nuclear import and export

Summary

The nuclear pore complex (NPC) controls the transport of molecules between the nucleus and cytoplasm in eukaryotic cells. Small molecules pass freely, but larger ones like proteins and RNA require active transport. Proteins with nuclear localization signals are recognized by importins, which guide them through the NPC. Inside the nucleus, Ran-GTP releases the cargo, and the Ran cycle ensures importins are recycled. Similarly, proteins with nuclear export signals are exported by exportins. Ran-GTP plays opposite roles for import and export, ensuring proper direction of transport across the nuclear membrane.

Takeaways

• 🔑 The nuclear pore complexes are the only channels allowing molecules to travel between the nucleus and cytoplasm.
• 🚪 Most proteins and RNAs are too large to passively diffuse through nuclear pore complexes and require active transport.
• 🧬 Proteins that are imported into the nucleus have nuclear localization signals (NLS), which consist of basic amino acid sequences.
• 🔍 NLS are recognized by nuclear transport receptors called importins, which transport cargo proteins into the nucleus.
• ⚙️ Importins bind to nuclear pore proteins, and the protein complex is translocated through the pore into the nucleus.
• 🧠 The complex is disrupted inside the nucleus by the binding of a protein called Ran, which carries GTP, releasing the cargo.
• ♻️ Ran plays a key role in protein import/export, and its state (GTP-bound or GDP-bound) dictates the binding and release of cargo.
• 🔄 Ran-GDP in the cytoplasm is transported back into the nucleus by its import receptor (NTF2), where it is recharged with GTP.
• 📤 Nuclear export signals (NES) direct proteins out of the nucleus, recognized by exportins, which facilitate export to the cytoplasm.
• 🔗 Ran-GTP promotes stable complexes for exportins and cargo proteins, guiding protein transport from the nucleus to the cytoplasm.

Q & A

What is the main function of nuclear pore complexes in eukaryotic cells?

Nuclear pore complexes control the traffic of small polar molecules, ions, and macromolecules between the nucleus and cytoplasm, playing a fundamental role in the physiology of eukaryotic cells.

Why can't most proteins and RNAs passively diffuse through the nuclear pore complex?

Most proteins and RNAs are too large to passively diffuse through the nuclear pore complex and must instead move through an active transport process.

What allows certain proteins to enter the nucleus from the cytoplasm?

Proteins that enter the nucleus from the cytoplasm have specific amino acid sequences called nuclear localization signals, which are recognized by nuclear transport receptors.

What are nuclear localization signals, and what do they consist of?

Nuclear localization signals are short segments of basic amino acids that allow specific proteins to be recognized for transport into the nucleus.

What role do importins play in nuclear protein import?

Importins are nuclear transport receptors that bind to cargo proteins with nuclear localization signals, facilitating their transport through the nuclear pore complex into the nucleus.

How does the protein Ran assist in the transport process?

Ran, when bound to GTP, binds to importins, causing a conformational change that releases the cargo protein into the nucleus. Ran also plays a key role in protein export.

What happens to the Ran-Importin complex after the cargo is released in the nucleus?

The Ran-Importin complex is re-exported through the nuclear pore to the cytoplasm, where Ran hydrolyzes its GTP to GDP, causing the release of importin for reuse.

What maintains the steep gradient of Ran-GTP and Ran-GDP across the nuclear membrane?

Ran-GTP is concentrated inside the nucleus, while Ran-GDP is in the cytoplasm, and this gradient is maintained by the actions of Ran-GEF in the nucleus and Ran-GAP in the cytoplasm.

How do proteins get exported from the nucleus?

Proteins destined for export contain nuclear export signals that are recognized by exportins. Ran-GTP promotes stable complexes between exportins and their cargo, allowing transport through the nuclear pore complex to the cytoplasm.

What role does Ran-GTP play in protein export versus import?

In protein export, Ran-GTP promotes stable complexes between exportins and their cargo, whereas in protein import, Ran-GTP causes importins to release their cargo once inside the nucleus.