Intermediate Filament

Intermediate filaments have a diameter of about 10 nm, which is in the middle of the two main major cytoskeletal elements, actin filaments (about 7 nm) and microtubules (about 10 nm) (about 25 nm). The intermediate filaments, unlike actin filaments and microtubules, are not directly engaged in cell motility. Instead, they appear to perform a structural role in cells and tissues, providing mechanical strength.

Intermediate Filament 

Intermediate filaments are thicker than actin filaments but thinner than microtubules or muscle myosin filaments, giving them their name. Intermediate filament subunits are elongated rather than globular, and they are antipolarly linked. As a result, there is no polarity in the total filament, and no motor proteins may move along intermediate filaments. Only complex multicellular creatures contain intermediate filaments. 

They are encoded by a variety of genes and are classified into groups based on their amino acid sequences. At different periods, cells in different body tissues express one or more of these genes. Over the course of a cell’s existence, it can even change the type of intermediate filament protein that it expresses.The varied kinds of intermediate filaments most likely have modest but significant changes in their functional features, which help determine the cell’s function. Intermediate filaments are structural components that assist cells keep their shape and integrity in general. Keratin filaments, for example, the intermediate filaments of epithelial cells that line the body’s surfaces, provide strength to the cell sheet that covers the surface. When the epithelial cell sheet is fragile and prone to rupture, keratin gene mutations can cause blisters. Hair, nail, and corneal deformations can all be caused by keratin mutations. The lamin family, which includes the nuclear lamina, a fibrous shell that underpins and maintains the nuclear membrane, is another example of an intermediate filament family.

Intermediate Filament Proteins

Actin filaments and microtubules are polymers of single proteins (actin and tubulin, respectively), whereas intermediate filaments are polymers of a range of proteins expressed in many cell types. There are more than 50 distinct intermediate filament proteins that have been found and grouped into six classes based on amino acid sequence similarity . Types I and II are two types of keratins that are expressed in epithelial cells and each consist of roughly 15 distinct proteins. At least one type I (acidic) and one type II (neutral/basic) keratin are synthesised by each kind of epithelial cell, which polymerised to produce filaments.

Hard keratins, which are type I and II keratins, are utilised to make structures like hair, nails, and horns. The cytoplasm of epithelial cells is rich in type I and II keratins (soft keratins), with distinct keratins expressed in different differentiated cell types

Vimentin, a type III intermediate filament protein present in fibroblasts, smooth muscle cells, and white blood cells, is one of the type III intermediate filament proteins.

The three neurofilament (NF) proteins are type IV intermediate filament proteins (designated NF-L, NF-M, and NF-H for light, medium, and heavy, respectively). Many types of adult neurons have substantial intermediate filaments made up of these proteins..

The nuclear lamins are type V intermediate filament proteins found in most eukaryotic cells. The nuclear lamins are components of the nuclear envelope rather than being part of the cytoskeleton.

Assembly of Intermediate Filaments

The production of dimers, in which the core rod domains of two polypeptide chains are looped around each other in a coiled-coil configuration identical to that created by myosin II heavy chains, is the initial stage of filament construction . The dimers then form tetramers, which can be assembled end to end to produce protofilaments, in a staggered antiparallel pattern. Approximately eight protofilaments are twisted around each other in a ropelike form in the final intermediate filament.

Both ends of intermediate filaments are equal because they are made up of antiparallel tetramers. As a result, intermediate filaments, unlike actin filaments and microtubules, are apolar, with no distinguishable plus and minus ends.

Interactions between specific types of intermediate filament proteins are required for filament assembly. Keratin filaments, for example, are always made up of heterodimers having one type I and one type II polypeptide. Type III proteins, on the other hand, can form filaments comprising only a single polypeptide (e.g., vimentin) or two separate type III proteins (e.g., vimentin plus desmin). The type III proteins, on the other hand, do not create copolymers with keratins. -internexin is the only type IV protein that can form filaments on its own, whereas the three neurofilament proteins copolymerize to produce heteropolymers.

Intracellular Organisation of Intermediate Filaments

Most cells’ cytoplasm contains an intricate network of intermediate filaments that extends from a ring encircling the nucleus to the plasma membrane . Keratin and vimentin filaments bind to the nuclear envelope, presumably positioning and anchoring the nucleus within the cell. Intermediate filaments can also associate with other cytoskeletal elements such as actin filaments and microtubules, in addition to the plasma membrane. Intermediate filaments serve as a scaffold that connects the cytoskeleton’s components and organises the cell’s internal structure.

Conclusion

 Introduction Intermediate filaments have a diameter of about 10 nm, which is in the middle of the two main major cytoskeletal elements, actin filaments and microtubules. The intermediate filaments, unlike actin filaments and microtubules, are not directly engaged in cell motility. Intermediate Filament Intermediate filaments are thicker than actin filaments but thinner than microtubules or muscle myosin filaments, giving them their name. Keratin filaments, for example, the intermediate filaments of epithelial cells that line the body’s surfaces, provide strength to the cell sheet that covers the surface. Intermediate Filament Proteins Actin filaments and microtubules are polymers of single proteins (actin and tubulin, respectively), whereas intermediate filaments are polymers of a range of proteins expressed in many cell types.