What is nucleo-cytoplasmic transport?
The movement of mRNA from nucleus to the cytoplasm for translation
Describe 3 traits of the nuclear pore complex
- Have inner and outer membrane
- have baskets used for movement of macromolecules in and out of nucleoplasm and cytoplasm
- Smaller moleciekls diffuse freely through the NPC, while bigger molecules must be transported
What makes nuclear pores?
- There is a gel like substance blocking diffusion of large molecules
What is needed to enter the nucleus? And to leave it?
NLS - nuclear localization signal
NES - nuclear export signal
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What are two important proteins in nuclear import?
- Ran - a G protein that exists in GTPbound or GDP bound conformation
- Nuclear transport receptors - importis bind to NLS on cargo proteins to facilitate transport through the pore by association with FG
Describe importin action
1)Importin forms a complex with NLS/cargo, this diffuses in due to a concentration gradient (more cargo on cytoplasmic side)
2) Importin facilitates this going through the pore with interactions with FG repeats
3) In nucleoplasm, the complex is greeted by Ran GEF (GTP bound form), removes the cargo from importin, and attaches to it
4) Now have a RanGTP:Importin complex, makes its way into cytoplams due to the RanGTP gradient
5) Greeted in cytoplasm by Ran GAP which removes Ran from importin
What are two key concentration gradients in nuclear IMPORT?
- High importin in cytoplasm
- High RanGTP in nucleus
Describe the steps in nuclear export
- Exportin+NESCargo+RanGTP forms a trimeric complex
- The complex makes its way out based on concentration gradient of the trimeric complex
- greeted in cytoplasm by ranGAP, which removes the cargo into cytoplasm
- Exportin and ran go back into the nucleus
(See graph to better visualize this)
What is the function of exportin t
Exports tRNAs
- binds fully processed tRNAs and Ran-GTP, passes through NPCs. Complex dissociates in cytosol when it interacts with Ran-GAP
Do we need Ran for exporting?
Can export in Ran independent manner using mRNA exporter
Explain the steps in ran-independent transport
Use of NFX1 and NXT1 - critical for movement of mature RNAs to export
Bind RNA with mRNP proteins:
- form a domain that interacts with FG repeats in nucleoporins
What is a Balbiani ring?
- Allows microscopic imaging of mRNP export
- mRNPs make way to nuclear basket that have polarity that allow mRNA to go through the pore , greeted by the ribosome
- ## mRNA exporter interacts with mature mRNAs
How does the mRNA exporter work?
- Interacts with mature mRNAs (proteins in nucleus must stay there! otherwise they must go back)
- RNA helicase knocks proteins off RNA as it leaves the nucleus (sends proteins back into nucleus), replaced by cytoplasmic proteins that protect RNA
- mRNA in cytoplasm now moves to formation of protein (PABPC!), cap protein
