16.6:
Directionality of Nuclear Transport
The directionality of transport is maintained by a small monomeric protein Ran, complexed with GTP. Ran-GTP helps nuclear export receptors or exportins to load the cargo, and binds to the import receptor or importin to unload the cargo in the nucleus.
Within the nucleus, Ran is regulated by the chromatin-bound Ran-Guanine nucleotide exchange factor or Ran-GEF, which converts RanGDP to RanGTP.
In the cytosol, Ran GTPase activating protein or Ran-GAP associates with Ran-binding protein RanBP1 and accelerates the hydrolysis of Ran GTP to Ran GDP.
The differential localization of Ran-specific regulators ensures a much higher concentration of Ran-GTP in the nucleus than in the cytosol.
During protein import, Ran-GTP binds with the incoming importin-cargo complex, facilitating the cargo release in the nucleus.
A higher concentration of the importin-cargo complex in the cytosol further drives cargo import into the nucleus.
In contrast, during nuclear export, Ran-GTP increases the cargo binding affinity of exportins inside the nucleus. A higher concentration of exportin-cargo complex in the nucleus compared to the cytosol ensures their unidirectional trafficking.
16.6:
Directionality of Nuclear Transport
Ras-related nuclear protein or Ran is a small G protein that cycles between its GTP and GDP bound states. Ran specific regulators, a Ran GTPase Activating Protein or RanGAP present in the cytosol and a Ran guanine nucleotide exchange factor or RanGEF present inside the nucleus regulate GTP/GDP exchange. A high concentration of GTP inside the cells, in addition to this asymmetric distribution of Ran-specific regulators, leads to a higher RanGTP concentration inside the nucleus. This concentration gradient of RanGTP formed across the nucleus helps in the unidirectional trafficking of nuclear proteins.
During nuclear export, RanGTP binds exportin and increases its affinity to bind the cargo. The Ran GTP-cargo-exportin complex transits out of the nucleus through the nuclear pore complex or NPCs. RanBP1 and RanBP2 present near the cytosolic side of the NPCs stimulate RanGAP activity, helping Ran undergo GTP hydrolysis to RanGDP. This weakens the cargo-exportin affinity, eventually leading to cargo release in the cytosol. Unlike RanGTP, RanGDP cannot bind receptors alone and releases the exportins to be recycled back into the nucleus for a second round of cargo export.
The continuous efflux and hydrolysis of RanGTP in the cytosol lowers the RanGTP levels in the nucleus. Nuclear transport factor 2 (NTF2) binds hydrolyzed RanGDP in the cytosol transporting them back to the nucleus to compensate for the loss. Inside the nucleus, RanGDP binds to the Regulators of Chromatin Condensation 1 (RCC1), a RanGEF, and undergoes a GDP/GTP exchange to restore the nuclear RanGTP concentrations.
Suggested Reading
- Mary Shannon Moore. Ran and Nuclear Transport, THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 273, No. 36, Issue of September 4, pp. 22857–22860, 1998
- Atlanta Cook et al., Structural Biology of Nucleocytoplasmic Transport. Annu. Rev. Biochem. 2007. 76:647–71.
- Petra Bjork et al., Mechanisms of mRNA transport. Seminars in Cell and Developmental biology 32 (2014) 47-54.