A Guide to the Structure, Components, and Function of Cytoskeleton

Cytoskeleton function
Cytoskeleton, as the name suggests, is a skeletal system within the cytoplasm of a cell, which consists of a variety of protein fibers that form a network and impart a certain shape and structure to the cell. Despite the rigidity implied by the term "structure", it is a dynamic complex that lends a certain amount of flexibility to the cell. This Buzzle post provides you with more information about this cellular complex.
Imperceptible Components
The constituting elements of the cytoskeleton are so minuscule, that their discovery was only made possible due to the very high resolving power of the electron microscope.
Nikolai Koltsov, a Russian biologist, was the first to propose the existence of a network of tubular structures within the cell that gave it a shape and form (1903). Later, this theory was improved upon by Rudolph Peters in 1929, who put forth the idea that the cytoplasmic biochemistry was orchestrated and regulated by a dynamic protein mosaic. The term to describe this system was later introduced by the French embryologist, Paul Wintrebert, in 1931. He coined the French term "cytosquelette", which was later translated in English as "cytoskeleton".

Each cell of all living organisms possesses a cytoskeleton, which is composed of three primary constituents - microfilaments, intermediate filaments, and microtubules. Additional elements may or may not be present, and this depends on the complexity and nature of the cell. These elements interact not only with the cytoplasm and the organelles and molecules contained in it, but also with the cellular membrane. They also interact with extracellular molecules, that bring about dynamic changes in the cell infrastructure. Eukaryotic and prokaryotic skeletons show a few variations with regards to the cytoskeletal makeup.
Eukaryotic Cytoskeleton
Microfilaments
Actin micro-filament
Actin Microfilament
They are the thinnest filaments found in the cytoskeleton, and are made up of F-actin protofilaments. Hence, they are also referred to as actin filaments. Each actin protofilament is a wound dimer of linear polymerized chains of G-actin subunits. F-actin refers to filamentous actin, and G-actin refers to globular actin. The individual strands of the F-actin protofilament are wound together with the help of a double-stranded alpha-helical coiled coil protein called tropomyosin, which regulates the function of the actin filaments. It bears a protein complex, called troponin, which is interspersed along the length of the coil. This complex functions to regulate the binding of myosin to the actin filament. It interacts with myosin via a cross-bridge formation that brings about actions of contraction and relaxation.

The complete microfilament has a diameter of around 7nm., and are generally found in the peripheral areas of the cell, where they anchor various membrane proteins. Many microfilaments can also bind to each other to form a stress fiber, that is capable of contractile action. This proves useful in adhering the cell to the surface or substrate on which it is growing. These filaments are also utilized by cellular elements like cilia and flagella in order to render the cell motile. By altering the microfilament interactions, a cell can regulate the microfilament network, and in turn control the physical properties of the cell such as viscosity and rigidity.

The actin-myosin complex is regulated by the Rho family of proteins, the lamellipodia (foot-like actin projection along the leading edge of cell for purpose of motility) are regulated by Rac proteins, and Cdc42 supervises the filopodia (actin bundle projections beyond the leading edge of the cell).
Intermediate Filaments
Intermediate filament
Intermediate Filament
These filaments are called intermediate as their diameter of 10nm is an intermediate between the diameter of the microfilaments and the microtubules. They are formed via a series of polymerizing steps. Initially the monomeric chain binds to another monomer to form a helically wound dimer. This dimer then again gets wound together with another dimer to give a tetramer formed of coiled coil dimer. These tetramers are then packed together in such a way as to produce sheets of eight tetramers. This sheet is then rolled and supercoiled to produce a tightly wound rope-like bundle of filaments.

Intermediate filaments are more stable as compared to the other cytoskeletal elements, and help in stabilizing cellular structural integrity. At the cell surface they attach to specific junctions, called desmosomes and hemidesmosomes, that help in the adhesion of neighboring cells to each other and to the extracellular matrix. They also serve as the structural backbone for the nuclear lamina. Their main function is to maintain the shape of the cell and provide tensile strength.

There are many different types of intermediate filaments, and these are classified into six major categories on the basis of their amino acid sequence and protein structure. They are as follows.
Type I
They are acidic in nature and have a low molecular weight. They are often arranged and coexpressed in pairs with type II basic keratins. This acidic-basic pairing of these cytokeratins lead to the formation of keratin filament. They are encoded for by genes on chromosome 17q. They include keratins 9 - 20.

Type II
They are basic or neutral in nature and possess a high molecular weight. They are encoded on chromosome 12q and include keratins 1 - 8.
NOTE
The type I and II are isoforms of each other and are often clubbed together and divided into two groups, namely, epithelial keratins and trichocyte keratins.
Type III
Desmin - It is a structural component of sarcomeres in muscles, that interacts with desmoplakin.

