EXOSC2

EXOSC2 (RRP4) is an essential cap subunit of the RNA exosome complex required for efficient RNA degradation and normal development (PMID:31628467, PMID:19053279). Through its archaeal ortholog, EXOSC2 directly binds RNA substrates along an extended ~50-A binding path on each cap protomer, recruiting substrates at the cap periphery and progressively releasing them as they enter the catalytic barrel to reduce friction during translocation toward the active sites (PMID:28288106). The EXOSC2-containing cap confers a substrate preference for poly(A) RNA and longer substrates, and is required for efficient phosphorolytic degradation of structured and natural RNAs that the catalytic hexameric ring cannot process alone, while being dispensable for polyadenylation (PMID:19053279, PMID:20488184). Integrity of EXOSC2 is required for assembly and stability of the entire exosome: the pathogenic G198D mutation abolishes binding to other exosome subunits and destabilizes the complex, and wild-type but not G198D human EXOSC2 rescues Drosophila rrp4 loss of function (PMID:31628467). Biallelic EXOSC2 missense mutations cause the SHRF syndrome, with loss of exosome function dysregulating autophagy pathway gene expression (PMID:26843489, PMID:31628467).

1. 2008 High

   Established that the Rrp4/EXOSC2 cap subunit is functionally required for efficient RNA degradation, answering whether the catalytic core alone suffices for exosome activity.

   Evidence In vitro reconstitution of archaeal exosome subcomplexes with RNA degradation and polyadenylation assays

   PMID:19053279

   Open questions at the time

   • Done in archaeal system; human EXOSC2-specific contributions not directly tested
   • Mechanism by which the cap enables degradation of structured substrates not resolved
2. 2010 High

   Defined that the Rrp4/EXOSC2 cap dictates substrate selectivity, showing it imparts poly(A)-RNA and long-substrate preference distinct from the Csl4 cap.

   Evidence In vitro RNA processing assays comparing hexameric core, Rrp4-exosome and Csl4-exosome with defined substrates

   PMID:20488184

   Open questions at the time

   • Archaeal model; selectivity in human exosome not directly confirmed
   • Structural basis of preference not addressed
3. 2016 Medium

   Connected EXOSC2 to human disease, establishing it as a developmentally essential exosome component by linking biallelic missense variants to the SHRF syndrome.

   Evidence Whole exome sequencing of affected patients with variant confirmation across families

   PMID:26843489

   Open questions at the time

   • No in vitro biochemical mechanism for the variants in this study
   • Cellular consequence of mutations not established
4. 2017 High

   Resolved how the cap physically channels substrates, mapping a 50-A RNA binding path per protomer and a binding-affinity gradient that hands substrates to the catalytic barrel.

   Evidence Methyl-TROSY NMR spectroscopy with quantitative substrate-binding measurements at defined steps

   PMID:28288106

   Open questions at the time

   • Archaeal Rrp4 structure; human EXOSC2 path not directly mapped
   • Coupling of cap binding to catalytic turnover kinetics not fully resolved
5. 2020 High

   Demonstrated the mechanism of pathogenicity, showing G198D abolishes exosome assembly and dysregulates autophagy genes, and that wild-type EXOSC2 integrity is required for complex stability.

   Evidence Co-IP in patient lymphoblasts, CRISPR knockout in keratinocytes, Drosophila rrp4 cross-species rescue, and rapamycin autophagy stimulation

   PMID:31628467

   Open questions at the time

   • Direct link between exosome loss and specific autophagy gene transcripts not fully defined
   • Tissue-specific basis of SHRF phenotypes unresolved

• Whether the substrate-channeling and selectivity properties characterized in archaeal Rrp4 operate identically in the human EXOSC2-containing exosome, and how these connect mechanistically to the autophagy dysregulation seen in SHRF, remains open.
• No human EXOSC2 structural binding-path study in the corpus
• Direct RNA substrate repertoire of human EXOSC2-exosome not defined
• Mechanistic link between exosome substrate handling and autophagy gene regulation unestablished