Large cargo is not imported into the nucleus without GTP (bottom).

The need for Ran and GTP in nuclear import reactions depends on the size of the imported cargo, according to new results by Lyman et al. (page 55), who show that large proteins in particular are susceptible to stalling mid-import when the small GTPase Ran is missing.

Imported proteins, in association with cargo receptors such as importin β or transportin, pass from one nucleoporin protein to another within the nuclear pore complex (NPC) on their way to the nucleoplasm. Ran was already known to help cargo detach from its receptor in the nucleus. The new results give Ran a second unexpected function within the center of the channel.

This function stems from the ability of RanGTP to bind to cargo receptors. When Ran or GTP was absent from importin α/β and transportin import reactions, or if the receptors were unable to bind to Ran, large cargoes got stuck on the cytoplasmic side of the NPC. Previous in vitro studies have shown that RanGTP binding to cargo receptors dissociates receptors from both cargo and nucleoporins. With their new results, the authors propose that RanGTP may also bind to receptors in a way that selectively dissociates nucleoporins but not cargo. Rapid subsequent dissociation of Ran from the receptor before the cargo also falls off would leave the receptor/cargo complex free to bind to another nucleoporin.

Receptor/cargo movement from one nucleoporin to the next within the NPC is thought to occur by passive diffusion. Increasing the off-rate of receptors from nucleoporins through RanGTP binding would be especially favorable for the import of large cargoes because they need more off-time to diffuse within the restricted space of the NPC to the next nucleoporin. The model also predicts that receptors with low affinities for nucleoporins will not rely on Ran for importing larger proteins, which may explain why other studies have shown that import via snurportin does not require Ran or GTP. Now, biophysical studies are needed to confirm that RanGTP favors dissociation of receptor from nucleoporin rather than receptor from cargo. ▪