NUCLEAR PORE: CHARACTERISTICS, FUNCTIONS, COMPONENTS - BIOLOGY - 2025

The nuclear pore (from the Greek, pores = passage or transit) is the "gate" of the nucleus that allows most of the transport between the nucleoplasm and the cytoplasm. The nuclear pore joins the inner and outer membranes of the nucleus to form channels, which serve to transport proteins and RNA.

The word pore does not reflect the complexity of the structure in question. Because of this, it is preferable to refer to nuclear pore complex (CPN), rather than nuclear pore. The CPN can undergo changes in its structure during transport or states of the cell cycle.

Source: RS Shaw at English Wikipedia.

Recently, it has been discovered that nucleoporins, proteins that make up the CPN, have an important role in the regulation of gene expression. Thus, when mutations occur that affect the function of nucleoporins, pathologies occur in humans, such as autoimmune diseases, cardiomyopathies, viral infections and cancer.

characteristics

Using electron tomography, it was determined that the CPN has a thickness of 50 nm, an external diameter between 80 and 120 nm, and an internal diameter of 40 nm. Large molecules such as the large ribosome subunit (MW 1590 kDa) can be exported out of the nucleus through the CPN. It is estimated that there are between 2,000 and 4,000 NPCs per nucleus.

The molecular weight of an individual CPN is approximately between 120 and 125 MDa (1 MDa = 10 ⁶ Da) in vertebrates. In contrast, the CPN is smaller in yeast, in which it is approximately 60 MDa. Despite the enormous size of the CPN, nucleoporins are highly conserved in all eukaryotes.

Translocation through the CPN is a fast process, the speed of which is 1000 translocations / second. However, the CPN does not itself determine the direction of the transport flow.

It depends on the RanGTP gradient, which is greater in the nucleus than in the cytosol. This gradient is maintained by a guanine exchange factor Ran.

During the cell cycle, CPNs undergo assembly and disunity cycles of their components. Assembly occurs at the interface and immediately after mitosis.

Features

Ribonucleic acid (small nuclear RNA, messenger RNA, transfer RNA), proteins, and ribonucleoproteins (RNPs) must be actively transported through CPN. This means that the energy from hydrolysis of ATP and GTP is required. Each molecule is transported in a specific way.

In general, RNA molecules are packed with proteins forming RNP complexes, being exported in this way. In contrast, proteins that are actively transported to the nucleus must possess a nucleus localization signal sequence (SLN), possessing positively charged amino acid residues (eg, KKKRK).

Proteins that are exported to the nucleus must have a nucleus export signal (NES) rich in the amino acid leucine.

In addition to facilitating transport between the nucleus and the cytoplasm, CPNs are involved in the organization of chromatin, the regulation of gene expression, and DNA repair. Nucleoporins (Nups) promote the activation or repression of transcription, regardless of the state of cell proliferation.

In yeast, Nups are found in the CNP of the nuclear envelope. In metazoans they are found inside. They carry out the same functions in all eukaryotes.

Import of substances

Through the CPN there is passive diffusion of small molecules in both directions and active transport, import of proteins, export of RNA and ribonucleoproteins (RNPs), and the bidirectional shuttle of molecules. The latter includes RNA, RNP, and proteins involved in signaling, biogenesis, and turnover.

The importation of proteins into the nucleus occurs in two steps: 1) binding of the protein to the cytoplasmic side of the CPN; 2) ATP-dependent translocation through the CPN. This process requires the hydrolysis of ATP and the exchange of GTP / GDP between the nucleus and the cytoplasm.

According to a transport model, the receptor-protein complex moves along the channel by binding, dissociating, and reattaching to the FG repeat sequences of nucleoporins. In this way, the complex moves from one nucleoporin to another within the CPN.

Export of substances

It is similar to import. Ran GTPase enforces directionality on transport through CNP. Ran is a molecular switch with two conformational states, depending on whether it is bound to GDP or GTP.

Two Ran-specific regulatory proteins trigger the conversion between the two states: 1) cytosolic GTPase activating protein (GAP), which causes GTP hydrolysis and thus converts Ran-GTP to Ran-GDP; and 2) nuclear guanine exchange factor (GEF), which promotes the exchange of GDP for GTP and converts Ran-GDP to Ran-GTP.

