Cell Test 3

1. Cargo contains a nuclear localization signal (NLS).
2. Importin (usually importin-α/β) binds the cargo via NLS.
3. The importin–cargo complex translocates through the nuclear pore complex (NPC).
4. In the nucleus, Ran-GTP binds to importin-β, triggering cargo release.
5. The importin–Ran-GTP complex is exported to the cytoplasm.
6. In the cytoplasm, Ran-GAP stimulates GTP hydrolysis → GDP, causing dissociation of Ran-GDP from importin.
7. Importin is free to bind new cargo.

1. Cargo has a nuclear export signal (NES).
2. Exportin (e.g., CRM1) recognizes NES and binds the cargo.
3. Ran-GTP binds and stabilizes the complex.
4. The ternary complex (Cargo + Exportin + Ran-GTP) exits via NPC.
5. In the cytoplasm, Ran-GAP hydrolyzes GTP → GDP.
6. Complex dissociates; cargo is released; exportin and Ran-GDP are recycled.

Structure: Small GTP-binding protein
Function: Imparts directionality to nuclear transport (import/export)
Location:
- Ran-GTP: Nucleus (due to Ran-GEF activity)
- Ran-GDP: Cytoplasm (due to Ran-GAP activity)

Function: Promotes exchange of GDP for GTP on Ran
Location: Nucleus
Importance: Maintains high concentration of Ran-GTP in the nucleus, ensuring correct transport directionality.

Function: Stimulates GTP hydrolysis on Ran
Location: Cytoplasm
Importance: Ensures Ran is predominantly in GDP-bound form outside the nucleus, supporting the export cycle.

Structure: Receptor protein (Importin-β can work with Importin-α)
Function: Binds cargo with NLS, transports it through NPC
Localization: Predominantly in the cytoplasm; concentrated around the nuclear envelope

Structure: Karyopherin family export receptor
Function: Recognizes NES; exports proteins and some RNAs (via adaptors)
Localization: Nucleus, exports cargo with Ran-GTP
Cargo examples: >1000 types including translation-related proteins

1. NF-κB is bound by IκB in the cytoplasm → NLS hidden.
2. TNF binding activates IκB kinase.
3. IκB is phosphorylated → ubiquitinated → degraded.
4. NF-κB’s NLS is now exposed → binds importin → enters nucleus.
5. In nucleus, NF-κB activates genes for apoptosis, immune response.

Dephosphorylation of Pho4 exposes its NLS, enabling importin binding and nuclear entry.

1. Processed mRNAs are bound by NXT1/NXF1 (heterodimeric complex).
2. They interact with FG-repeats in NPC proteins.
3. At cytoplasmic face, Gle1 + IP6 activate Dbp5, an ATPase.
4. Dbp5 hydrolyzes ATP → triggers release of mRNA and export machinery.
5. Directionality ensured by irreversible dissociation.

NXT1/NXF1: mRNA transport complex
Gle1: Binds IP6, activates Dbp5
Dbp5: ATP-dependent helicase, disassembles export complex at cytoplasmic NPC face
CBC (cap-binding complex): Binds 5’ cap; must be removed for translation
HuR, eIF4E: CRM1 adapters for some mRNA export (non-primary pathway)

mDia2 localizes to nuclear envelope in cytoplasm.
siRNA against importin-β reduces both importin and mDia2 envelope localization.
Conclusion: mDia2 associates with importin-β but is not imported—shows how localization can reflect interaction without transport

Normally: CRM1 exports proteins → mostly cytoplasmic localization.
After inhibition: Proteins accumulate in the nucleus → nuclear retention
Conclusion: Export requires active CRM1 + NES

- It will be recognized by importin and transported into the nucleus.
- You can also mutate an NLS to prevent nuclear import.