RANBP6

Model summary

A soluble transport receptor that shuttles between cytoplasm and nucleus to drive nuclear import of NLS-bearing proteins. Its extended HEAT/armadillo-repeat scaffold forms a flexible binding cradle that captures cargo in the cytosol, translocates through the nuclear pore, and releases substrates in the nucleus upon RanGTP-driven allosteric switching. By coupling cargo recognition to the RanGTP gradient, it ensures efficient and directional delivery of macromolecules required for nuclear functions.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide is dominated by tandem helical repeat scaffolds: IPR011989 (Armadillo-like helical homologous superfamily, residues 9–1103) and IPR016024 (Armadillo-type fold homologous superfamily, residues 17–946) define a solenoidal array of HEAT/armadillo repeats that create an extended, flexible binding surface. This scaffold is refined by multiple repeat modules diagnostic of karyopherin/importin-β family transport receptors: IPR057672 (IPO4/5-like, TPR repeats domain, residues 110–269) and IPR058584 (Importin subunit beta-1/Transportin-1-like, TPR repeats, residues 512–645) indicate internal TPR-like arrays that tune cargo and RanGTP recognition. The internal repeat annotations IPR041653 (Importin repeat 4, residues 279–365) and IPR041389 (Importin repeat 6, residues 773–880) map additional HEAT/HEAT-like units that typically form the cargo- and Ran-interaction cradle. A terminal IPR000357 (HEAT repeat, residues 917–943) near the C-terminus further supports a HEAT-repeat superhelix. The overall identity is cemented by IPR040122 (Importin beta family, residues 21–1076), which specifies a transport receptor that binds cargo in the cytosol and releases it in the nucleus under RanGTP control.

2. 02

   This repeat-rich architecture causes avid, multivalent protein binding rather than catalysis, aligning with GO:0005515 protein binding as the molecular function.

   HEAT/armadillo solenoids generate conformationally adaptable grooves that clamp onto basic nuclear localization signals (NLSs) and Ran-family GTPases. The IPO4/5-like and importin-β/transportin-like TPR segments provide discrete binding platforms that coordinate cargo capture in the cytosol and RanGTP-triggered cargo release in the nucleus.

3. 03

   From this binding logic, the biological role follows: the solenoidal importin-β family scaffold mediates directional macromolecular trafficking through nuclear pores.

   By recognizing NLS-bearing substrates and coupling their transport to the RanGTP gradient, the protein executes GO:0006606 protein import into nucleus. The HEAT-repeat core is known to undergo allosteric transitions upon RanGTP binding; this switching mechanism ensures vectorial movement of cargo from cytosol to nucleoplasm.

4. 04

   Cellular localization emerges from the transport cycle.

   Cargo loading occurs in the soluble phase of the cytoplasm, consistent with GO:0005829 cytosol, and the receptor shuttles to and within the nucleus to complete delivery. The absence of transmembrane segments and the presence of soluble HEAT/TPR arrays reinforce a cytosolic-to-nuclear shuttling behavior rather than membrane association.

5. 05

   Mechanistically, the protein likely binds NLS-bearing cargo via internal HEAT/TPR grooves, traverses the nuclear pore complex by transiently engaging FG-repeat nucleoporins, and then releases cargo upon RanGTP binding in the nucleus.

   The RanGTP gradient imposes directionality: cytosolic RanGDP favors cargo loading, while nuclear RanGTP drives cargo unloading and receptor recycling. I therefore hypothesize interactions with Ran GTPase (for allosteric control), classical NLS-bearing cargos such as nucleic acid–associated factors and ribosomal proteins, and FG-nucleoporins that guide transit. Together, the HEAT/armadillo superhelix and importin-β family modules define a soluble transport receptor that binds cargo and Ran to power nuclear import.