importin 4 (importin 4b, Ran-binding protein 4, RanBP4, IPO4)

Function

• nuclear protein import as nuclear transport receptor
• receptor for nuclear localization signals (NLS) in cargo substrates
• thought to mediate docking of importin/substrate complex to the nuclear pore complex through binding to nucleoporin
• the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism
• at the nucleoplasmic side of the nuclear pore complex, Ran binds to the importin, the importin/substrate complex dissociates & importin is re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran
• directionality of nuclear import is thought to be conferred by an asymmetric distribution of the GTP- & GDP-bound forms of Ran between the cytoplasm & nucleus
• mediates the nuclear import of RPS3A
• binds with high affinity to RPS3A
• the binding is coupled to RanGTP cycles

Structure

• belongs to the importin beta family
• contains 6 HEAT repeats
• contains 1 importin N-terminal domain

Compartment

cytoplasm (putative), nucleus

Alternative splicing

named isoforms=2

Pathology

• in vitro, mediates the nuclear import of human cytomegalovirus UL84 by recognizing a non-classical NLS

More general terms

• importin beta subunit

Additional terms

• importin complex (NLS-binding protein)

Database

• Kegg: http://www.genome.jp/dbget-bin/www_bget?hsa:7971
• UniProt: http://www.uniprot.org/uniprot/Q8TEX9.html