SKIC8

SKIC8 (Ski8/WDR61) is a WD-repeat scaffold protein that operates in mechanistically distinct cytoplasmic and nuclear contexts to support RNA turnover, genome stability, and meiotic recombination (PMID:27980209, PMID:14992724, PMID:8442386, PMID:3029964). In the cytoplasm it is a core subunit of the Ski2–Ski3–Ski8 helicase complex: docking of this complex onto 80S ribosomes displaces the autoinhibitory domain of Ski2 and positions the helicase channel at the ribosomal mRNA entry tunnel so that an mRNA 3' overhang is threaded directly from the small subunit into Ski2 for exosome-mediated 3'–5' degradation (PMID:27980209). Consistent with this role in RNA metabolism, the protein is essential for repressing M double-stranded RNA virus replication in yeast (PMID:8442386, PMID:3029964). During meiosis SKIC8 relocalizes from cytoplasm to nucleus and physically associates with Spo11 on meiotic chromosomes, acting as a scaffold that recruits additional double-strand break proteins; this meiotic function is independent of its RNA-metabolism partners and is conserved in the fission yeast ortholog Rec14 (PMID:14992724, PMID:9258671). In a nuclear context distinct from RNA decay, the protein associates with the Paf1 transcription elongation complex, and loss of WDR61 causes R-loop accumulation, DNA damage, and impaired proliferation, defining a role in maintaining genomic stability (PMID:22511887, PMID:38708718).

1. 1993 Medium

   Established the molecular nature of the gene product, defining SKI8 as a WD-repeat scaffold protein and linking it to control of an RNA virus.

   Evidence DNA sequencing, sequence analysis, and genetic disruption in yeast

   PMID:3029964 PMID:8442386

   Open questions at the time

   • Did not resolve the biochemical mechanism of viral repression
   • No direct partners or complex membership identified
2. 1997 Medium

   Resolved whether the protein's recombination role is meiosis-specific by showing the fission yeast ortholog Rec14 is required for meiotic but not mitotic recombination.

   Evidence Genetic deletion and quantitative recombination assays across multiple intervals in S. pombe, with complementation cloning

   PMID:9258671

   Open questions at the time

   • Did not identify the molecular partner mediating the recombination function
   • Mechanistic link between this WD-repeat protein and DSB machinery undefined
3. 2004 High

   Connected the meiotic recombination defect to a direct physical and scaffolding role, showing Ski8 partners with Spo11 and relocalizes to recruit DSB proteins independently of its RNA-metabolism functions.

   Evidence Two-hybrid mapping, chromosome immunolocalization, relocalization assay, and genetic epistasis with RNA-metabolism partner deletions

   PMID:14992724

   Open questions at the time

   • Structural basis of the Ski8–Spo11 interaction not resolved
   • Full set of recruited DSB proteins and order of assembly not defined
4. 2012 Medium

   Demonstrated dual subcellular partitioning of SKI8 homologs, separating a cytoplasmic SKI-complex role from a nuclear Paf1 complex association and extending the latter to human SKIC8.

   Evidence GFP localization, biochemical co-purification, RNA-seq of mutants, and complementation in Arabidopsis VIP3, with note of human SKIC8–Paf1c association

   PMID:22511887

   Open questions at the time

   • Human SKIC8–Paf1c association not independently replicated in this study
   • Functional consequence of nuclear Paf1c association in animals not established
5. 2016 High

   Defined how the Ski complex physically couples to translation, showing SKIC8 is part of a complex that docks on 80S ribosomes and channels mRNA 3' overhangs into the Ski2 helicase for exosome degradation.

   Evidence Cryo-EM of an endogenous ribosome–Ski complex at ~4–10 Å resolution

   PMID:27980209

   Open questions at the time

   • Specific SKIC8 contacts within the ribosome-bound complex not delineated at high resolution
   • Regulation of complex recruitment to ribosomes undefined
6. 2024 Medium

   Assigned a genome-stability function to human WDR61, showing it suppresses R-loop accumulation to prevent DNA damage and sustain proliferation.

   Evidence Conditional knockout in a mouse tumor model, siRNA knockdown, R-loop interaction and DNA damage assays in breast tumor cells

   PMID:38708718

   Open questions at the time

   • Mechanism linking WDR61 to R-loop resolution not defined
   • Relationship between R-loop suppression and Paf1c association not established
7. 2024 Medium

   Identified SKIC8 as the direct target of the antiviral compound UMB18, linking compound binding to SREBP/SCAP-driven cholesterol upregulation that restricts viral replication.

   Evidence Direct binding assay, transcriptomics in A549 cells, siRNA knockdown of SREBPs/SCAP, and cholesterol extraction (preprint)

   PMID:bio_10.1101_2024.12.03.626536

   Open questions at the time

   • Preprint not yet peer-reviewed
   • How UMB18 binding to SKIC8 mechanistically triggers the mevalonate pathway is unresolved
   • Binding site on SKIC8 not mapped

• How a single WD-repeat scaffold is partitioned among the cytoplasmic SKI/ribosome complex, the meiotic Spo11 DSB machinery, and the nuclear Paf1c/R-loop functions remains unresolved.
• No structural model of SKIC8 within Paf1c
• Regulatory switch controlling cytoplasmic vs nuclear deployment unknown
• Whether R-loop suppression depends on Paf1c association untested