Structure of Nucleus

Structure of Nucleus

Structure of Nucleus

Nucleus

In 1833 Robert Brown discovered a prominent body within the cell and termed it nucleus.

Number, shape, size and of the nucleus-

Generally there is a single nucleus per, cell (mononucleate conditions), but more than one nucleus (polynucleate condition) may be found in certain special cases. There may be many nuclei in a syncytium, which is formed due to fusion of cells. A similar multinucleate situation is found in coenocytes commonly found in plants. A coenocyte results by repeated nuclear divisions without cytokinesis.

There are also variations with respect to shape and size of nucleus. It may be spherical, oval to flattened lobe or irregular in shape. In most cases it has a regular outline, but may also have irregular outline. Shape of nucleus also depends on the cell. In spheroid, cuboid or polyhedral cells, nucleus is usually spheroid. In cylindrical, prismatic or fusiform cells, nucleus is ellipsoid. Irregular nuclei are sometimes found in some neutrophylls or leukocytes and branched nuclei are sometimes found in glandular cells: In still other cases like spermatozoa, pyriform or lanceolate nuclei may be found. Size of the nucleus would also vary not only depending upon the type of cell involved but also according to activity of cell. The nucleus will be larger in an active cell, but will be smaller in a resting cell.

Nuclear Envelope-

Nuclear boundary of interphase and prophase nuclei is called nuclear membrane or nuclear envelope. It breaks down at the end of prophase and is reformed at the end of the nuclear division. It consists of a double membrane having two unit membranes. The space between two unit membranes varies in width and is known as perinuclear space. Outer membrane is continuous with the endoplasmic reticulum.

Each unit membrane is 7.5 nm (1 nm = 10A) in diameter and perinuclear space may vary from regular 15nm wide spaces to irregular cavities several hundred times wide. Outline of nuclear envelope is smooth and interrupted by pores which appear circular in surface view. Diameter of these pores varies from 30nm to 100nm. In sections, it is obvious that at the boundary of these pores, outer and inner unit membranes are joined. These pores provide direct contact between nucleus and cytoplasm and allow import and export of protein and RNA (particularly export of messenger RNA, which is synthesized in the nucleus and then reaches cytoplasm for protein synthesis). The double membrane also rakes part in giving rise to the initials of cell organelles like mitochondria or plastids.

The inner nuclear membrane is lined by the nuclear lamina, which is composed of A and B type lamins (a specialized type of intermediate filament protein). Similarly, the outer membrane is surrounded by a network of intermediate filaments, which are not as well organized as the nuclear lamina.

(I) Disassembly and reassembly of nuclear envelope-

As mentioned above, the nuclear envelope disassembles at the onset of mitosis and is reassembled at the end of mitosis. The mechanism of assembly involves: (i) the attachment of vesicles to the chromatin, followed by (ii) the fusion of vesicles to form a double membrane system. The binding of vesicles to the chromatin requires both chromatin and membrane bound proteins, but does not require ATP. On the other hand, the fusion of membrane bound vesicles to form the nuclear membrane does require ATP, and GTP hydrolysis, that is also required in membrane fusion events during exocytosis and endocytosis. The vesicles used for assembly of nuclear envelope form a subset of ER-derived vesicles, and are distinct from the majority of ER-derived vesicles (COPII). Lamin depoymerization and polymerization during disassembly and reassembly of the nuclear envelope also involves reversible phosphorylation.

(II) Nuclear pore complex-

Nuclear pore occupies a central position among the major cellular structures, but remained one of the least understood structures. However, during the late 1980s and early 1990s, significant progress has been made towards a better understanding of the structure and function of the nuclear pore. During this recent past, new pore proteins have been identified (particularly in yeast), the genes for several of these proteins have been cloned, a number of mutants in these pore proteins have been isolated and detailed mechanism of nucleocytoplasmic traffic has been proposed. Further, the pore has been reconstituted in vitro, a number of ‘signal sequences’ and one or more ‘signal sequence receptors’ have been identified, and new ‘basket-like structure’ has been found attached to the inner side of the nuclear pore.

The nuclear pore is a large complex structure of 125 million daltons or 30 times the size of a eukaryotic ribosome. The pore is 120 nm in diameter and 50 nm in thickness. It consists of four separate elements:

  • the scaffold, which includes majority of the pore,
  • the central hub or transporter, which carries out active transport (both import and export) of proteins and RNAs,
  • short thick filaments attached to the cytoplasmic side of the pore and
  • a newly discovered basket attached to the nucleoplasmic side of the pore.

The scaffold is a stack of three closely apposed rings, namely the cytoplasmic ring, the nucleoplasmic ring and a central ring of thick spokes. Each ring has a eightfold symmetry. The spokes of central ring are attached to the transporter of the inner side, and to the nucleoplasmic and cytoplasmic rings on the outer side. Interspersed between the spokes are aqueous channels, 9 nm wide, which allow diffusion of proteins and metabolites between the nucleus and the cytoplasm.

