Ran is a small GTPase involved in transport through the nuclear pore complex and in organization of microtubules. It switches between GDP- and GTP-bound states. The spatial organization of its regulators makes its nucleotide state informative about location, rather than simply reporting the total amount of the protein. [ref]
NCBI Gene by the National Library of Medicine. Human RAN, Gene ID 5901, Summary: nuclear transport, RCC1 and microtubule organization
In nuclear import, a high nuclear Ran-GTP activity promotes release of cargo from transport receptors, while the cytosolic side supports recycling through a different nucleotide state. This directional cycle depends on separated regulator activities and a supply of chemical energy. Equalizing all conditions on both sides would remove the same spatial cue even if the total Ran abundance stayed fixed. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 12, The Transport of Molecules between the Nucleus and the Cytosol: the Ran GTPase cycle
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