Book : Cell Biology
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Posted by: CHELSEA
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Writer CHELSEA

Nucllear Pore

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Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are about an average of 2000 nuclear pore complexes (NPCs), in the nuclear envelope of a vertebrate cell, but it varies depending on cell type and the stage in the life cycle. The proteins that make up the nuclear pore complex are known as nucleoporins. About half of the nucleoporins typically contain solenoid protein domains—either an alpha solenoid or a beta-propeller fold, or in some cases both as separate structural domains. Each NPC contains at least 456 individual protein molecules and is composed of 30 distinct proteins (nucleoporins). The other half show structural characteristics typical of "natively unfolded" or intrinsically disordered proteins, i.e. they are highly flexible proteins that lack ordered secondary structure.  These disordered proteins are the FG nucleoporins, so called because their amino-acid sequence contains many phenylalanines—glycine repeats.

Nuclear pore complexes allow the transport of molecules across the nuclear envelope. This transport includes RNA and ribosomal proteins moving from nucleus to the cytoplasm and proteins (such as DNA polymerase and lamins), carbohydrates, signaling molecules and lipids moving into the nucleus. It is notable that the nuclear pore complex (NPC) can actively conduct 1000 translocations per complex per second. Although smaller molecules simply diffuse through the pores, larger molecules may be recognized by specific signal sequences and then be diffused with the help of nucleoporins into or out of the nucleus. It has been recently shown that these nucleoporins have specific evolutionary conserved features encoded in their sequences that provide insight into how they regulate the transport of molecules through the nuclear pore. Nucleoporin-mediated transport is not directly energy requiring, but depends on concentrations gradients associated with the RAN cycle. Each of the eight protein subunits surrounding the actual pore (the outer ring) projects a spoke-shaped protein over the pore channel. The center of the pore often appears to contain a plug-like structure. It is yet unknown whether this corresponds to an actual plug or is merely cargo caught in transit.

The entire nuclear pore complex has a diameter of about 120 nanometers in vertebrates. The diameter of the channel ranges from 5.2 nanometers in humans to 10.7 nm in the frog Xenopus laevis, with a depth of roughly 45 nm. mRNA, which is single-stranded, has a thickness of about 0.5 to 1 nm. The molecular mass of the mammalian NPC is about 124 megadaltons (MDa) and it contains approximately 30 different protein components, each in multiple copies. In contrast, the yeast Saccharomyces cerevisiae is smaller, weighing only 66 MDa.

Function of Nuclear Pore:

  • Nuclear pores are protein-based channels in the nuclear envelope. They regulate the movement of molecules from the nucleus to the cytoplasm, and vice versa. In most eukaryotic cells, the nucleus is enclosed by this nuclear membrane in order to separate it from the cytoplasm.
  • Many higher eukaryotic cells have as many as 2,000 nuclear pore complexes in the nuclear membrane of each cell. This membrane or envelope keeps the DNA safe, and contains it within the nucleus. Despite the presence of this barrier, communication still has to take place between the nucleus and the cytoplasm, so the nuclear pores serve as transportation and communication channels.
  • This transportation and communication has to be able to happen quickly for cell regulation and health. Some molecules are simply small enough to pass through the pores, but larger molecules must be recognized by different signal sequences before being allowed to diffuse through the nuclear pores via a concentration gradient.
  • Some substances, like carbohydrates, lipids, and even ribosomes are able to pass through quite easily, while RNA and some proteins must be "cleared" for release through signal sequences within the nucleus. 



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