Practice question

Most nuclear import of NLS proteins using importin-alpha-beta heterodimer and export of NES proteins via CRM1 strictly consume Ran-GTP cycles, with RCC1 generating Ran-GTP inside nucleus and RanGAP1 hydrolyzing outside. Each cycle consumes one GTP and relies on NTF2 recycling of Ran-GDP. Messenger RNA departure follows divergent energetic logic. Bulk poly(A) mRNA transcribed, spliced and polyadenylated recruits TREX complex and adaptor Aly/REF that loads heterodimer NXF1-NXT1 which binds FG repeats directly without Ran. At cytoplasmic filaments of pore, DEAD-box helicase Dbp5/DDX19 activated by Gle1 and IP6 hydrolyzes ATP to displace NXF1 and associated factors, remodeling particle and preventing back-diffusion, thus providing directionality. Because Ran not involved, dominant-negative Ran mutants or RCC1 inhibition arrest protein transport but leave mRNA export largely intact. Golgi transport uses Rab and Arf GTPases instead of Ran. Hence among nuclear processes listed, mRNA export represents classic Ran-independent pathway illustrating ATP-driven remodeling can substitute for GTP gradient to achieve vectorial movement across envelope while supporting continuous gene expression even during mitotic remodeling of envelope.