Nuclear pores are one of the most important intracellular components, as they are involved in molecular transport. Despite advances in biological research, not all questions regarding these structures have been fully explored. Some scientists believe that the nuclear pore complex can be attributed to cell organelles in terms of the significance of functions and structural complexity.
Nuclear shell
A characteristic feature of eukaryotic cells is the presence of a nucleus, which is surrounded by a membrane that separates it from the cytoplasm. The membrane consists of two layers - inner and outer, interconnected by a large number of pores.
The significance of the nuclear envelope is very high - it allows you to delimit the processes of protein synthesis and nucleic acids necessary to regulate the functional activity of genes. The membrane controls the process of transporting substances inward, into the cytoplasm, and vice versa. It is also the skeletal structure that supports the shape of the nucleus.
Between the outer and inner membrane is the perinuclear space, the width of which is 20-40 nm. Externally, the nuclear envelope looks likedouble layer bag. The presence of pores in its structure is a significant difference between this structure and similar ones found in mitochondria and plastids.
Structure of nuclear pores
Channels are perforations about 100 nm in diameter, passing through the entire nuclear envelope. In cross section, they are characterized by the shape of a polygon with symmetry of the eighth order. The substance-permeable channel is in the center. It is filled with complexly organized globular (in the form of a coil) and fibrillar (in the form of a twisted thread) structures that form a central granule - a “plug” (or a transporter). In the figure below, you can clearly study what a nuclear pore is.
Microscopic examination of these structures shows that they have an annular structure. Fibrillar outgrowths extend both outward, into the cytoplasm, and inward, towards the nucleus (filaments). The latter form a kind of basket (called "basket" in foreign literature). In the passive pore, the basket fibrils close the channel, while in the active pore they form an additional formation about 50 nm in diameter. The ring on the side of the cytoplasm consists of 8 granules interconnected like beads on a string.
The totality of these perforations in the shell of the nucleus is called the complex of nuclear pores. Thus, biologists emphasize the interconnection between individual holes, working as a single well-coordinated mechanism.
The outer ring is connected to the central conveyor. Lower eukaryotes (lichens and others) do not have cytoplasmicand nucleoplasmic rings.
Structure features
The structure and functions of nuclear pores have the following features:
- Channels are numerous copies of about 30-50 nucleoporins (for a total of about 1000 proteins).
- The mass of complexes ranges from 44 MDa in lower eukaryotes to 125 MDa in vertebrates.
- In all organisms (humans, birds, reptiles and other animals), in all cells, these structures are arranged in a similar way, that is, pore complexes are a strictly conservative system.
- The components of nuclear complexes have a subunit structure, due to which they have high plasticity.
- The diameter of the central channel varies between 10-26 nm, and the height of the pore complex is about 75 nm.
The parts of the nuclear pores far from the center are not symmetrical. Scientists attribute this to various mechanisms of regulation of the transport function at the initial stages of cell development. It is also assumed that all pores are universal structures and ensure the movement of molecules both into the cytoplasm and in the opposite direction. Nuclear pore complexes are also present in other membrane-bearing cell components, but in rarer cases (reticulum, fenestrated cytoplasmic membranes).
Number of pores
The main factor that determines the number of nuclear pores is the metabolic activity in the cell (the higher it is, the morenumber of tubules). Their concentration in the thickness of the membrane can change several times during different periods of the functional state of the cells. The first increase in the number of pores occurs after division - mitosis (during the reconstruction of nuclei), and then during the period of DNA growth.
Different animal species have different numbers. It also depends on where the sample was taken. So, in a human tissue culture, there are about 11 pcs/µm2, and in an immature xenopus frog egg cell - 51 pcs/µm2. On average, their density varies between 13-30 pieces/µm2.
The distribution of nuclear pores over the surface of the shell is almost uniform, but in places where the substance of the chromosomes approaches the membrane, their concentration decreases sharply. Lower eukaryotes do not have a rigid fibrillar network under the nuclear membrane, so the pores can move along the nuclear membrane, and their density in different areas varies significantly.
Functions
The main function of the nuclear pore complex is passive (diffusion) and active (requiring energy costs) transfer of molecules through the membrane, that is, the exchange of substances between the cell nucleus and the cytoplasm. This process is vital and is governed by three systems that are in constant interaction with each other:
- a complex of biologically active substances-regulators in the nucleus and cytoplasm - importin α and β, Ran-protein, guanosine triphosphate (purine nucleotide) and other inhibitors and activators;
- nucleoporins;
- structural components of the porous nuclear complex, which are able to change their shape and ensure the transfer of substances in the right direction.
Proteins necessary for the functioning of the nucleus come from the cytoplasm through the nuclear pores, and various forms of RNA are excreted in the opposite direction. The pore complex not only performs purely mechanical transport, but also serves as a sorter that "recognizes" certain molecules.
Passive transfer occurs for those substances whose molecular weight is low (no more than 5∙103 Yes). Substances such as ions, sugars, hormones, nucleotides, adenosine triphosphoric acid, which are involved in energy exchange, freely enter the nucleus. The maximum size of proteins that can penetrate through the pores into the nucleus is 3.5 nm.
During the synthesis of a daughter DNA molecule, the transport of substances reaches a peak of activity - 100-500 molecules through 1 nuclear pore in 1 minute.
Pore proteins
Channel elements are of protein nature. The proteins of this complex are called nucleoporins. They are collected in approximately 12 subcomplexes. Conventionally, they are divided into three groups:
- compounds with specific repeat sequences recognizable by biochemical factors;
- not having sequences;
- integral proteins that are located in the area of the membrane that forms the pore, or in the pore itself in the space between the layers of the nuclear envelope.
Studies have shown that nucleoporins are able to formrather complex complexes, including up to 7 proteins, and are also directly involved in the transport of substances. Some of them can directly bind to molecules moving through the nuclear pore.
Export of substances to the cytoplasm
The same pore can take part in both the withdrawal and import of substances. The reverse translation of RNA from the cytoplasm into the nucleus does not occur. Nuclear complexes recognize export signals (NES) carried by ribonucleoproteins.
NES-sequence of signaling substances is a complex complex of amino acids and proteins, which, after being removed from the nucleus into the cytoplasm, dissociate (break down into separate components). Therefore, similar particles introduced artificially into the cytoplasm do not penetrate back into the nucleus.
The process of mitosis
During cell division (mitosis), the nuclear pore complex is "dismantled". Thus, complexes with a molecular weight of 120 mDa decompose into subcomplexes of 1 mDa each. After the end of the division, they reassemble. In this case, the nuclear pores do not move separately, but in arrays. This is one of the proofs that the nuclear pore complex is a well-coordinated system.
The ruptured membrane turns into a bubble cluster that surrounds the core area in the interphase period. In metaphase, when the chromosomes are held in the equatorial plane, these elements are pushed to the peripheral zones of the cell. At the end of anaphase, this cluster begins to contact chromosomes and growth starts.rudiments of the nuclear membrane.
Bubbles turn into vacuoles, which gradually envelop the chromosomes. Then they merge and fence off the new interphase nucleus from the cytoplasm. The pores appear already at the very early stage, when the closure of the shells has not yet occurred.
