SKIC3

SKIC3 (TTC37/SKI3) is a tetratricopeptide-repeat scaffold protein that forms the structural core of the trimeric Ski complex (SKIC2–SKIC3–SKIC8) governing cytoplasmic 3'-to-5' mRNA surveillance and decay (PMID:27980209). Within this complex, SKIC3 provides the architectural scaffold while the assembled complex docks directly onto 80S ribosomes that present a short mRNA 3' overhang; ribosome binding displaces the autoinhibitory domain of the SKIC2 helicase and threads the mRNA 3' overhang from the small subunit into the helicase channel, positioning the substrate for exosome-mediated degradation (PMID:27980209). Genetic work in the yeast ortholog established that this activity extends beyond antiviral dsRNA control: SKI3 is synthetically lethal with the 5'-to-3' exoribonuclease xrn1/sep1 independently of killer viruses, and the double mutant arrests at Start in late G1, consistent with SKI3 acting to block translation of transcripts targeted for degradation (PMID:7739552). Loss-of-function mutations in human TTC37/SKIC3 cause trichohepatoenteric syndrome, a multisystem disorder in which specific enterocyte brush-border transporters (NHE2, NHE3, AQP7, sodium iodide symporter, H/K ATPase) show reduced expression or mislocalization while basolateral Na/K ATPase is unaffected, indicating a requirement for SKIC3 in the stability and/or localization of particular target proteins (PMID:20176027).

1. 1989 Medium

   Established the basic cellular properties of the gene product, defining it as a large nucleus-associated protein whose essentiality is conditional on the M dsRNA virus, framing it initially as an antiviral factor.

   Evidence Cell fractionation, NLS mapping with beta-galactosidase fusion reporters, and gene deletion in yeast

   PMID:2660461

   Open questions at the time

   • Nuclear association in yeast is not reconciled with later cytoplasmic ribosome-associated function
   • No molecular activity or partners identified at this stage
   • Mechanism of conditional essentiality with M virus unexplained
2. 1995 Medium

   Demonstrated that the gene functions in a general mRNA decay process rather than purely antiviral defense, by showing virus-independent synthetic lethality with the 5'-to-3' exoribonuclease and a cell-cycle arrest phenotype linking it to translational repression of decay-targeted transcripts.

   Evidence Synthetic lethality screen with sep1/xrn1, virus-cured strains, and temperature-sensitive double-mutant cell cycle analysis in yeast

   PMID:7739552

   Open questions at the time

   • Genetic interaction does not define the biochemical mechanism of translation blockage
   • Did not identify the physical complex or ribosome association
   • No direct substrate transcripts identified
3. 2010 Medium

   Connected the human ortholog to disease, showing TTC37 loss-of-function causes trichohepatoenteric syndrome and is required for stability/localization of specific enterocyte transporter proteins, extending its role to epithelial protein homeostasis.

   Evidence Autozygosity mapping, candidate gene sequencing, and immunohistochemistry of patient hepatocytes and intestinal epithelium

   PMID:20176027

   Open questions at the time

   • How an mRNA decay scaffold causes selective transporter mislocalization is not mechanistically resolved
   • Direct molecular link between TTC37 and the affected target proteins not established
   • Single study
4. 2011 Low

   Expanded the mutational spectrum and tissue expression profile, showing no recurrent mutational hotspot and broad expression notably excluding liver.

   Evidence Patient sequencing and RT-PCR expression profiling across human tissues

   PMID:21120949

   Open questions at the time

   • No direct functional mechanistic assay performed
   • Tissue expression does not explain organ-specific pathology
   • Liver-absent expression unexplained relative to hepatic phenotype
5. 2016 High

   Resolved the molecular mechanism: SKIC3 is the scaffold of the trimeric Ski complex that engages 80S ribosomes and threads the mRNA 3' overhang into the SKIC2 helicase, defining the structural basis of co-translational 3'-to-5' mRNA surveillance.

   Evidence Cryo-EM of the endogenous yeast ribosome-Ski complex at ~4 Å core resolution

   PMID:27980209

   Open questions at the time

   • Structure determined for yeast complex; human SKIC3 architecture not directly resolved
   • Does not explain disease-specific transporter phenotypes in humans
   • Coupling to the exosome captured only by inference from positioning
6. 2018 Low

   Compared the clinical consequences of losing the two Ski-complex components, showing SKIV2L loss is more severe than TTC37 loss, implying non-equivalent functional contributions of complex subunits in humans.

   Evidence Genotype-phenotype cohort comparison of 96 THES patients

   PMID:29527791

   Open questions at the time

   • No biochemical or cellular assay of SKIC3 protein function
   • Mechanistic basis for differential severity not determined
   • Clinical correlation only

• It remains unresolved how the cytoplasmic ribosome-associated mRNA-decay scaffold function of SKIC3 mechanistically produces the selective enterocyte transporter mislocalization seen in trichohepatoenteric syndrome.
• No direct demonstration that human SKIC3 controls the implicated transporter mRNAs or proteins
• Nuclear localization of yeast ortholog versus cytoplasmic ribosome function not reconciled
• Tissue-specific basis of disease phenotype unknown