Importin β-type nuclear transport receptors have distinct binding affinities for Ran-GTP

Cargos destined to enter or leave the cell nucleus are typically transported by receptors of the importin β family to pass the nuclear pore complex. The yeast Saccharomyces cerevisiae comprises 14 members of this protein family, which can be divided in importins and exportins. The Ran GTPase regulates the association and dissociation of receptors and cargos as well as the transport direction through the nuclear pore. All receptors bind to Ran exclusively in its GTP-bound state and this event is restricted to the nuclear compartment. We determined the Ran-GTP binding properties of all yeast transport receptors by biosensor measurements and observed that the affinity of importins for Ran-GTP differs significantly. The dissociation constants range from 230 pM to 270 nM, which is mostly based on a variability of the off-rate constants. The divergent affinity of importins for Ran-GTP suggests the existence of a novel mode of nucleocytoplasmic transport regulation. Furthermore, the cellular concentration of β-receptors and of other Ran-binding proteins was determined. We found that the number of β-receptors altogether about equals the amounts of yeast Ran, but Ran-GTP is not limiting in the nucleus. The implications of our results for nucleocytoplasmic transport mechanisms are discussed.