Practice question

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 by facilitated diffusion without ATP consumption. Inside nucleus, binding of Ran-GTP to importin-beta N-terminal arch induces large conformational opening, releasing importin-alpha-cargo assembly. Nup50 competes for NLS binding groove to liberate cargo, while CAS exportin plus Ran-GTP returns importin-alpha to cytosol for another cycle. This mechanism concentrates DNA polymerases, histones, transcription factors and spliceosomal components inside nucleus where they function. Export of NES proteins, chromatin modification or ribosome anchoring are performed by unrelated factors, highlighting importins as dedicated carriers for positively charged NLS recognition during nuclear import and gene regulation.