Question

Ran is a GTPase. Ran in the GTP bound form is concentrated in the nucleus while Ran in the GDP bound form is concentrated in the cytosol. This compartmentalization of Ran-GTP versus Ran-GDP is essential for nuclear-cytosolic protein transport. Please select which of the following statements best describes how the Ran-GTP/Ran GDP gradient is established. Select one: 1. It's another trick question. Ran has nothing to do with nuclear import. Ran is for signaling purposes. The small GTPase, Ras, drives nuclear import. 2. Ran-GTP is physically anchored in the nucleus and cannot diffuse out while Ran GDP is physically anchored in the cytosol and cannot diffuse into the nucleus. 3. Ran-GTP can only diffuse into the nucleus. Ran GDP can only diffuse out of the nucleus. 4. Ran can diffuse across the pore in either direction. If it diffuses into the nucleus, then Ran-GEF loads Ran with GTP. If Ran is in the cytosol, Ran-GAP catalyzes GTP hydrolysis to make Ran GDP. 5. Fast shuttling of Ran-GEF and Ran-GAP back and forth across the nuclear are required for the constant maintenance of the Ran gradient.

Answer 1

Ans- Ran-GEF in the nucleoplasm maintains the Ran in the Ran-GTP state where it promotes dissociation of the cargo protein. Ran-GAP on the cytoplasmic side of the nuclear pore complex converts Ran-GTP to Ran-GDP dissociating the importin- Ran-GTP complex and releasing the free importin into the cytosol.

Hence option 4 is correct