Let's consider the functions of non-histone proteins, their importance for the body. This topic is of particular interest and deserves detailed study.
Main chromatin proteins
Histone and non-histone proteins are directly linked to DNA. Its role in the composition of interphase and mitotic chromosomes is quite large - the storage and distribution of genetic information.
When carrying out such functions, it is necessary to have a clear structural base that allows long DNA molecules to be arranged in a clear order. This action allows you to control the frequency of RNA synthesis and DNA replication.
Its concentration in the interphase nucleus is 100 mg/ml. One mammalian nucleus accounts for approximately 2 m of DNA, localized in a spherical nucleus with a diameter of about 10 microns.
Protein groups
Despite the diversity, it is customary to single out two groups. The functions of histone and non-histone proteins have certain differences. About 80 percent of all chromatin proteins are histones. They interact with DNA through ionic and s alt bonds.
Despite a significant amount, histones and non-histone proteins of chromatinrepresented by an insignificant variety of proteins, eukaryotic cells contain about five to seven types of histone molecules.
Nonhistone proteins in chromosomes are mostly specific. They interact only with certain structures of DNA molecules.
Histone features
What are the functions of histone and non-histone proteins in the chromosome? Histones bind in the form of a molecular complex with DNA, they are subunits of such a system.
Histones are proteins characteristic only of chromatin. They have certain qualities that allow them to perform specific functions in organisms. These are alkaline or basic proteins, characterized by a fairly high content of arginine and lysine. Due to the positive charges on the amino groups, an electrostatic or s alt bond is caused with opposite charges on the phosphate structures of DNA.
This bond is quite labile, it is easily destroyed, and dissociation into histones and DNA occurs. Chromatin is considered to be a complex nucleic-protein complex, inside which there are highly polymeric linear DNA molecules, as well as a significant number of histone molecules.
Properties
Histones are fairly small proteins in terms of molecular weight. They have similar properties in all eukaryotes and are found by similar classes of histones. For example, types H3 and H4 are considered rich in arginine, since they contain a sufficient amount of thisamino acids.
Varieties of histones
Such histones are considered conservative, since the amino acid sequence in them is similar even in distant species.
H2A and H2B are considered to be moderate lysine proteins. Different objects within these groups have some variations in the primary structure, as well as in the sequence of amino acid residues.
Histone H1 is a class of proteins in which amino acids are arranged in a similar sequence.
They show more significant intertissue and interspecies variations. A significant amount of lysine is considered as a general property, as a result of which these proteins can be separated from chromatin in dilute saline solutions.
Histones of all classes are characterized by a cluster distribution of the main amino acids: arginine and lysine at the ends of the molecules.
H1 has a variable N-terminus that interacts with other histones, and the C-terminus is enriched with lysine, it is he who interacts with DNA.
Histone modifications are possible during the life of cells:
- methylation;
- acetylation.
Such processes lead to a change in the number of positive charges, they are reversible reactions. When serine residues are phosphorylated, an excess negative charge appears. Such modifications affect the properties of histones and their interaction with DNA. For example, when histones are acetylated, gene activation is observed, and dephosphorylation causes decondensation and condensationchromatin.
Synthesis features
The process occurs in the cytoplasm, then it is transported to the nucleus, binding to DNA during its replication in the S-period. After the termination of DNA synthesis by the cell, the breakdown of informational histone RNAs occurs within a few minutes, the synthesis process stops.
Division into groups
There are different types of non-histone proteins. Their division into five groups is conditional, it is based on internal similarity. A significant number of distinctive properties have been identified in higher and lower eukaryotic organisms.
For example, instead of H1, characteristic of the tissues of lower vertebrate organisms, histone H5 is found, which contains more serine and arginine.
There are also situations associated with the partial or complete absence of histone groups in eukaryotes.
Functionality
Similar proteins have been found in bacteria, viruses, mitochondria. For example, in E. coli, proteins were found in the cell, the amino acid composition of which is similar to histones.
Nonhistone chromatin proteins perform important functions in living organisms. Prior to the discovery of nucleosomes, two hypotheses were used regarding the functional significance, regulatory, and structural role of such proteins.
It was found that when RNA polymerase is added to the isolated chromatin, a template for the transcription process is obtained. But his activity is estimatedonly 10 percent of that for pure DNA. It increases with the removal of histone groups, and in their absence it is the maximum value.
This indicates that the total content of histones allows you to control the transcription process. Qualitative and quantitative changes in histones affect the activity of chromatin, the degree of its compactness.
The question of the specificity of the regulatory characteristics of histones during the synthesis of specific mRNAs in different cells has not been fully studied.
