EXOSC6

EXOSC6 (yeast Mtr3p) is an essential structural subunit of the conserved 3'→5' RNA exosome, an enzymatic ribonucleoprotein machine that performs nuclear RNA surveillance and turnover of coding and non-coding transcripts (PMID:8534909, PMID:36861343). Within the ten-subunit exosome it occupies the lower PH-like ring together with EXOSC4/7/8/9/5, the catalytic core that, via the EXOSC2 cap subunit, docks the RNA helicase MTR4 to feed substrates into the complex (PMID:36861343). Loss of Mtr3p function produces a coherent set of RNA-processing defects: nucleolar accumulation of poly(A)+ RNA with disorganized nucleolar morphology and blocked rRNA synthesis/processing (PMID:8534909), accumulation of deadenylated and 3'-truncated mRNAs distinct from the Rrp6p-dependent pathway (PMID:16260613), and impaired degradation of cryptic non-coding transcripts arising from telomeric, rDNA intergenic spacer, and centromeric repressed chromatin loci (PMID:18007593). Beyond RNA metabolism, mtr3 mutants also display microtubule and spindle-orientation abnormalities, linking exosome RNA-processing activity to cell-cycle progression (PMID:21919057). The available corpus characterizes EXOSC6 largely through its yeast ortholog; no human-specific biochemical or disease-causative finding is present in the timeline.

1. 1995 Medium

   Established Mtr3p as an essential nucleolar factor whose loss blocks nuclear RNA export and ribosome biogenesis, first placing the gene in nuclear RNA metabolism.

   Evidence Temperature-sensitive mtr3-1 mutant analysis with poly(A)+ RNA localization and nucleolar morphology readouts in S. cerevisiae

   PMID:8534909

   Open questions at the time

   • Did not define Mtr3p as an exosome subunit
   • No biochemical demonstration of catalytic role
   • Phenotypes are pleiotropic, leaving direct substrates unidentified
2. 2005 Medium

   Resolved whether Mtr3p acts in the core degradation pathway by showing its inactivation yields deadenylated/3'-truncated mRNAs distinct from the nuclear-specific Rrp6p, positioning it in core exosome turnover.

   Evidence Conditional inactivation of core exosome subunits and Northern analysis of mRNA species in a pap1-5 background

   PMID:16260613

   Open questions at the time

   • Does not establish whether Mtr3p itself is catalytic or purely structural
   • Single-lab genetic epistasis without reconstituted enzymology
3. 2007 Medium

   Extended Mtr3p substrate range to non-coding cryptic transcripts from repressed chromatin, showing the exosome degrades RNAs from telomeric, rDNA spacer, and centromeric loci.

   Evidence Northern blot analysis in mtr3 mutant strains compared with TRAMP and Rrp6 mutants

   PMID:18007593

   Open questions at the time

   • Mechanism of ncRNA targeting to the exosome not defined
   • Relationship to chromatin silencing left correlative
4. 2011 Medium

   Connected exosome RNA-processing activity to cytoskeletal/cell-cycle control by demonstrating microtubule and spindle defects in mtr3 mutants.

   Evidence Genetic analysis of mtr3 mutants with microtubule fluorescence imaging and RNA-seq of cell-cycle/MT transcripts

   PMID:21919057

   Open questions at the time

   • Causal RNA target linking exosome loss to MT phenotype not identified
   • Direct versus indirect contribution unresolved
5. 2013 Low

   Probed how core ring subunits maintain complex integrity, showing coordinated co-evolution of Rrp41 and Mtr3 preserves exosome activity.

   Evidence Systematic mutagenesis and activity assays of mutant exosome complexes with experimentally validated co-evolution analysis

   PMID:23852635

   Open questions at the time

   • Limited mechanistic detail on which interfaces drive co-evolution
   • Not independently confirmed beyond single study
6. 2023 Medium

   Placed EXOSC6 structurally in the lower PH-like ring and clarified that the EXOSC2 cap mediates MTR4 helicase recruitment required for substrate delivery.

   Evidence Structural modeling of a patient mutation, yeast complementation, Co-IP interaction assay, and RNA target accumulation assays

   PMID:36861343

   Open questions at the time

   • MTR4 interaction characterized via EXOSC2, not EXOSC6 directly
   • No human EXOSC6 disease mutation or biochemical characterization provided

• Whether EXOSC6 contributes catalytically or solely structurally to RNA degradation, and its specific role in human cells and disease, remains unresolved in this corpus.
• No human EXOSC6 loss-of-function phenotype
• No reconstituted activity assay isolating EXOSC6 contribution
• No EXOSC6-specific structural data beyond ring assignment