Intermediate filaments are a characteristic structure of eukaryotic cells. They are self-assembling and chemically resistant. The structure and functions of intermediate filaments are determined by the features of bonds in protein molecules. They serve not only for the formation of the cell framework, but also provide the interaction of organelles.
general description
Filaments are filamentous protein structures that take part in the construction of the cytoskeleton. In accordance with their diameter they are divided into 3 classes. Intermediate filaments (PF) have an average value in cross section - 7-11 nm. They occupy an intermediate position between microfilaments Ø5-8 nm and microtubules Ø25 nm, for which they got their name.
There are 2 types of these structures:
- Laminate. They are in the core. All animals have laminar filaments.
- Cytoplasmic. They are located in the cytoplasm. There are nematodes, mollusks, vertebrates. The latter may be absent in some types of cells (for example, in glial cells).
Location
Intermediate filaments are one of the main elements of the cytoskeleton of living organisms whose cells contain nuclei (eukaryotes). In prokaryotes, analogues of these fibrillar structures are also found. They were not found in plant cells.
Most of the filaments are located in the perinuclear zone and bundles of fibrils, which are located under the plasma membrane and extend from the center to the edges of the cells. Especially a lot of them in those species that are subjected to mechanical stress - in muscle, epithelial, as well as in cells of nerve fibers.
Types of Proteins
As studies show, the proteins that make up the intermediate filaments are distinguished depending on the type of cells and the stage of their differentiation. However, they are all related.
Proteins of intermediate filaments are divided into 4 types:
- Keratins. They form polymers of two subtypes - acidic and neutral. The molecular weight of these compounds ranges from 40,000 to 70,000 a. E. m. Depending on the tissue source, the number of diverse heterogeneous forms of keratins can reach several tens. They are divided into 2 groups according to isoform - epithelial (most numerous) and horny, of which the hair, horns, nails and feathers of animals are composed.
- In the second type, 3 types of proteins are combined that have almost the same molecular weight (45,000-53,000 a.u. m.). These include: vimentin (connective tissue, flat cells lining the surface of blood and lymph vessels; blood cells); desmin (muscle tissue); peripheral (peripheral and central neurons); glial fibrillar acidic protein (highly specific brain protein).
- Proteins of neurofilaments found in neurites - cylindrical processes along which impulses pass between nerve cells.
- Proteins of the nuclear lamina underlying the nuclear membrane. They are the precursors of all other PFs.
Intermediate filaments may consist of several types of the above substances.
The properties
Characteristics of PF are determined by their following features:
- a large number of polypeptide molecules in cross section;
- strong hydrophobic interactions, which play an important role in the assembly of macromolecules by the type of twisted superhelix;
- the formation of tetramers with high electrostatic interaction.
As a result, intermediate filaments acquire the properties of a strong twisted rope - they bend well, but do not tear. When treated with reagents and strong electrolytes, these structures are the last to pass into solution, that is, they are characterized by high chemical stability. So, after complete denaturation of protein molecules in urea, the filaments can independently collect. Proteins introduced from outside quickly integrate into the existing structure of these compounds.
Structure
In their structure, intermediate filaments are non-branched polymers that are capable of both the formation of high molecular weight compounds and depolymerization. Their structural instability helps cells to change their shape.
Despite the fact that filaments have a diverse composition according to the type of proteins, they have the same structural plan. At the center of the molecules is an alpha helix in the shape of a right-handed helix. It is formed due to contacts between hydrophobic structures. There are 4 spiral segments in its structure, separated by short non-spiral sections.
At the ends of the alpha helix are domains with an indefinite structure. They play an important role in the assembly of filaments and interaction with cellular organelles. Their sizes and protein sequence vary greatly among different types of FS.
Building protein
The main building material for PFs are dimers - complex molecules composed of two simple ones. Usually they include 2 different proteins interconnected by rod-shaped structures.
The cytoplasmic type of filament consists of dimers that form strands 1 block thick. Since they are located in parallel, but in the opposite direction, there is no polarity. From these dimeric molecules, more complex molecules can subsequently form.
Functions
The main functions of intermediate filaments are as follows:
- ensuring the mechanical strength of cells and their processes;
- adaptation to stress factors;
- participation in contacts providing a strong connection of cells (epithelial and muscle tissue);
- intracellular distribution of proteins and organelles (localization of the Golgi apparatus, lysosomes, endosomes, nuclei);
- involved in the transport of lipids and the transmission of signals between cells.
PFs also affect mitochondrial function. As laboratory experiments on mice show, in those individuals that lack the desmin gene, the intracellular arrangement of these organelles is disrupted, and the cells themselves are programmed for a shorter life span. As a result, tissue oxygen consumption is reduced.
On the other hand, the presence of intermediate filaments reduces the mobility of mitochondria. If vimentin is artificially introduced into the cell, then the PF network can be restored.
Value in medicine
Violations of the synthesis, accumulation and structure of PF leads to the emergence of some pathological conditions:
- The formation of hyaline drops in the cytoplasm of liver cells. They are also called Mallory bodies. These structures are epithelial type PF proteins. They are formed with prolonged exposure to alcohol (acute alcoholic hepatitis), as well as metabolic processes in primary hepatocellular liver cancer (in patients with viral hepatitis B and cirrhosis), and stagnation of bile in the liver and gall bladder. Alcoholic hyaline has immunogenic properties, which determines the development of systemic pathology.
- With mutations in the genes responsible for the production of keratins, a hereditary skin disease occurs - epidermolysis bullosa. In this case, there is a violation of the attachment of the outer layer of the skin to the basement membrane that separates it from the connective tissue. As a result, erosion and bubbles form. The skin becomes very sensitive to the slightest mechanical damage.
- The formation of senile plaques and neurofibrillary tangles in brain cells in Alzheimer's disease.
- Some types of cardiomyopathy associated with excessive accumulation of PF.
We hope that our article answered all your questions.