GFAP - It refers to glial fibrillary acidic protein that is found in astrocytes and other cells of the central nervous system. It imparts mechanical strength.

Peripherin - It is found in peripheral neurons, where it brings about neural elongation and axonal regeneration.
Vimentin - It is the most abundant protein, and is found in fibroblasts, leukocytes, and endothelial cells. It functions to provide support to the cellular membranes, while keeping the organelles fixed in the cytoplasm. It also transmits signals between the membrane receptors to the nucleus.
Type IV
α-Internexin - It is found in developing neuroblasts and in adult central nervous system. Exact function is as yet unknown.

Neurofilaments - It is found in the axons of vertebrate neurons, where it provides structural support to the axon and regulates axon diameter.

Synemin - It is also called desmuslin and functions to maintain structural integrity in cells.

Syncoilin - It binds to desmin, but the exact function has not yet been deciphered.
Type V
Nuclear Lamins - They exhibit a structural function and transcriptional regulation in the cell nucleus. They are also involved in the formation of the nuclear lamina.

Type VI
Nestin - They are present in the nerve cells, where they are involved in the radial growth of axons.
Microtubules
Microtubules
Microtubule
They are hollow tubular structures composed of polymerized α- and β-tubulin dimers that are wound together. The dimers form linear protofilaments that are arranged in a bundle-like pattern, giving rise to a hollow, tube-like structure with a 25nm diameter. Each microtubule consists of 13 protofilaments that are organized by the centrosome.

They are involved in maintaining the structure of a cell, and provide paths for intracellular transport of molecules. They are also involved in the movement of secretory vesicles, organelles, and other such macromolecules. They also play an important role in cellular and chromosomal separation during the processes of mitosis and meiosis. They also impart motility to cells via the disassembly of focal adhesion points. Microtubule assembly is essential during embryogenesis in order to determine the axis of the egg. They are also involved in the synthesis of cell wall in plant cells.
Microtubule motility
9+2 Arrangement of Microtubules in Cilia/Flagella
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Cilia and flagella are considered as part of the cytoskeleton as they are maintained by microtubules and they act as structural components of a cell. The tubulin units are present in a 9+2 arrangement in these structures, where the 9 dimers are arranged in a circle with two monomers placed at the center. The molecules are all connected to each other via the help of dyenin arms (above image). This structure forms the axoneme of cilia and flagella that helps in its movement in order to bring about cell migration.
Septins
They are highly conserved GTP binding proteins that assemble into rings and filaments. Its primary function is to serve as a localized attachment site for other proteins, and to prevent the free and unchecked diffusion of molecules throughout the cells. In yeast cells, they provide the scaffold required for the cell division to occur. In humans, studies have found that these proteins immobilize pathogens and prevent them from invading other cells.
Prokaryotic Cytoskeleton
Initially, it was believed that the prokaryotic cells lacked a cytoskeleton, however recent research has found that homologous structures of eukaryotic cytoskeleton exist in the prokaryotic cells.

FtsZ
It functions like tubulin in the presence of GTP, but does not form tubules. It is vital for the recruitment of other proteins to initiate cell wall synthesis during cell division.

MreB
It determines and maintains cellular shape and also forms a helical network beneath the cell membrane so as to guide the synthesis of cell walls.

ParM
Its function is analogous to that of microtubules during mitosis. It is involved in the partitioning and separation of new daughter cells during cell division.

Crescentin
It maintains the cell shape in helical and vibrioid bacteria, but the exact mechanism is unknown.

Bactofilin
It regulates cell shape in rod-shaped bacteria and also maintains cell wall integrity.

Crenactin
It is homologous to actin and unique to kingdom Archaea. It causes the cells to form rod or needle-like shapes, thereby implying a role in shape determination.
Functions of Cytoskeleton
★ Determines, imparts, and maintains shape of cell.

★ Provides mechanical resistance to deformation.

★ Interacts with extra cellular matrix and provides overall stability.

★ Exhibits contractile nature which is used by the cell for the purpose of migration.

★ Transmits and transports intracellular signals and molecules.

★ Segregates chromosomes during cell division, and later separates the daughter cells during cytokinesis.

★ Provides a framework for organization of cellular components.

★ Forms specialized structures like flagella, cilia, podosomes, etc..

★ Allows cells to adhere to surroundings.

★ Provides a support for cell wall synthesis.

In 1970s, Keith Porter proposed the existence of a fourth eukaryotic cytoskeletal element called "microtrabeculae." Recent investigations of this claim have led to the conclusion that these structures are not cytoskeletal elements but are mere artifacts of the fixation treatment (for their visualization) given to the cells. Despite this, the complexity and interactions of the cytoskeleton as a whole are yet to be fully deciphered.
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