TNPO1

TNPO1 (transportin-1/karyopherin-β2) is a RanGTP-regulated nuclear import receptor that recognizes non-classical proline-tyrosine nuclear localization signals (PY-NLS) and mediates translocation of diverse cargoes—including shuttling hnRNPs, ribosomal proteins, Gli/Ci transcription factors, and LINE-1 ribonucleoprotein complexes—through the nuclear pore complex (PMID:8808633, PMID:9687515, PMID:24413177, PMID:28974576). Its 18 HEAT-repeat structure forms two orthogonal arches: RanGTP binds the N-terminal arch and allosterically triggers cargo release from the C-terminal arch, with a critical loop in HEAT repeat 7 coupling these events (PMID:10353245). The M9 signal of hnRNP A1, the founding PY-NLS, functions as both a nuclear import and nuclear export signal, establishing a bidirectional transport pathway (PMID:8521471, PMID:8808633). Familial ALS mutations in the PY-NLS of FUS impair TNPO1-mediated import in proportion to disease severity, leading to cytoplasmic FUS accumulation and stress-granule recruitment (PMID:20606625).

1. 1995 High

   Demonstration that the M9 domain of hnRNP A1 functions as both a nuclear import and export signal established the existence of a bidirectional transport pathway distinct from the classical NLS route, raising the question of which receptor mediates this pathway.

   Evidence Microinjection of M9-tagged fusion proteins into Xenopus oocyte nuclei with temperature-sensitivity controls

   PMID:8521471

   Open questions at the time

   • Identity of the receptor recognizing M9 was unknown
   • Mechanism of bidirectional signaling by a single peptide was unexplained
2. 1996 High

   Identification of transportin (TNPO1) as the ~90 kDa import receptor that directly binds the M9 signal answered the receptor-identity question and defined a non-classical nuclear import pathway parallel to importin-α/β.

   Evidence Co-immunoprecipitation of transportin with M9-containing proteins and reconstituted in vitro nuclear import assays in permeabilized cells

   PMID:8808633

   Open questions at the time

   • Structural basis of cargo recognition was unknown
   • Range of substrates beyond hnRNP A1 was uncharacterized
   • Mechanism of cargo release in the nucleus was unclear
3. 1998 High

   Showing that transportin directly imports ribosomal proteins (rpL23a, rpS7, rpL5) via basic-region signals expanded the substrate repertoire well beyond shuttling hnRNPs and indicated that TNPO1 recognizes diverse signal types.

   Evidence Reconstituted nuclear import assays in digitonin-permeabilized mammalian cells with deletion mapping of import signals

   PMID:9687515

   Open questions at the time

   • Consensus features distinguishing transportin-recognized signals from classical NLSs were not yet defined
   • Relative contribution of transportin versus other importin-β family members for ribosomal protein import was uncertain
4. 1999 High

   The 3.0 Å crystal structure of TNPO1 bound to RanGppNHp revealed the 18-HEAT-repeat, two-arch architecture and showed how RanGTP binding to the N-terminal arch allosterically promotes cargo release from the C-terminal arch, providing the first structural explanation for the import cycle.

   Evidence X-ray crystallography of karyopherin-β2–RanGppNHp complex with structure-based mapping of functional domains

   PMID:10353245

   Open questions at the time

   • No co-crystal with a PY-NLS cargo was available
   • Role of the HEAT-repeat 7 loop in substrate dissociation was inferred but not directly tested by mutagenesis in this study
5. 2010 High

   Mapping the PY-NLS of FUS as the transportin-binding element and showing that ALS-causing mutations within it impair nuclear import in proportion to clinical severity directly linked TNPO1-dependent transport to neurodegenerative disease pathogenesis.

   Evidence Nuclear import assays, co-immunoprecipitation, mutagenesis, live-cell imaging, siRNA knockdown of transportin, and immunofluorescence in patient tissue

   PMID:20606625

   Open questions at the time

   • Whether restoring TNPO1-mediated import could rescue disease phenotypes in vivo was untested
   • Contribution of stress-granule sequestration versus loss of nuclear function to neurodegeneration was unresolved
6. 2014 High

   Identification of a conserved PY-NLS in Ci/Gli transcription factors and demonstration that Sufu masks this signal to block TNPO1-mediated nuclear entry connected transportin to Hedgehog signaling and illustrated a regulatory strategy in which co-factors modulate import by NLS masking.

   Evidence PY-NLS mutagenesis, co-immunoprecipitation of Trn with Ci, genetic epistasis in Drosophila Hedgehog pathway, and mammalian Gli validation

   PMID:24413177

   Open questions at the time

   • Structural basis of Sufu masking of the PY-NLS was not determined
   • Whether other signaling pathways regulate cargo access to TNPO1 by analogous masking was unknown
7. 2017 Medium

   Demonstrating that TNPO1 mediates nuclear import of LINE-1 ribonucleoprotein complexes and that miR-128 represses TNPO1 expression to restrict retrotransposition revealed a role for transportin in transposable element biology and a post-transcriptional regulatory layer controlling its abundance.

   Evidence TNPO1 knockdown/overexpression, L1 retrotransposition reporter assays, ORF1p nuclear import assays, and 3′ UTR luciferase reporters for miR-128 targeting

   PMID:28974576

   Open questions at the time

   • Direct physical interaction between TNPO1 and L1-RNP was not demonstrated with purified components
   • Whether TNPO1 recognizes a PY-NLS-like motif in L1 ORF1p or ORF2p was not mapped
   • Findings from a single laboratory

• A comprehensive structural picture of TNPO1 bound to its diverse PY-NLS cargoes (FUS, Gli, ribosomal proteins, L1-RNP) is still incomplete, and whether therapeutic modulation of TNPO1-mediated import can ameliorate ALS or other proteinopathies remains an open question.
• No co-crystal structures with FUS, Gli, or ribosomal protein PY-NLS cargoes described in this timeline
• In vivo therapeutic relevance of restoring TNPO1-mediated FUS import in ALS models is untested
• Regulatory mechanisms controlling TNPO1 expression beyond miR-128 are uncharacterized