During nuclear export of cargo proteins, what promotes GTP hydrolysis in the nucleus?

The correct answer highlights the role of Ran-binding protein and Ran-GAP in promoting GTP hydrolysis during the nuclear export of cargo proteins. In the context of nuclear transport, Ran is a small GTPase that exists in two forms: Ran-GTP, which is predominantly found in the nucleus, and Ran-GDP, which is more prevalent in the cytoplasm.

For proper transport, cargo proteins are bound to transport receptors in the nucleus; these receptors facilitate the recognition of nucleoporins at the nuclear pore complex for export. Upon binding, Ran-GAP (GTPase-activating protein) is located in the cytoplasm, where it promotes the hydrolysis of GTP to GDP. This hydrolysis of GTP to GDP leads to a conformational change in the transport receptor, causing the release of the cargo protein into the cytoplasm.

This mechanism is crucial because it allows for the recycling of transport receptors back into the nucleus, where they can bind to new cargo, and it maintains the concentration gradient of Ran-GTP and Ran-GDP that is essential for effective nuclear transport.

In contrast, Ran-GEF (guanine nucleotide exchange factor) is responsible for converting Ran-GDP back to Ran-GTP in the nucleus,