16.4
Nucleaire lokalisatiesignalen en import
Eiwitten die op de kern zijn gericht, dragen korte stukken aminozuursequenties die het nucleaire lokalisatiesignaal of NLS worden genoemd. Klassieke n…
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.
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Q1: What are nuclear localization signals and why do proteins need them?
Nuclear localization signals (NLS) are short stretches of amino acid sequences that mark proteins for nuclear import. Proteins destined for the nucleus contain these signals, which are recognized by specialized import receptors called importins. Without NLS, proteins cannot be directed to and transported across the nuclear envelope.
Q2: How do importin alpha and importin beta work together during nuclear import?
Importin alpha recognizes and binds the NLS of cargo proteins, forming a cargo-receptor complex. Importin beta then binds to cytoplasmic fibrils extending from the nuclear pore complex and docks the cargo-receptor complex at the channel opening. Together, these subunits facilitate transport of the cargo protein into the nucleus.
Q3: What role do FG-rich repeats play in nuclear pore transport?
The inner channel of the nuclear pore complex is lined with phenylalanine-glycine (FG) repeats that form a gel-like selective barrier. As the cargo-receptor complex moves through, the importin beta subunit makes weak contacts with these FG repeats and hops across the channel, dissolving the gel-like barrier and allowing passage of the cargo protein.
Q4: What are the different types of classical nuclear localization signals?
Classical NLS exist in two forms: monopartite and bipartite. Monopartite NLS consists of a single cluster of 4-8 amino acids rich in lysine and arginine. Bipartite NLS contains two clusters of 2-3 amino acids separated by a 9-12 residue proline-rich linker. Both types are recognized by importin alpha for nuclear targeting.
Q5: How does Ran-GTP release cargo proteins inside the nucleus?
Once inside the nucleus, the GTP-bound protein Ran binds to importin beta and induces a conformational change in the receptor. This conformational change causes the importin alpha-cargo complex to dissociate, releasing the cargo protein into the nucleus. The importin-Ran-GTP complex is then transported back to the cytosol for recycling.
Q6: What happens to importins after they return to the cytosol?
When the importin-Ran-GTP complex returns to the cytosol, GTP is hydrolyzed to GDP, releasing the importins. The freed importin alpha and beta subunits dissociate and become available as soluble dimers in the cytosol for another round of cargo import, enabling continuous nuclear protein transport.
Q7: What are non-classical and cryptic nuclear localization signals?
Beyond classical monopartite and bipartite NLS, cells use non-classical NLS, putative NLS, spatial epitope NLS, and cryptic NLS for nuclear targeting. These alternative signal types provide flexibility in protein recognition and allow importins to transport diverse cargo proteins. Some proteins contain multiple NLS to enhance nuclear import efficiency.