Question

You express in a cell a constitutive active form of Ran. Which of the following is a likely consequence?

1) Ran will not bind to Import Receptors.
2) Export Receptors will not bind to their Cargo.
3) Import Receptors will not bind to their Cargo.
4) Ran will contain GDP instead of GTP.
5) Ran will be found only in the Nucleus.

Answer 1

Expressing a constitutively active form of Ran bound to GTP leads to import receptors being unable to bind to their cargo, due to Ran-GTP's high affinity which prevents cargo release and receptor resetting for another transport cycle. The correct option is (4)

Step-by-step explanation:

If constitutively active Ran that is always bound to GTP were expressed in a cell, several consequences regarding nuclear transport could follow, but among the options provided the most directly related effect would be that Import Receptors will not bind to their Cargo. Normally, Ran-GTP interacts with Import Receptors after they ferry their cargo into the nucleus and causes them to release their cargo. In its GTP-bound state, Ran promotes the release of the cargo from the import receptors, allowing them to return to the cytoplasm to bind new cargo. If Ran cannot hydrolyze GTP to GDP and become Ran-GDP (which has a different conformation and a different affinity for import and export receptors), then the import receptors will remain bound to Ran-GTP and unable to release their cargo or bind new cargo, therefore Import Receptors will not bind to their Cargo.