Sandbox Reserved 709: Difference between revisions
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Marmin Lucas (talk | contribs) Part 3 |
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* '''Step 6 :''' '''[[Nup50|Nup50]]''' (also called '''[[Nup50|Npap60]]'''), a nucleoplasmin, weakens the link between importin α and the NLS of the cargo. Thus the cargo protein is now free in the nucleoplasm. | * '''Step 6 :''' '''[[Nup50|Nup50]]''' (also called '''[[Nup50|Npap60]]'''), a nucleoplasmin, weakens the link between importin α and the NLS of the cargo. Thus the cargo protein is now free in the nucleoplasm. | ||
* '''Step 7 :''' Importin α can’t leave the nucleus only with the help of Ran-GTP. It needs also interaction with '''[[CAS|CAS]]''' (''Cellular Apoptosis Susceptibility'' protein), an '''exportin''' which is similar in shape to importin β. Binding between CAS and importin α is favoured by interactions with Nup50. | * '''Step 7 :''' Importin α can’t leave the nucleus only with the help of Ran-GTP. It needs also interaction with '''[[CAS|CAS]]''' (''Cellular Apoptosis Susceptibility'' protein), an '''exportin''' which is similar in shape to importin β. Binding between CAS and importin α is favoured by interactions with Nup50. | ||
* '''Step 8 :'''The newly formed '''importin α:CAS:Ran-GTP''' complex is able to interact with nucleoplasmins of NPC, and once in the cytoplasm, Ran-GTP is hydrolysed into Ran-GDP (''see '''Step 5''' for details''). The complex splits, and importin α is now ready for a new import cycle.<ref>PMID:16222336</ref> | * '''Step 8 :'''The newly formed '''importin α:CAS:Ran-GTP''' complex is able to interact with nucleoplasmins of NPC, and once in the cytoplasm, Ran-GTP is hydrolysed into Ran-GDP (''see '''Step 5''' for details''). The complex splits, and importin α is now ready for a new import cycle.<ref name ="NUP50">PMID:16222336</ref> | ||
==Importin α structure== | ==Importin α structure== | ||
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Nup50 (''Nucleoporin 50 kDa'', about 450 aminoacids) is a part of the NPCs, in the nucleoplasmic side. It is able to bind to <scene name='Sandbox_Reserved_709/Importin/2'> importin α </scene>thanks to <scene name='Sandbox_Reserved_709/Nup50/3'>two binding segments</scene> on its N-terminus (''only 40 first aminoacids represented on the structure''). | Nup50 (''Nucleoporin 50 kDa'', about 450 aminoacids) is a part of the NPCs, in the nucleoplasmic side. It is able to bind to <scene name='Sandbox_Reserved_709/Importin/2'> importin α </scene>thanks to <scene name='Sandbox_Reserved_709/Nup50/3'>two binding segments</scene> on its N-terminus (''only 40 first aminoacids represented on the structure''). | ||
* The first one (''1-15'') binds to the '''minor NLS binding site''' of importin α. This binding is allowed by the '''two basic aminoacids'''<scene name='Sandbox_Reserved_709/Nup_bindingnls/1'>Lys3 and Arg4</scene> of Nup50. In fact, the positive charge will establish bonds with acidic aminoacids of the NLS binding site, just as an NLS-containing cargo would do. In presence of Nup50, the complex importin α:NLS dissociation rate is higher than the one of spontaneous dissociation : there is a '''competition''' between Nup50 and NLS, whose affinities for the NLS minor binding site are in the same order of magnitude. | * The first one (''1-15'') binds to the '''minor NLS binding site''' of importin α. This binding is allowed by the '''two basic aminoacids'''<scene name='Sandbox_Reserved_709/Nup_bindingnls/1'>Lys3 and Arg4</scene> of Nup50. In fact, the positive charge will establish bonds with acidic aminoacids of the NLS binding site, just as an NLS-containing cargo would do. In presence of Nup50, the complex importin α:NLS dissociation rate is higher than the one of spontaneous dissociation : there is a '''competition''' between Nup50 and NLS, whose affinities for the NLS minor binding site are in the same order of magnitude. <ref name ="NUP50"/> | ||
* The second one (''24-46'') binds to the '''C-terminus''' of importin α. Nup50 basic residues 41-46 ('''KKAKRR''') are directed by the formation of two turns of an α helix (''31-36'' SEEVMK) towards an <scene name='Sandbox_Reserved_709/Nup50_bindingcas/1'>acidic surface</scene> on ARM10. Those electrostatic interactions are essential for the binding of Nup50. Indeed, replacing residues 41-46 by Ala makes the binding undetectable. Furthermore, this binding overlaps the CAS and the Ran binding sites, explaining why interaction between Nup50 and importin is crucial for building the export complex. | * The second one (''24-46'') binds to the '''C-terminus''' of importin α. Nup50 basic residues 41-46 ('''KKAKRR''') are directed by the formation of '''two turns of an α helix''' (''31-36'' SEEVMK) towards an <scene name='Sandbox_Reserved_709/Nup50_bindingcas/1'>acidic surface</scene> on ARM10. Those electrostatic interactions are essential for the binding of Nup50. Indeed, replacing residues 41-46 by Ala makes the binding undetectable. Furthermore, this binding overlaps the '''CAS''' and the '''Ran binding sites''', explaining why interaction between Nup50 and importin is crucial for building the '''export complex'''.<ref name ="NUP50"/> | ||
The chain between the two binding segments is rich in acidic residues, and seems to work as a flexible linker between those two sites. | The chain between the two binding segments is rich in acidic residues, and seems to work as a flexible linker between those two sites. | ||
