The major energy source for nuclear transport is:
Energy input driving selective accumulation of proteins inside nucleus does not come from kinesin ATP hydrolysis used for microtubule transport or proton motive force powering mitochondrial import, but from compartmentalized GTP turnover of Ran. RCC1 guanine exchange factor bound to nucleosomes continuously recharges Ran-GDP to Ran-GTP using nuclear GTP pool, while cytoplasmic RanGAP1 SUMOylated and anchored to RanBP2/Nup358 plus co-activator RanBP1 hydrolyze GTP outside. Each import cycle consumes one GTP upon export complex disassembly in cytosol, and each export cycle consumes GTP as Ran-GTP hydrolyzed after cargo release, with turnover estimated thousands per minute in active cell. NTF2 imports Ran-GDP to sustain supply. Although DEAD-box helicases consume ATP to remodel exported mRNPs via Dbp5, primary cost for karyopherin-mediated protein flux remains GTP hydrolysis. Ubiquitin-mediated proteolysis and ATP-dependent chaperones act elsewhere. Thus GTP hydrolysis by Ran uniquely powers directionality, accumulation against gradient and receptor recycling, analogy to ATP in vesicle budding but chemically distinct using small GTPase asymmetry across nuclear envelope to bias cargo-receptor affinity transitions and compartment identity.
Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: GTP Hydrolysis by Ran as Energy Source.