KPNA1

KPNA1 (importin α5/NPI-1/hSRP1) is a nuclear localization signal (NLS) receptor that recognizes NLS-bearing cargo through its ARM-repeat domain and adapts them to the classical nuclear import pathway (PMID:8052633, PMID:16439554). It engages a broad range of cargo, including the V(D)J recombinase RAG-1 via four of its ARM repeats (PMID:8052633), EBV nuclear antigen EBNA-1—whose import KPNA1 accelerates upon phosphorylation of Ser385 within the EBNA-1 NLS (PMID:16439554)—influenza A nucleoprotein through a nonconventional NLS motif (PMID:7831767, PMID:9032315), and HIV-1 Vpr, which binds selectively to the ninth ARM repeat in a manner that distinguishes KPNA1 from other importin α isoforms (PMID:22110766). After cargo delivery, the exportin CAS releases cargo such as Vpr from KPNA1 to recycle the adaptor (PMID:22110766). A C-terminal region of KPNA1 binds phosphorylated STAT1 (PY-STAT1), and this interferon-signaling node is a recurrent target of viral antagonism: Ebola VP24 occupies the PY-STAT1-binding region to competitively block PY-STAT1 import and ISG expression (PMID:17928350), while PEDV nsp7 sequesters the STAT1 DNA-binding domain to disrupt KPNA1-mediated nuclear translocation of the ISGF3 complex without altering STAT1 phosphorylation (PMID:35442061). Beyond classical transport, KPNA1 has a neuronal role: its knockout in mice produces neuropsychiatric-relevant behavioral and neuroendocrine phenotypes (PMID:34767585), and in axons KPNA1 co-migrates with endosome/lysosome-associated factors rather than forming a steady-state importin β1 complex, with a schizophrenia-associated E448X truncation mislocalizing to the nucleus and losing axonal targeting unless a nuclear export signal is restored (PMID:40010609).

1. 1994 High

   Established that human KPNA1 physically engages an endogenous nuclear protein, RAG-1, and mapped the interaction to specific ARM repeats, defining KPNA1 as an NLS-cargo adaptor with a discrete cargo-binding surface.

   Evidence Yeast two-hybrid, Co-IP after co-transfection in 293T cells, and deletion mapping

   PMID:8052633

   Open questions at the time

   • Did not establish import kinetics or dependence on importin β
   • Functional consequence for V(D)J recombination not tested
2. 1995 High

   Showed KPNA1 directly binds a viral cargo (influenza NP) in infected cells, demonstrating that pathogens recruit KPNA1 for nuclear access.

   Evidence GST pulldown and reciprocal Co-IP from virus-infected MDBK cells

   PMID:7831767

   Open questions at the time

   • NLS motif within NP not yet defined
   • Import efficiency relative to other importin α isoforms unknown
3. 1997 High

   Defined the nonconventional NLS motif on influenza NP recognized by KPNA1, refining the cargo-recognition specificity of the adaptor.

   Evidence Yeast two-hybrid alanine-scanning mutagenesis and NLS-fusion nuclear localization assay in HeLa cells

   PMID:9032315

   Open questions at the time

   • Structural basis of nonconventional NLS recognition not resolved
   • Did not address which ARM repeats bind this motif
4. 2000 High

   Extended KPNA1 cargo repertoire to EBV EBNA-1 using endogenous protein from B cells, showing redundancy with karyopherin α2.

   Evidence GST pulldown, Co-IP of endogenous EBNA-1 from Raji B cell extracts, yeast two-hybrid

   PMID:10612665

   Open questions at the time

   • Did not quantify isoform preference
   • Phospho-regulation of the NLS not yet examined
5. 2006 High

   Demonstrated that NLS phosphorylation tunes KPNA1 binding affinity and import rate, establishing post-translational control of cargo selection.

   Evidence Microinjection of NLS peptide-BSA conjugates, GFP-EBNA-1 Ser-to-Ala mutants, and in vitro binding with phospho-NLS peptides across importin α isoforms

   PMID:16439554

   Open questions at the time

   • Kinase responsible for Ser385 phosphorylation not identified
   • Generality of phospho-tuning to other cargo not tested
6. 2007 High

   Identified the C-terminal PY-STAT1-binding region of KPNA1 as the node targeted by Ebola VP24 to block interferon signaling, linking KPNA1 to innate immune nuclear import.

   Evidence KPNA1 mutational mapping, Co-IP of endogenous/overexpressed proteins, and ISG expression assays

   PMID:17928350

   Open questions at the time

   • Did not resolve VP24 binding stoichiometry or structure
   • Contribution of KPNA5/KPNA6 redundancy in vivo unquantified
7. 2008 Medium

   Showed KPNA1 itself relocalizes from cytoplasm to nucleus under oxidative stress, indicating its distribution is stimulus-regulated.

   Evidence Immunofluorescence with a monospecific monoclonal antibody in H2O2-treated HeLa cells

   PMID:18707546

   Open questions at the time

   • Single immunofluorescence experiment without functional consequence for KPNA1
   • Mechanism driving stress-induced relocalization unknown
8. 2011 High

   Reconstituted isoform-selective import of HIV-1 Vpr through the ninth ARM repeat of KPNA1 and showed CAS-dependent cargo release, mechanistically connecting cargo binding to recycling.

   Evidence Digitonin-permeabilized cell import assay, surface plasmon resonance, and CAS siRNA knockdown with functional rescue

   PMID:22110766

   Open questions at the time

   • Structural basis for ninth-ARM-repeat selectivity not solved
   • Whether CAS regulates release of all KPNA1 cargo not addressed
9. 2021 Medium

   Revealed a physiological role for KPNA1 beyond transport biochemistry, with knockout mice showing neuropsychiatric-relevant behavioral and neuroendocrine deficits.

   Evidence Kpna1 knockout mouse behavioral battery plus plasma hormone/cytokine measurement

   PMID:34767585

   Open questions at the time

   • Molecular cargo underlying behavioral phenotypes not identified
   • Single lab; cell-type-specific contributions unresolved
10. 2022 Medium

    Defined a distinct viral strategy in which PEDV nsp7 sequesters the STAT1 DNA-binding domain to disrupt KPNA1-mediated ISGF3 import without affecting STAT1 phosphorylation, separating import blockade from upstream signaling.

    Evidence Co-IP, nuclear translocation assays, and ISG expression assays

    PMID:35442061

    Open questions at the time

    • Single lab without reciprocal structural confirmation
    • Quantitative effect on KPNA1 occupancy not measured
11. 2025 Medium

    Characterized a non-canonical axonal function of KPNA1 dependent on endosomal transport rather than steady-state importin β1 complexing, and showed a schizophrenia-associated truncation mislocalizes by losing nuclear export.

    Evidence Live-cell imaging of KPNA1-GFP in neurons, co-migration with endosome/lysosome markers, import complex analysis, and NES rescue of the E448X mutant

    PMID:40010609

    Open questions at the time

    • Axonal cargo carried by KPNA1 not identified
    • Link between axonal mislocalization and disease phenotype not functionally proven
    • Single study

• The endogenous physiological cargo of KPNA1 that drive its neuronal and neuroendocrine functions, and the structural basis of its isoform-selective cargo recognition, remain undefined.
• No native neuronal cargo identified for axonal KPNA1
• No high-resolution structure of KPNA1 with cargo or viral antagonists
• Relative in vivo contribution of NPI-1 subfamily redundancy unresolved