understood
   • EXOSC7 was not individually distinguished from other core subunits in the ZAP-mediated antiviral pathway
6. 2014 High

   A 3.3 Å crystal structure of the ten-subunit exosome (Exo9 + Rrp6) revealed the RNA channeling path through the barrel and showed that cap-ring integrity is required for directing substrates to Rrp6, clarifying how the barrel scaffold including EXOSC7 organizes RNA processing.

   Evidence X-ray crystallography (3.3 Å, yeast) with validation using human exosome complexes in solution RNA-binding assays and mutagenesis

   PMID:25043052

   Open questions at the time

   • Individual contributions of barrel subunits (including EXOSC7) to RNA threading were not dissected
   • Structural basis for how the exosome core directs AID activity was not addressed
7. 2025 Medium

   Systematic CRISPR knockout revealed that EXOSC7, together with EXOSC2 and EXOSC4, acts as an early nucleator of hierarchical exosome assembly, with orphan subunits degraded by the ubiquitin–proteasome system — establishing a quality-control pathway for exosome biogenesis.

   Evidence Inducible CRISPR/Cas9 knockout of individual exosome subunits in mESCs; quantitative mass spectrometry; proteasome inhibition (preprint)

   PMID:bio_10.1101_2025.03.14.643291

   Open questions at the time

   • Preprint awaiting peer review
   • Whether the nucleation hierarchy is conserved in differentiated human cell types is untested
   • The E3 ligase(s) responsible for ubiquitination of orphan subunits were not identified

• It remains unresolved how the structural barrel, and EXOSC7 specifically, contributes to substrate selectivity among diverse RNA and DNA substrates processed by the exosome, and whether EXOSC7 mutations cause human disease.
• No structure of a human exosome–AID complex exists
• No disease-causing mutations in EXOSC7 have been reported
• Individual contribution of EXOSC7 versus other barrel subunits to RNA threading specificity is unknown