IPO8

IPO8 is an importin-β superfamily nuclear transport receptor that delivers diverse cargoes into the nucleus, functioning as a hub coupling cytoplasmic signaling to nuclear gene-regulatory programs (PMID:11682607, PMID:23906023). Its earliest defined cargo was the signal recognition particle subunit SRP19, which IPO8 imports in a reconstituted system consistent with nuclear assembly of the SRP (PMID:11682607). IPO8 also mediates nuclear translocation of NF-κB/p65 through a route independent of the canonical p65 NLS, with knockdown reducing nuclear p65 and NF-κB transcriptional output after TNF-α stimulation (PMID:23906023); during enterovirus infection IPO8 assembles into a trimeric complex with the lncRNA LINC1467 and p65 that promotes p65 phosphorylation, nuclear entry, and pro-inflammatory cytokine expression (PMID:41156681). IPO8 is additionally required for proper TGF-β/SMAD signaling: bi-allelic loss-of-function causes thoracic aortic aneurysm, and Ipo8-knockout mice show aortic-wall pSmad2 accumulation, reduced Smad6/7, and elastic fiber disorganization (PMID:34010605). IPO8 transcription is itself directly activated by RUNX2 binding to a defined promoter element, linking IPO8 expression to osteoblast differentiation programs (PMID:27277970).

1. 2001 High

   Established IPO8 as a functional nuclear import receptor by assigning it a first defined cargo, addressing whether this importin-β family member carries protein substrates into the nucleus.

   Evidence In vitro reconstituted nuclear import assay with recombinant importin 8 and SRP19, plus endogenous SRP19 localization

   PMID:11682607

   Open questions at the time

   • RanGTP dependence not directly dissected in this entry
   • Does not address physiological cargo range beyond SRP19
   • No structural basis for cargo recognition
2. 2013 High

   Showed IPO8 imports NF-κB/p65 by an NLS-independent route, answering how p65 reaches the nucleus when the canonical NLS pathway is bypassed.

   Evidence siRNA screen of importin-β family, Co-IP of NLS-mutant p65 with IPO8, nuclear fractionation, and NF-κB reporter assay

   PMID:23906023

   Open questions at the time

   • Binding interface between IPO8 and NLS-mutant p65 not mapped
   • Does not establish whether import is RanGTP-dependent
   • Relationship to canonical importin-α/β p65 import unresolved
3. 2016 Medium

   Identified an upstream transcriptional regulator of IPO8, addressing how IPO8 expression is controlled in a differentiation context.

   Evidence ChIP-on-chip, luciferase reporter promoter-deletion analysis, RUNX2 siRNA knockdown with RT-qPCR, and osteoblast differentiation assay in human mesenchymal stem cells

   PMID:27277970

   Open questions at the time

   • Single lab without independent confirmation
   • Functional consequence of RUNX2-driven IPO8 induction on nuclear transport not tested
   • Cell-type specificity of the regulation unknown
4. 2021 High

   Connected IPO8 loss to a Mendelian disease and a specific signaling defect, defining its physiological role in TGF-β/SMAD nuclear transport and aortic homeostasis.

   Evidence Ipo8 knockout mouse with histology, pSmad2 immunostaining, biomechanical compliance assays, aortic-wall RT-qPCR, and human bi-allelic loss-of-function variants across families

   PMID:34010605

   Open questions at the time

   • Direct demonstration that IPO8 imports specific SMAD proteins not shown biochemically
   • Mechanism linking IPO8 loss to elevated pSmad2 (rather than reduced) not fully resolved
   • Tissue specificity of the aortic phenotype unexplained
5. 2025 Medium

   Refined the IPO8/p65 axis by defining a lncRNA-containing ternary complex, addressing how IPO8-mediated p65 import is regulated during infection.

   Evidence Co-IP of LINC1467/IPO8/p65 ternary complex, viral replication and cytokine assays, and LINC1467 loss-of-function in a mouse viral infection model

   PMID:41156681

   Open questions at the time

   • Single lab, no structural validation or in vitro reconstitution
   • Stoichiometry and direct binding contacts within the trimeric complex not resolved
   • Whether LINC1467 is required for p65 import in non-infection contexts unknown

• Whether IPO8 cargo recognition shares a unified structural basis and how RanGTP-dependent release operates across its diverse substrates (SRP19, p65, SMADs) remains unresolved.
• No structural model of IPO8-cargo complexes in the corpus
• Direct biochemical demonstration of SMAD import by IPO8 lacking
• Full cargo repertoire and selectivity determinants uncharacterized