16.4
Señales de localización nuclear e importación.
Las proteínas dirigidas al núcleo transportan tramos cortos de secuencias de aminoácidos llamados señal de localización nuclear o NLS. Las señales de…
Las proteínas importadas del citosol al núcleo contienen tramos cortos de secuencias de aminoácidos llamadas señales de localización nuclear o NLS.
Las importinas, receptores especializados en importaciones nucleares, reconocen el NLS de las proteínas de carga y las transportan al núcleo a través de los complejos de poros nucleares o NPC.
Las importinas están presentes en el citosol como dímeros solubles de una subunidad alfa y una subunidad beta. La importina alfa se une al NLS de una proteína de carga para formar un complejo de receptor de carga.
La importina beta se une a las fibrillas citoplasmáticas que se extienden desde la NPC y acopla el complejo receptor-carga a la abertura del canal.
El canal interno del NPC está recubierto con fenilalanina-glicina o repeticiones ricas en FG que forman una barrera selectiva similar a un gel.
A medida que el complejo carga-receptor se mueve a través del canal, la subunidad beta hace múltiples contactos débiles con las repeticiones FG y salta a través del canal, rompiendo las interacciones entre las repeticiones FG y disolviendo la barrera gelatinosa. A través de estos contactos repetidos y disolución, el complejo carga-receptor viaja dentro del núcleo.
Dentro del núcleo, una proteína unida a GTP llamada Ran se une a la importina beta e induce un cambio conformacional en el receptor para liberar la proteína de carga.
A continuación, el complejo importina-Ran-GTP se transporta de vuelta al citosol, donde se hidroliza el GTP, liberando las importinas para otra ronda de importación de carga.
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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.