Nuclear transport: Signalling and transport converge

signaling update home

SOURCE:

RSK- and AKT-mediated phosphorylation of Ran-binding protein-3 (RanBP3) increases the efficiency of nuclear import.

Researchers have found an unexpected link between the Ras–extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K) signalling pathways and the regulation of nucleocytoplasmic transport.


DIGITAL VISION

Using a two-hybrid screen to identify new targets of the ribosomal S6 kinase (RSK), a downstream effector of the Ras–ERK signalling pathway, Blenis and colleagues identified Ran-binding protein-3 (RanBP3), a regulator of Ran GTPase, which coordinates the nucleocytoplasmic transport of many proteins and RNAs. The authors showed that activated RSK phosphorylates residue Ser58 of RanBP3 in cells. Ser58 is also phosphorylated by activated AKT/PKB, a kinase of the PI3K signalling pathway, but not by other kinases.

So, what is the effect of RanBP3 phosphorylation in vivo? To address this question, Blenis and co-workers expressed endogenous levels of RanBP3 and a GTP-bound Ran mutant that associates weakly with RanBP3, in cells lacking the two endogenous proteins. Upon RanBP3 phosphorylation, the interaction between the two proteins increases, which suggests that the serum-induced Ser58 phosphorylation increases the affinity of RanBP3 for Ran. Indeed, a RanBP3 mutant that cannot be phosphorylated (S58A) did not show an increased interaction with Ran.

GTP-bound Ran is predominantly nuclear owing to the compartmentalization of its regulators; RCC1, a guanine nucleotide-exchange factor, is restricted to the nucleus, whereas RanGAP, a GTPase-activating protein, is in the cytoplasm. To test whether RanBP3 phosphorylation affects the Ran gradient, which is important for nucleocytoplasmic transport, the authors knocked down RanBP3 and showed that Ran relocates to the cytoplasm, where it is GDP-bound. Expressing wild-type RanBP3 but not the S58A mutant restores the Ran gradient. Furthermore, the S58A mutant reduces the efficiency of GDP release by nuclear RCC1 in vitro, which further confirms that phosphorylated RanBP3 can modulate the Ran gradient. Finally, by visualizing the nuclear import of a tagged Ran target in wild-type or S58A RanBP3-expressing cells, Blenis and colleagues demonstrated that phosphorylation of RanBP3 increases the efficiency of nuclear import.

This work reveals how cellular signals might affect the Ran gradient and how the Ras–ERK and PI3K signalling pathways can converge to modulate nucleocytoplasmic protein transport.

Francesca Cesari

References

Yoon, S-O. et al. Ran-binding protein 3 phosphorylation links the Ras and PI3-kinase pathways to nucleocytoplasmic transport. Mol. Cell 29, 362–375 (2008) | Article | PubMed |