16.4
Sinais de Localização Nuclear e Importação
As proteínas direcionadas ao núcleo carregam pequenos trechos de sequências de aminoácidos chamados de sinal de localização nuclear, ou NLS. Os sinais…
As proteínas importadas do citosol para o núcleo contêm pequenos trechos de sequências de aminoácidos chamados sinais de localização nuclear ou NLS.
As importinas, receptores de importação nuclear especializados, reconhecem o NLS das proteínas de carga e as transportam para o núcleo através dos complexos de poros nucleares ou NPCs.
As importinas estão presentes no citosol como dímeros solúveis de uma subunidade alfa e beta. A importina alfa se liga ao NLS de uma proteína de carga para formar um complexo receptor de carga.
A importina beta liga-se às fibrilas citoplasmáticas que se estendem do NPC e encaixa o complexo receptor de carga na abertura do canal.
O canal interno do NPC é revestido com repetições ricas em fenilalanina-glicina ou FG que formam uma barreira seletiva semelhante a um gel.
À medida que o complexo receptor de carga se move através do canal, a subunidade beta faz vários contatos fracos com as repetições FG e salta através do canal, quebrando as interações entre as repetições FG e dissolvendo a barreira semelhante a gel. Por meio de tais contatos repetidos e dissolução, o complexo carga-receptor viaja dentro do núcleo.
Dentro do núcleo, uma proteína ligada ao GTP chamada Ran se liga à importina beta e induz uma mudança conformacional no receptor para liberar a proteína de carga.
O complexo importina-Ran-GTP é então transportado de volta para o citosol, onde o GTP é hidrolisado, liberando as importinas para outra rodada de importação 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.