An ATP-dependent activity that releases RanGDP from NTF2

The small GTPase Ran functions in several critical processes in eukaryotic cells including nuclear transport, nuclear envelope formation, and spindle formation. A RanGDP-binding protein, NTF2, facilitates translocation of RanGDP through the nuclear pore complex and also acts to stabilize RanGDP against nucleotide exchange. Here, we identify a novel activity that stimulates release of GDP from Ran in the presence of NTF2. Hydrolyzable ATP enhances the GDP dissociation activity, and this enhancement is inhibited by nonhydrolyzable ATP analogues. In contrast, neither hydrolyzable ATP nor nonhydrolyzable ATP analogues affect GDP dissociation from Ran catalyzed by recombinant RCC1 or inhibition of GDP dissociation from Ran by recombinant NTF2. The ATP-dependent RanGDP dissociation activity therefore has the properties of a RanGDP dissociation inhibitor (GDI) displacement factor (RanGDF) where the GDI is NTF2. A protein phosphatase inhibitor mixture stimulates the RanGDF activity, suggesting the activity is regulated by phosphorylation. We propose that the ATP-dependent NTF2 releasing factor may have a role in the RanGDP/GTP cycle.