Nuclear fission, or nucleopore, is a large protein complex that passes through a nucleolem (a double membrane that encloses a nucleus in a eukaryotic cell) and forms a permeable channel therein. Typically, about 2000 nucleopores are found on the surface of the vertebral nucleus, but this number can vary widely depending on the number of transcripts that have occurred with the cell. The proteins that form the nuclear pore are called nucleoporins. About half of the nucleoporins contain types of folded tertiary structure, such as the alpha solenoid and the beta propeller, and in some cases both. The rest of the nucleoporins do not have folded structures in their composition, so they are characterized by considerable variability of the structure (due to the absence of a rigid secondary structure) . These disordered proteins are called FG-nucleoporins, given the frequent repetition in their primary structure of phenylalanine-glycine amino acids (F is a one-letter designation of phenylalanine, G is glycine) . . Also, structures such as RNA (mRNA, ribosome components), proteins (DNA polymerases, laminas), hydrocarbons, small molecules, and lipids can be transported through nucleopores. It is important to note that each so-called nuclear pore complex (NPC) is capable of performing about 1000 movements per second. In this case, the small molecules are able to move freely through the pore due to diffusion, and the large molecules must be recognized by specific signal sequences and then displaced by nucleoproteins within the nucleus or outward using energy-absorbing mechanisms. This mechanism of large molecule movement is known as the "Ran-GTP phase" (RAN cycle).
Each of the eight protein subunits that make up the outer outer ring of a nuclear pore has a lever-like structure (see figure) extending into the pore channel . The central part of the pore most often contains a cortical locking formation. It is unknown at present whether this formation is a truly functional locking element, or simply a mass transported through time and stopped during the preparation of the test specimens.
1 Dimensions and structure
2 Transport through nucleopores
2.1 Importing Proteins
2.2 Exporting Proteins
2.3 Exporting RNA
3 Additional Images
Dimensions and Structure
The entire nucleopore complex has a diameter of about 120 nanometers, the pore channel diameter is about 50 nanometers, the longitudinal channel length is approximately 200 nanometers. The molecular weight of a nucleopore complex is about 50 mD and consists of about 30 different protein subunits . Transport via nucleopore
More: Nuclear transport
Objects less than 30 kDa are able to pass through a nucleopore due to passive diffusion. Larger objects can also sometimes pass through the pore opening, but in negligible amounts . Effective transport through the nucleopore complex requires the presence of several protein structures . Karyoferrins, which can function as both importins (structures that provide transport within the nucleus) and exportins (structures that provide transport from the nucleus to the outside), are the most studied of these critically important structures; they are all representatives of the β-importin superfamily, which has a common three-dimensional structure.
Currently, there are three models that explain the mechanism of translocation (active non-diffusion movement) through nucleopores:
Affinity gradients in the direction of the sensory cortex
Brownian affine shutter
Any macromolecule that has a nuclear localizing amino acid sequence (YLP) is able to undergo rapid transport through the nucleolome due to the import of the import. There are several YLPs; all of them have a conserved polypeptide sequence with basic amino acid residues, type PKKKRKV.
The classical scheme of transport of YALP proteins begins with the binding of importin-α to YALP, which provides further binding in the formed structure of importin-β. The importin-α-importin-β-protein complex is directed to and passes through the nucleopore. Once inside the nucleus, RanGTP separates importinβ from it. Subsequently, the apoptosis-sensitive cellular protein (CAS), a RanGTP-binding exporter, detaches importin-α. The YLP protein is thus free in the nucleoplasm. The importin-β-RanGTF and importin-α-CAS-RanGTP complexes diffuse back into the cytoplasm where GTP is hydrolyzed to GDF, which in turn leads to the release of importin-α and importin-β, which may later participate in new transport cycles.
Although the transport of macromolecules through nucleopores occurs with the participation of accompanying proteins, this movement is not in itself energy-dependent. But the whole import cycle requires the hydrolysis of two GTP molecules; thus, it is energy dependent and is classified as a form of active cellular transport. The import cycle is ensured by the presence of a concentration nucleo-cytoplasmic gradient of RanGTP. This gradient occurs due to the localization solely in the nucleus of the so-called RanGEF proteins, which replace GDF with GTP on Ran molecules. Thus, in the nucleus, compared to the cytoplasm, under normal conditions there is an increased concentration of RanGTP.
Export of proteins
Some nuclear proteins, such as ribosome subunits and RNA messengers, are synthesized in the nucleus and then transported to the cytoplasm through nucleopores . This is provided by a mechanism similar in principle to the mechanism of import.
In the classical export scheme, proteins containing a specific nuclear export sequence (NNP) of amino acids bind to exportin and RanGTP to form a heteromeric complex. This complex is able to move into the cytoplasm by diffusion, after which the GTP is hydrolyzed and the NFP protein is released. RanGDF diffuses back into the kernel, where GDF is replaced by GTP due to RanGEF. This process is also energy dependent, since it requires the hydrolysis of GTP. Exports of proteins with the participation of exporters can be inhibited (up to blocking) by the antibiotic leptomycin.
RNA Exports For each type of RNA, there are specific routes of transport from the nucleus. This transport is also NNP-dependent, with NNP present in the protein structures that RNA binds to during synthesis (with the exception of transport RNA that does not have such adapter proteins). It is important to note that all types of viral RNA and all types of cellular RNA, except for matrix RNA, are dependent on RanGTP in their transport. Inactivated template RNA is transported with the participation of its specific mRNA transport protein factors. These factors are Mex67 / Tap (large subunit) and Mtr2 / p15 (small subunit). In addition, mRNA transport requires binding to a large subunit of a specific protein adapter.
RanGTP - Transport Cycle
↑ Denning D, Patel S, Uversky V, Fink A , Rexach M (2003). Disorder in the nuclear pore complex: the FG repeat regions of nucleoporins are natively unfolded. Proc Natl Acad Sci U S A 100 (5). with. 2450–5. PMID 12604785.
↑ Peters R (2006). Introduction to nucleocytoplasmic transport: molecules and mechanisms. Methods Mol Biol 322. p. 235–58. PMID 16739728.
↑ Alber F, Dokudovskaya S, Veenhoff L, Zhang W, Kipper J, Devos D, Suprapto A, Karni-Schmidt O, Williams R, Chait B, Rout M, Sali A (2007). Determining the architectures of macromolecular assemblies. Nature 450 (7170). with. 683–94. PMID 18046405.
↑ Rodriguez M, Dargemont C, Stutz F (2004). Nuclear export of RNA. Biol Cell 96 (8). with. 639–55. PMID 15519698.
↑ Reed R, Hurt E (2002). A conserved mRNA export machinery coupled to pre-mRNA splicing. Cell 108 (4). with. 523–31. PMID 11909523.
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