The transporter is a proteinaceous ring, 36-38 nm is diameter and consists of two irises of eight arms each. The two irises are assumed to be stacked atop one another and open sequentially. each like the diaphragm of a camera, to let a nuclear protein or RNA pass through from the nucleus to the cytoplasm. On the cytoplasmic side of the pore, thick fibres (3.3 nm in diameter) extend into the cytoplasm. On the nuclear side, a large basket like structure is found, which consists of eight filaments (each 100 nm long), extending from nucleoplasmic ring of the pore and meeting a smaller ring (60 nm in diameter) within the nucleus. This basket may play an important role in RNA export.

(III) Nucleocytoplasmic transport-

Nuclear pore complexes (NPCs) are the sites of exchange of macromolecules between the cytoplasm and the nucleus. NPCs have a mass of 125 megadaltons in higher eukaryotes and contain about 100 different polypeptides called Nup (nuclear pore proteins or nucleoporins). Following are some of the characteristic features of many of these nucleoportins:

  • modification of O-linked N-acetyl glucosamine (N=asparagine);
  • presence of short degenerate repeats (e.g. FXFG repeats in Nup153p and p62 and GLFG repeats in Nup98p; in FXFG and GLFG repeats, each letter stands for an amino acid.

The nuclear pore complex has passive diffusion channel, 9 nm in diameter, permitting, diffusion of many but not all small proteins including cytochrome c. Several small proteins like histones (<9 nm) and other larger proteins (>9 nm and upto 25 nm) pass through NPC by an active transport process. The active process is facilitated by (i) energy, (ii) signal sequence and (iii) saturability, so that the process is mediated by carrier molecules.

The transport through NPCs involves both import to and export from the nucleus. All nuclear proteins are imported from the cytoplasm where they are synthesized and all tRNAs/mRNAs are exported to the cytoplasm, where they are used for protein synthesis. There are also molecules that are first exported to the cytoplasm and then reimported. For instance, some of the small nuclear RNAs (snRNAs) involved in RNA splicing (Ch. 20: Genetics) include U1, U2 U4, and U5. They are exported out of the nucleus after transcription. In the cytoplasm, they assemble with Sm core proteins and undergo a number of modifications such as cap hypermethylation involving conversion of the cap m7 GpppN5′ to m2’27 GpppN. These partly mature U snRNPS re-enter the nucleus and associate with proteins specific for U snRNPs. In case of U4, the U4 snRNP re-enters the nucleus and associates with U6 snRNA. Thus the snRNPs complete their assembly by export followed by reimport in the nucleus. The situation is reverse in case of ribosomal proteins. They enter the nucleus, get incorporated into ribosomal subunits. And are then re-exported as part of ribosome subunits. It has been estimated in HeLa cells, that 100 ribosomal proteins and 3 ribosomal subunits travel through each pore each minute. This demonstrates that transport through NPC is a major activity. The details of components of the transport machinery and the mechanism involved in this transport will be briefly discussed in this section. For a detailed account, the readers may consult two recent reviews.

  1. Signals for transport across the pore-

    The import and export of proteins and RNPs across the NPCs are facilitated by the presence of signal sequences. These signal sequences include nuclear localization sequences (NLSS) and nuclear export signal (NES).

  2. Import of nuclear proteins-

    The following four factors are known to be required in nuclear protein import: (i) importin- α (also called NLS receptor; called SRP1p in yeast) (ii) importin-ß (also called p97 or PTC97), (iii) the small GTPase Ran, and (iv) pp 15 (also called pº or NTF2). Importin is also called karyophorin.

The import actually involves several stages including the following: (i) the protein to be imported has an NLS- which helps in binding of protein to importin α-ß heterodimer, the latter thus acting as a receptor, the receptor site being present in subunit importin-ß. An importin-α binding (IBB) domain in importin- subunit not only helps in interaction with importin-ß, but also facilitates translocation through NPC to the nucleus. The NLS-protein-receptor complex docks to the nuclear pore complex via importin-ß and is subsequently  translocation through the pore by an energy-dependent mechanism. After the translocation, the constituents of NLS-protein-receptor complex become separated, of which the protein remains in the nucleus and importin-ß and importin-α are recycled to the cytoplasm.

  1. Export of RNA from the nucleus-

    RNA export from the nucleus across NPC is also mediated by some signal sequences in the proteins that associate with RNA to form RNPs. The best studied such protein is HIV-1Rey protein. The RNA to be exported may also contain response elements for the proteins that associate to form RNPS.

  2. Export and reimport of RNAs-

    An example of export followed by reimport is 5S rRNA, and two proteins, TFIIIA and ribosomal protein L5 seem to be involved as mediators of export step. TFIIIA seems to contain a nuclear export signal (NES).

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