Dissecting the interactions between NTF2, RanGDP, and the nucleoporin XFXFG repeats.

We have used a range of complementary biochemical and biophysical methods to investigate the interactions between nuclear transport factor 2 (NTF2), the Ras family GTPase Ran, and XFXFG nucleoporin repeats that are crucial for nuclear trafficking. Microcalorimetry, microtiter plate binding, and fluorescence quenching in solution are all consistent with the binding constant for the NTF2-RanGDP interaction being in the 100 nM range, whereas the interaction between NTF2 and XFXFG repeat-containing nucleoporins such as Nsp1p is in the 1 microM range. Although the accumulation of NTF2 at the nuclear envelope is enhanced by RanGDP, we show that Ran binding does not alter the affinity of NTF2 for nucleoporins nor does the binding of nucleoporins alter the affinity of NTF2 for RanGDP. These results indicate that, instead, Ran increases the binding of NTF2 to nucleoporins by another mechanism, most probably by Ran itself binding to nucleoporins and NTF2 binding to this nuclear pore-associated Ran.