Practice question

Barrier between genome and cytoplasm is perforated by nuclear pore complexes large supramolecular structures of approximately 125 megadaltons composed of about 30 distinct nucleoporins present in multiple copies arranged with eightfold rotational symmetry spanning double nuclear membranes. Outer scaffold comprises Y-shaped Nup107-160 subcomplex forming two concentric rings, inner ring composed of Nup205, Nup188, Nup155 anchoring central channel filled with disordered FG repeats rich in phenylalanine-glycine motifs that form selective phase limiting diffusion. Molecules smaller than 40 kilodalton diffuse passively, larger cargo requires transport receptors karyopherin family interacting via hydrophobic patches with FG repeats. Import receptors importin alpha/beta recognize basic nuclear localization signals with clusters of lysine and arginine, while export receptor CRM1/XPO1 binds leucine-rich nuclear export signals. Ran-GTP gradient provides directionality dissociating import and promoting export complexes. NPC therefore gates transcription factors, histones, ribosomal subunits, and messenger ribonucleoproteins, controlling gene expression, DNA repair factor availability, and development, with dysfunction linked to cancer and viral infections.