Intermediate filaments

The local structure of protein molecules of PF is quite conservative. The polypeptide usually has two globular domains at the N- and C-ends, which are connected by an extended supercoiled rod-shaped domain consisting of alpha-helices . The main building block of the filament is a dimer , not a monomer. It is formed by two polypeptide chains, usually two different proteins, which interact with each other with their rod-shaped domains, forming a double supercoiled helix. Cytoplasmic PFs are formed from such dimers forming nonpolar filaments, one block thick. The absence of polarity in PFs is due to the antiparallel orientation of the dimers in the tetramer. From them, more complex structures are formed in which PFs can become denser, as a result of which they have a variable diameter.

Unlike actin and tubulin , PF proteins do not have a nucleoside triphosphate binding site.

Not all eukaryotes have cytoplasmic PFs; they are found only in some groups of animals. So, nematodes have PF. mollusks and vertebrates. but not found in arthropods and echinoderms . In vertebral PFs, they are absent in some cells (for example, oligodendrocytes). PFs were not found in plant cells. ^{[2] [3]}
In most animal cells, PFs form a “basket” around the nucleus , from where they are directed to the periphery of the cells. There are especially many PFs in cells subject to mechanical stress: in epithelia , where PFs are involved in connecting cells to each other through desmosomes , in nerve fibers , in cells of smooth and striated muscle tissue.

Unlike other basic elements of the cytoskeleton, PF in the cytoplasm of cells of different tissues consist of different, albeit similar in structure proteins. All human PF proteins encode about 70 genes. Based on the characteristics of the amino acid composition and structure, five main groups of PF proteins are distinguished.

Type I - Keratins

The most diverse group of PFs consists of keratins with a molecular weight of 40–70 kDa. This type of protein is divided into 2 subfamilies:

• acid keratins
• neutral and basic keratins.

Keratin dimer consists of one acidic and one basic keratin. Among the numerous isoforms of keratin, two main groups are distinguished: epithelial keratins (see cytokeratin ), which includes about 20 types of keratins, and hair keratins (about 10 species), from which nails , horns and reptile scales are also built.

Type II

The second type of PF proteins includes 4 types of proteins:

• vimentin - a protein with a mass of 45 - 53 kDa, characteristic of cells of mesenchymal origin: it is part of the cells of connective tissue , endothelium , blood cells;
• desmin ;
• glial fibrillar acidic protein ;
• peripheral .

Type III

• Alpha internexin
• Neurofilament Proteins
• Nestin
• Cinemin
• Syncoilin

Type IV

• Nuclear Laminates

PF in some cases provide mechanical strength of cells, their processes or epithelial layers. They participate in the formation of intercellular contacts - desmosomes and hemidesmosomes .

Mutations of the keratin genes krt5 and krt14 are associated with the development of hereditary skin disease, epidermolysis bullosa (Epidermolysis bullosa simplex). In this disease, the attachment of the epidermis to the basal plate is impaired, blisters filled with serous contents form on the skin.

• Cytoskeleton
• Keratin
• Vimentin
• Desmosome

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