Import of molecules into the nucleus requires the use of importin and RAN-GTP. When RAN-GTP binds to importin, what happens to the molecule?

• A. The molecule is released into the cytoplasm
• B. The molecule is released into the nucleus
• C. The molecule is bound in the cytoplasm
• D. The molecule is bound in the nucleus

Answer: (B)

Verified step by step guidance

1. Understand the role of importin: Importin is a transport receptor that facilitates the import of proteins into the nucleus by recognizing nuclear localization signals (NLS) on the cargo proteins.
2. Learn about RAN-GTP: RAN is a small GTPase that exists in two forms, RAN-GTP and RAN-GDP. RAN-GTP is predominantly found in the nucleus and plays a crucial role in the directionality of nuclear transport.
3. Recognize the interaction between importin and RAN-GTP: When importin, bound to its cargo, enters the nucleus, it encounters RAN-GTP.
4. Analyze the effect of RAN-GTP binding: The binding of RAN-GTP to importin causes a conformational change in importin, leading to the release of the cargo molecule into the nucleus.
5. Conclude the process: After the cargo is released, the importin-RAN-GTP complex is transported back to the cytoplasm, where RAN-GTP is hydrolyzed to RAN-GDP, allowing importin to be reused for another round of transport.