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 characteristics of bonds in protein molecules. They serve not only to form the cell scaffold, but also ensure the interaction of organelles.
General Description
Filaments are filamentous protein structures that take part in the construction of the cytoskeleton. According to the diameter they are divided into 3 classes. Intermediate filaments (IF) have an average cross-sectional value of 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:
- Lamine. They are in the core. All animals have laminar filaments.
- Cytoplasmic. They are located in the cytoplasm. Available in nematodes, mollusks, vertebrates. In the latter, some types of cells may be absent (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). Prokaryotes also have analogues of these fibrillar structures. They are 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. There are especially many of them in those species that are subjected to mechanical stress - in muscle, epithelial, and also in the cells of nerve fibers.
Protein types
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.
Intermediate filament proteins are divided into 4 types:
- Keratins. They form polymers from two subtypes - acidic and neutral. The molecular weight of these compounds ranges from 40,000-70,000 amu. m. Depending on the tissue source, the number of various heterogeneous forms of keratins can reach several tens. They are divided into 2 groups according to isoform - epithelial (the most numerous) and horny, which make up the hair, horns, nails and feathers of animals.
- In the second type, 3 types of proteins are combined, having almost the same molecular weight (45,000-53,000 amu). These include: vimentin (connective tissue, squamous cells,lining the surface of blood and lymphatic vessels; blood cells) desmin (muscle tissue); peripherin (peripheral and central neurons); glial fibrillar acidic protein (highly specific brain protein).
- Neurofilament proteins found in neurites, cylindrical processes that carry impulses between nerve cells.
- Proteins of the nuclear lamina that underlies the nuclear membrane. They are the forerunners of all other PFs.
Intermediate filaments can consist of several types of the above substances.
Properties
The characteristics of the PF are determined by their following features:
- large number of polypeptide molecules in cross section;
- strong hydrophobic interactions that play an important role in the assembly of macromolecules in the form of a twisted supercoil;
- formation of tetramers with high electrostatic interaction.
As a result, the intermediate filaments acquire the properties of a strong twisted rope - they bend well, but do not break. When treated with reagents and strong electrolytes, these structures are the last to go into solution, that is, they are characterized by high chemical stability. So, after the complete denaturation of protein molecules in urea, the filaments can independently assemble. Proteins introduced from outside are quickly integrated into the already existing structure of these compounds.
Structure
By their structure, intermediate filaments are non-branchingpolymers that are capable of both the formation of macromolecular compounds and depolymerization. Their structural instability helps cells change their shape.
Despite the fact that the filaments have a diverse composition according to the type of proteins, they have the same structural plan. In the center of the molecules there is an alpha helix, which has the shape of a right-handed helix. It is formed by contacts between hydrophobic structures. Its structure contains 4 spiral segments 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 filament assembly and interaction with cell organelles. Their size and protein sequence vary greatly among different IF species.
Building protein
The main building material for PF are dimers - complex molecules composed of two simple ones. Usually they include 2 different proteins connected by rod-shaped structures.
The cytoplasmic type of filaments consists of dimers that form threads 1 block thick. Since they are parallel but in opposite direction, there is no polarity. These dimeric molecules can later form more complex ones.
Functions
The main functions of intermediate filaments are as follows:
- ensuring the mechanical strength of cells and their processes;
- adaptation to stressors;
- participation incontacts that provide a strong connection of cells (epithelial and muscle tissue);
- intracellular distribution of proteins and organelles (localization of the Golgi apparatus, lysosomes, endosomes, nuclei);
- participation in lipid transport and signaling between cells.
PF 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 disturbed, and the cells themselves are programmed for a shorter lifespan. As a result, tissue oxygen consumption is reduced.
On the other hand, the presence of intermediate filaments contributes to a decrease in mitochondrial mobility. If vimentin is artificially introduced into the cell, then the IF network can be restored.
Medicine Significance
Violations in the synthesis, accumulation and structure of PF leads to the emergence of some pathological conditions:
- Formation of hyaline drops in the cytoplasm of liver cells. In another way, they are called Mallory bodies. These structures are IF proteins of the epithelial type. They are formed with prolonged exposure to alcohol (acute alcoholic hepatitis), as well as a violation of metabolic processes in primary hepatocellular liver cancer (in patients with viral hepatitis B and cirrhosis), with stagnation of bile in the liver and gallbladder. Alcoholic hyaline has immunogenic properties, which predetermines the development of systemic pathology.
- When genes mutate,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 are formed. The skin becomes very sensitive to the slightest mechanical damage.
- 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.