With the gradual addition of a histone fraction to solutions containing pure DNA, precipitation is observed in the form of a DNP complex. When histones are removed from the chromatin solution, a complete transition into a soluble base occurs.
The functions of non-histone proteins are not limited to the construction of molecules, they are much more complex and multifaceted.
The structural significance of nucleosomes
In the first electromicroscopic and biochemical studies, it was proved that there are filamentous structures in DPN preparations, the diameter of which is in the range of 5-50 nm. With the improvement of ideas about the structure of protein molecules, it was possible to find out that there is a direct relationship between the diameter of the chromatin fibril and the method of drug isolation.
On thin sections of mitotic chromosomes and interphase nuclei, after detection with glutaraldehyde, chromated fibrils were found, the thickness of which is 30 nm.
Fibrils have similar sizeschromatin in the case of physical fixation of their nuclei: during freezing, chipping, taking replicas from similar preparations.
The non-histone proteins of chromatin have been discovered in two different ways by chromatin particle nucleosomes.
Research
When chromatin preparations are deposited on a substrate for electron microscopy under alkaline conditions with insignificant ionic strength, chromatin strands similar to beads are obtained. Their size does not exceed 10 nm, and the globules are interconnected by DNA segments, the length of which does not exceed 20 nm. In the course of observations, it was possible to establish a connection between the structure of DNA and decay products.
Interesting information
Non-histone proteins make up about twenty percent of chromatin proteins. They are proteins (except those that are secreted by chromosomes). Non-histone proteins are a combined group of proteins that differ from each other not only in properties, but also in functional importance.
Most of them refer to nuclear matrix proteins, which are found both in the composition of interphase nuclei and in mitotic chromosomes.
Non-histone proteins can include about 450 individual polymers with different molecular weights. Some of them are soluble in water, while others are soluble in acidic solutions. Due to the fragility of the connection with the chromatin of the ongoing dissociation in the presence of denaturing agents, there are significant problems with the classification and description of these protein molecules.
Nonhistone proteins are regulatory polymers,stimulating transcription. There are also inhibitors of this process that bind in a specific sequence on DNA.
Nonhistone proteins can also include enzymes involved in the metabolism of nucleic acids: RNA and DNA methylases, DNases, polymerases, chromatin proteins.
The environment of many similar polymeric compounds is considered to be the most studied non-histone proteins with high mobility. They are characterized by good electrophoretic mobility, extraction in a solution of common s alt.
HMG proteins come in four types:
- HMG-2 (m.w.=26,000),
- HMG-1 (m.w.=25,500),
- HMG-17 (m.w.=9247),
- HMG-14 (m.w.=100,000).
A living cell of such structures contains no more than 5% of the total amount of histones. They are especially common in active chromatin.
HMG-2 and HMG-1 proteins are not included in nucleosomes, they only bind to linker DNA fragments.
Proteins HMG-14 and HMG-17 are able to bind to heart-like polymers of nucleosomes, resulting in a change in the assembly level of DNP fibrils, they will be more accessible for reaction with RNA polymerase. In such a situation, HMG proteins play the role of regulators of transcriptional activity. It was found that the chromatin fraction, which has an increased sensitivity to DNase I, is saturated with HMG proteins.
Conclusion
The third level of structural organization of chromatin is the loop domains of DNA. In the course of the research, it was found that onlydeciphering the principle of chromosomal elementary components, it is difficult to get a complete picture of chromosomes in mitosis, in interphase.
DNA densification by 40 times is obtained due to maximum spiralization. This is not enough to get a real idea of the size and characteristics of chromosomes. It can be logically concluded that there must be even higher levels of DNA assembly, with the help of which it would be possible to unambiguously characterize chromosomes.
Scientists have been able to detect similar levels of chromatin organization as a result of its artificial decondensation. In such a situation, specific proteins will bind to certain sections of DNA that have domains in the places of association.
The principle of DNA loop packing was also discovered in eukaryotic cells.
For example, if the isolated nuclei are treated with a solution of table s alt, the integrity of the nucleus will be preserved. This structure became known as a nucleotide. Its periphery includes a significant number of closed DNA loops, the average size of which is 60 kb.
With preparative isolation of chromomeres, subsequent extraction of histones from them, looped rosette-like structures will be visible under an electron microscope. The number of loops in one socket is from 15 to 80, the total length of DNA reaches 50 microns.
The ideas about the structure and main functional characteristics of protein molecules, obtained in the course of experimental activities, allow scientists to develop drugs, create innovativemethods of effective fight against genetic diseases.