The cytosol contains mainly Ran-GDP. The kernel mainly contains Ran-GTP. This gradient of the two conformational forms of Ran directs the transport in the appropriate direction.

Importation of the receptor, attached to the cargo, is facilitated by attachment to the FG-repeats. If it reaches the nuclear side of the CNP, Ran-GTP joins the receiver in releasing his position. Thus, Ran-GTP creates the direction of the import process.

Nuclear export is similar. However, Ran-GTP in the nucleus promotes the binding of cargo to receptor export. When the export receptor moves through the pore into the cytosol, it encounters Ran-GAP, which induces the hydrolysis of GTP to GDP. Finally, the receptor is released from its post and Ran-GDP in the cytosol.

RNA transport

The export of some classes of RNA is similar to the export of proteins. For example, tRNA and nsRNA (small nuclear) use the RanGTP gradient and are transported through the CPN by carioferin exportin-t and Crm, respectively. The export of mature ribosomes is also dependent on the RanGTP gradient.

MRNA is exported in a very different way than proteins and other RNAs. For its export, mRNA forms a complex of messenger RNP (mRNP), in which one mRNA molecule is surrounded by hundreds of protein molecules. These proteins are responsible for the processing, capping, splicing and polyadenylation of mRNA.

The cell must be able to distinguish between mRNA with mature mRNA and mRNA with immature mRNA. The mRNA, which forms the RPNm complex, could adopt topologies that need to be remodeled for transport. Before mRNP enters the CPN, a control step occurs carried out by TRAMP and exosome protein complexes.

When the mature RNPm is assembled, the RPNm is transported through the channel by a transport receptor (Nxf1-Nxt1). This receptor requires the hydrolysis of ATP (not a RanGTP gradient) to establish the remodeling directionality of the mRNP, which will reach the cytoplasm.

The nuclear pore complex and the control of gene expression

Some studies indicate that CPN components could influence the regulation of gene expression by controlling the structure of chromatin and its accessibility to transcription factors.

In more recently evolved eukaryotes, heterochromatin is preferentially located at the periphery of the nucleus. This territory is interrupted by euchromatin channels, which are maintained by the nuclear basket of the CPN. The nuclear basket association with euchromatin is related to gene transcription.

For example, activation of transcription at the CPN level involves the interaction of components of the nuclear basket with proteins such as histone SAGA acetyltransferase and RNA export factors.

Thus, the nuclear basket is a platform for numerous highly transcribed housekeeping genes and genes strongly induced by changes in environmental conditions.

The nuclear pore complex and virology

Viral infection of eukaryotic cells is dependent on the CPN. In each case of viral infection, its success depends on the DNA, RNA, or RPN passing through the CPN to reach its ultimate goal, which is virus replication.

The simian virus 40 (SV40) has been one of the most studied models to investigate the role of the CPN in translocation within the nucleus. This is because SV40 has a small genome (5,000 bases).

It has been shown that the transport of virus DNA is facilitated by virus coat proteins, which protect the virus until it reaches the nucleus.

Components

The CPN is embedded inside the nuclear envelope and is made up of between 500 and 1000 Nups, approximately. These proteins are organized into structural subcomplexes or modules, which interact with each other.

The first module is a central component, or ring, within the hourglass-shaped pore, which is limited by another ring 120 nm in diameter on both faces, intranuclear and cytoplasmic. The second module is the nucleus and cytoplasm rings (120 nm in diameter each) located around the hourglass shaped component.

The third module is eight filaments that project from the 120 nm ring into the nucleoplasm and form a basket-shaped structure. The fourth module is made up of the filaments that project towards the side of the cytoplasm.

The Y-shaped complex, made up of six Nups and the proteins Seh 1 and Sec 13, is the largest and best characterized complex of CNP. This complex is the essential unit that is part of the scaffolding of the CPN.

Despite the low similarity between the sequences of the Nups, the CPN scaffold is highly conserved in all eukaryotes.

References

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4. Knockenhauer, KE, Schwartz, TU 2016. The nuclear pore complex as a flexible and dynamic gate. Cell, 164, 1162-1171.
5. Ptak, C., Aitchison, JD, Wozniak, RW 2014. The multifunctional nuclear pore complex: a platform for controlling gene expression. Current Opinion of Cell Biology, DOI: 10.1016 / j.ceb.2014.02.001.
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