Chapter 16: Intracellular Compartments and Protein Sorting

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16.4:

Signal de localisation nucléaire et importation

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JoVE Core Cell Biology

Nuclear Localization Signals and Import

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16.3: Nuclear Protein Sorting

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16.5: Nuclear Export

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Proteins imported from the cytosol to the nucleus contain short stretches of amino acid sequences called nuclear localization signals or NLS. Importins, specialized nuclear import receptors, recognize the NLS of cargo proteins and transport them to the nucleus through the nuclear pore complexes or NPCs. Importins are present in the cytosol as soluble dimers of an alpha and a beta subunit. Importin alpha binds the NLS of a cargo protein to form a cargo-receptor complex. Importin beta binds cytoplasmic fibrils extending from the NPC and docks the cargo-receptor complex onto the channel's opening. The inner channel of the NPC is lined with phenylalanine-glycine or FG-rich repeats that form a gel-like selective barrier. As the cargo-receptor complex moves through the channel, the beta subunit makes multiple weak contacts with the FG repeats and hops across the channel, breaking the interactions between the FG repeats and dissolving the gel-like barrier. Through such repeated contacts and dissolution, the cargo-receptor complex travels inside the nucleus. Within the nucleus, a GTP-bound protein called Ran binds importin beta and induces a conformational change in the receptor to release the cargo protein. The importin-Ran-GTP complex is then transported back to the cytosol, where GTP is hydrolyzed, releasing the importins for another round of cargo import.

16.4:

Signal de localisation nucléaire et importation

Proteins targeted to the nucleus carry short stretches of amino acid sequences called the nuclear localization signal or NLS. Classical nuclear localization signals are of two types: monopartite and bipartite NLS. Monopartite classical NLS (cNLS) consists of a single cluster of 4-8 amino acids. Bipartite cNLS consists of two clusters of 2-3 amino acids and a 9-12 residue long proline-rich linker bridging the two clusters. Signal clusters are rich in positively charged amino acids such as lysine and arginine and are present as loops or signal patches. In addition to cNLS, other classes of NLS include:
1) Non-classical NLS,
2) Putative NLS,
3) Spatial epitope NLS,
4) Cryptic NLS, and
5) Multiple NLS.

NLS is recognized by soluble nuclear import receptors called importins that are made of alpha and beta subunits. Importin alpha binds cargo proteins to form a receptor-cargo complex. Importin beta interacts with FG nucleoporins and shuttles the receptor-cargo complex across the nucleus. Once inside the nucleus, importin beta interacts with the GTP-bound small monomeric Ran GTPase, which leads to dissociation of the importin alpha-cargo complex. Cas-Ran GTP, the export receptor for importin alpha, associates with Nup 50 and importin alpha to release the cargo into the nucleus. Importin beta-Ran GTP complex and CAS-Ran GTP-Importin alpha complex shuttles back to the cytosol for another round of nuclear import.

Suggested Reading

Alberts, Bruce, et al. Molecular Biology of the Cell. 6th ed. Garland Science, 2017. pp 650-652
Lodish, Harvey, et al. Molecular Cell Biology. 8th ed. W.H. Freeman and Company, 2016. Pp 624-625
Lange, et al. Classical nuclear localization signals: Definition, function, and interaction with importin alpha, J Biol Chem, 2007, 282(8): 5101-5105. Pp 2-3
Lodish et al. Signal mediated transport through nuclear pore complexes, NCBI Bookshelf. Pp 5-6
Lu et al., Types of nuclear localization signals and mechanisms of protein import into the nucleus. Cell Commun Signal (2021) 19:60.