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A nuclear transport factor carrying suitable precursor microRNA hairpins into the cytoplasm.

Transport is coupled to the Ran nucleotide system, with different binding conditions in nucleus and cytoplasm helping release cargo on the appropriate side. Export is one step of microRNA biogenesis rather than a direct measure of final target repression. ^[ref]

An importin is a protein involved in selective import through a nuclear pore complex.

It recognizes suitable cargo directly or through adaptors and participates in passage through the selective pore environment. Directionality depends on the Ran nucleotide cycle and compartment-specific interactions rather than on a permanently one-way opening. ^[ref]

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]

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]