What is the role of importins in nuclear transport?
Importins act as soluble chaperones that decode nuclear localization signals and ferry cargo through nuclear pore complex while keeping hydrophobic patches shielded. Classical monopartite NLS such as SV40 large T antigen PKKKRKV or bipartite nucleoplasmin KRPAATKKAGQAKKKK are highly enriched in basic lysine and arginine side chains that bind armadillo repeats of importin-alpha via electrostatic interactions and tryptophan-mediated cation-pi stacking. Importin-beta then wraps around importin-alpha IBB domain and mediates transient hydrophobic contacts with FG nucleoporins, moving complex inward
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: Importin Alpha-Beta and NLS Recognition Mechanism.