NUCLEAR ENVELOPE

Nuclear envelope encloses the nucleus. So it separates nuclear contents from the cytoplasm. It is continuous with the ER at number of points.

12

Structure

.1.1ic nuclear envelope is composed of double membrane. The two membranes are separated by a space called perinuclear space. It is about 20 to 40nm. A layer of protein is present closely associated with the nuclear side of the inner membrane. It is called nuclear lamina. Nuclear lamina is a network of intermediate filaments. It helps to maintain the shape of the nucleus and maintain the organization of the genetic material. It also plays a role in mitosis

and meiosis. The lamina acts as a site for the attachment for chromosomes.

During prophase in mitosis. the chromatids condense to form chromosomes and nuclear envelope disintegrates. During metaphase, the nuclear envelope is completely disintegrated.

12

Nuclear pore

‘Hie nuclear envelope is perlbrated by pores. The whole pore complex has a diameter of about I 50mn, and the diameter of the opening is about 50 nm wide. There are about 3.000-4,000 nuclear pore complexes in the nuclear envelope of an animal cell. The pore complex regulates the movement of macromolecules and particles. These pores give the nucleus direct contact with the ER. The size of pores prevents the DNA from leaving. But it allows RNA and proteins to move out.

Nuclear Pore Complex (NPC) is cylindrical. It has an 8-fold symmetry. It is composed of large proteins. These proteins contain an alpha solenoid or a beta-propeller fold. In some cases, both are present. Eight protein molecules are called annular subunits. The

annular subunits form spoke assembly ring. It surrounds the actual pore called outer ring. The center of the pore contains a plug-like structure. The 8 annular units are attached to the membrane by luminar subunits. Two rings are attached to each luminar subunits. One faces the nucleus and the other faces the cytoplasm. The nucleoplasmic side of the NPC has fibrils. A cage-like assembly is attached on the nucleoplasmic side. The nucleoplasmic side and the cytoplasmic side of the NPC are very different. This causes the differences in the selective transport in the two directions

Transport of material through nuclear pore

Nuclear pores allow the transport of water-soluble molecules across the nuclear envelope. This transport includes RNA and ribosomes. .1hey move from nucleus to the cytoplasm. Proteins (DNA polymerase). carbohydrates. signal molecules and lipids also move into the nucleus. ma I ler molecules simply diffuse through the pores. But larger molecules are recognized by specific signal sequences and then they are diffused with the help of nucleoporins into or out of the nucleus. This is known as RAN cycle.

(a)         Import of proteins

The proteins which are to be imported into nucleus contain Nuclear Localization Signal (NLS). ‘Nese signals allow quick and efficient transport through the pore. There are several kinds of NTS sequences. There is following mechanism of import of proteins:

I. I mportins (carrier) are imolyed in the transport of protein with an NTS into the nucleus. Importin-u binds first to the NTS sequence. It acts as a bridge for the attachment of Importin-p.


----------------------------


----------------------------

  1. The complex importinh-importinu-complex is then directed towards the nuclear pore and diffuses through it.
  2. RanGTP binds to !moottin-13 in nucleus and displaces it from the complex.
  3. Then CAS (Cellular Apoptosis Susceptibility) protein displaces Importin-u. The NTS-protein is thus free in the nucleoplasm.
  4. The Importinh-RanGTP and Importinu-CAS-RanGTP complex diffuses back into the cytoplasm. Where GTPs are hydrolysed to GDP and release Import inh and Importinu

Export of proteins

Some nuclear proteins are also exported to the cytoplasm. Sometimes. Pre-ribosomal particles and mRNAs are also exported. There is following mechanism of their export:

annular subunits form spoke assembly ring. It surrounds the actual pore called outer ring. The center of the pore contains a plug-like structure. The 8 annular units are attached to the membrane by luminar subunits. Two rings are attached to each luminar subunits. One faces the nucleus and the other faces the cytoplasm. The nucleoplasmic side of the NPC has fibrils. A cage-like assembly is attached on the nucleoplasmic side. The nucleoplasmic side and the cytoplasmic side of the NPC are very different. This causes the differences in the selective transport in the two directions

Transport of material through nuclear pore

Nuclear pores allow the transport of water-soluble molecules across the nuclear envelope. This transport includes RNA and ribosomes. .1hey move from nucleus to the cytoplasm. Proteins (DNA polymerase). carbohydrates. signal molecules and lipids also move into the nucleus. ma I ler molecules simply diffuse through the pores. But larger molecules are recognized by specific signal sequences and then they are diffused with the help of nucleoporins into or out of the nucleus. This is known as RAN cycle.

(a)         Import of proteins

The proteins which are to be imported into nucleus contain Nuclear Localization Signal (NLS). ‘Nese signals allow quick and efficient transport through the pore. There are several kinds of NTS sequences. There is following mechanism of import of proteins:

I. I mportins (carrier) are imolyed in the transport of protein with an NTS into the nucleus. Importin-u binds first to the NTS sequence. It acts as a bridge for the attachment of Importin-p.

  1. The complex importinh-importinu-complex is then directed towards the nuclear pore and diffuses through it.
  2. RanGTP binds to !moottin-13 in nucleus and displaces it from the complex.
  3. Then CAS (Cellular Apoptosis Susceptibility) protein displaces Importin-u. The NTS-protein is thus free in the nucleoplasm.
  4. The Importinh-RanGTP and Importinu-CAS-RanGTP complex diffuses back into the cytoplasm. Where GTPs are hydrolysed to GDP and release Import inh and Importinu

Export of proteins

Some nuclear proteins are also exported to the cytoplasm. Sometimes. Pre-ribosomal particles and mRNAs are also exported. There is following mechanism of their export:

I. Proteins which are to be exported contain NES (Nuclear Export Sequence). It binds with an exportin and RanGTP in the nucleus. Thus a heterotrimeric complex is formed.

2. The complex is then diffused into the cytoplasm where GTP is hydrolysed and the NES-protein is released. RanGDP diffuses back to the nucleus.

12

Similar Articles:

Leave a Reply

Your email address will not be published. Required fields are marked *