NTF2 mediates nuclear import of Ran

Abstract

Importin β family transport receptors shuttle between the nucleus and the cytoplasm and mediate transport of macromolecules through nuclear pore complexes (NPCs). The interactions between these receptors and their cargoes are regulated by binding RanGTP; all receptors probably exit the nucleus complexed with RanGTP, and so should deplete RanGTP continuously from the nucleus. We describe here the development of an in vitro system to study how nuclear Ran is replenished. Nuclear import of Ran does not rely on simple diffusion as Ran's small size would permit, but instead is stimulated by soluble transport factors. This facilitated import is specific for cytoplasmic RanGDP and employs nuclear transport factor 2 (NTF2) as the actual carrier. NTF2 binds RanGDP initially to NPCs and probably also mediates translocation of the NTF2-RanGDP complex to the nuclear side of the NPCs. A direct NTF2-RanGDP interaction is crucial for this process, since point mutations that disturb the RanGDP-NTF2 interaction also interfere with Ran import. The subsequent nuclear accumulation of Ran also requires GTP, but not GTP hydrolysis. The release of Ran from NTF2 into the nucleus, and thus the directionality of Ran import, probably involves nucleotide exchange to generate RanGTP, for which NTF2 has no detectable affinity, followed by binding of the RanGTP to an importin β family transport receptor.

Hyperosmotic Stress Signaling to the Nucleus Disrupts the Ran Gradient and the Production of RanGTP

“…Biochemical, genetic, and cell biological data provide compelling evidence for the involvement of Mog1, NTF2, and RCC1 in the maintenance of the Ran gradient (Bischoff and Ponstingl, 1991;Tachibana et al, 1994;Paschal et al, 1997;Oki and Nishimoto, 1998;Ribbeck et al, 1998;Smith et al, 1998;Baker et al, 2001). Our data support an interdependence of the localization and function of these proteins during sorbitol stress-induced disruption and recovery of the Ran protein gradient as well.…”

“…RanGTP is constantly exported to the cytoplasm in association with export complexes. Cytoplasmic RanGDP is imported into the nucleus by its transport receptor, NTF2 (Ribbeck et al, 1998;Smith et al, 1998). Once in the nucleus, RCC1 binds RanGDP and catalyzes exchange of GDP for GTP.…”

Inhibition of active nuclear transport is an intrinsic trigger of programmed cell death in trypanosomatids

“…It is noteworthy that RanBP1, CAS and CRM1 participate in the diverse functions of Ran in the cell, whereas NTF2 constitutes a Ran partner solely involved in nucleocytoplasmic transport. 23,24 All in all, these data show that, in trypanosomatids, PCD can be directly triggered by the inhibition of NTF2 expression, therefore by the inhibition of the sole nucleocytoplasmic transport.…”

“…By contrast, NTF2 is solely involved in the nuclear import of RanGDP. 23,24 Therefore, NTF2 depletion acts primarily on the nucleocytoplasmic gradient of RanGTP/ RanGDP and it causes PCD. This shows that the disruption of this gradient is an intrinsic factor sufficient for triggering PCD.…”

Reduction of exportin 6 activity leads to actin accumulation via failure of RanGTP restoration and NTF2 sequestration in the nuclei of senescent cells

“…We also immunocytochemically examined intracellular distribution of NTF2 under an assumption that if NTF2, a regulator of Ran translocation from cytoplasm to nuclei [51][52][53], was blocked or its expression was decreased, Ran distribution would also be changed. As expected, NTF2 was observed in the cytoplasm and perinuclei of young cells, whereas it was clearly stained in the Fig.…”