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CELL BIOLOGY

Cytoskeleton types, Structure & function

What is Cytoskeleton?

types of cytoskeletal fibers
Types of cytoskeletal fibers

The cytoskeleton structure is a filamentous protein network present in the protoplasm of the cell. It provides shape and support to the cell. Major Cytoskeleton types include microtubules, microfilaments, and intermediate filaments. These are chemically composed of proteins like tubulin, actin, myosin, troponin, and Tropomyosin. Their types, Structure & function is given right below.

Types of Cytoskeleton                                                              

There are three types of Cytoskeleton

1-Microfilaments (made of Actin protein)            

2-Intermediate filaments

3-Microtubules (made of tubulin protein)

1-Microfilaments

  • Microfilaments are the narrowest type, out of three types of cytoskeletal protein fibers.
  • They have about 7 nm in diameter. These are formed by many joined monomers of a protein (Globular actin, or G actin), which combine in a structure to form a double helix.
  • Because microfilaments are formed of Actin monomers, these are also called Actin filaments.
  • These are directionality distinct mean that they have structurally two different ends and show polarity. The growth rate of microfilaments is affected by their polarity because their one positive end is rapid while the other negative end is slow.
  • Like microtubules, microfilaments are also nucleated at the cell membrane. Therefore the highest concentration of microfilaments is present at the edges of a cell.
  • Several external factors and a group of special proteins effect on the characteristics of microfilament. The microfilament continuously disassembled in one region of the cell but reassembled in another place.
  • Microfilaments are supposed to be part of the cell cortex. These regulate the movement and shape of the cell’s surface. 
  • Microfilaments play a key role in the development of various cell surface projections including stereocilia, lamellipodia, and filopodia.
microfilaments diagram
Microfilaments diagram

Functions of Microfilaments

1- Cytoskeleton support the transportation of vesicles into and out of a cell.

2-It makes cell migration possible like cell motility which is needed to build up tissues.

3- Microfilaments are involved in endocytosis and exocytosis.

4-The cytoskeleton help in the transportation of communication signals between cells.

5-Muscle contraction is caused by actin and myosin filaments.

6-After nuclear division, cleavage is brought about by constriction of a ring of microfilament.

7-The microtubules and microfilaments can be assembled or disassemble and allow cells to contract, migrate and crawl e.g. amoeboid movement.

8-Microfilaments also help in cytoplasmic streaming. The flow of cytoplasm is called Cytoplasmic streaming.

9- Microfilaments allow, traveling cell organelles, nutrients, and waste products, from one part of the cell to another.

10-Microfilaments attach to a cell organelle and then contract hence pull the organelle to a different area in the cell.

11- Actin filaments develop a track system in non-muscle cells, for cargo transport that is powered by myosins i.e. myosin V and VI. Myosin utilizes the energy from ATP which hydrolysis to transport cargo much rapidly than diffusion.

2-Intermediate  filament

Intermediate filaments have different types, each type formed with a different protein. One of these proteins is keratin that is a fibrous protein also found in our skin, nails, and hair. For example, you may have seen shampoo commercial which claims to smooth your hair by keratin.                       

Functions of Intermediate  filament

  • Intermediate filaments play an essential structural role in the cell which is more permanent.
  • They maintain the shape and bear tension, and berth the nucleus and other cell organelles in place.

3-Microtubules

 Microtubules are the largest among the three types of cytoskeleton protein fibers. Their diameter is about 25 nm and formed of tubulin proteins. These look straw-like tubes.  Two subunits are present in each tubulin protein (α-tubulin and β-tubulin).

  • Like actin filaments, microtubules have dynamic structures. They can shrink and grow fastly because of the removal or addition of proteins (tubulin).
  • Microtubules show directionality means that they have different structures at different ends. Different ends make microtubules polar molecules.
  • They have a negatively charged end that grows relatively slow and another positively charged end that grows relatively fast. The positive end of the microtubule is always exposed by beta subunits, while the alpha subunits expose the negative end of microtubules. Their polarity permits to arrange them in a specific way and function correctly.
  • Microtubules have an important structural role in a cell i.e. help in resisting compression forces.
  • Microtubules are present adjacent to the nucleus. These are arranged by microtubule organizing centers (MTOCs).  Important MTOCs include the basal bodies found in flagella and cilia, Centrosome present in the center of the animal cell and spindle pole bodies (SPBs) in most of the fungi.
  •  Motor proteins dynein, kinesin and, many other proteins like katanin bound microtubules. These protein are important for regulating microtubule dynamics.
  • Currently, a gram-positive bacterium, Bacillus thuringiensis contain actin-like protein. This protein forms a microtubule-like structure and is involved in plasmid segregation.

Do You know Microtubules are dynamically unstable?

“Marc Kirschner and Tim Mitchison described Dynamic instability in 1984. It results in the continual and rapid turnover of most microtubules within the cell which has only half-lives with several minutes. This rapid turnover of microtubules is critical for the remodeling of the cytoskeleton which occurs during mitosis. Drugs that affect microtubular arrangement, are helpful not only as experimental tools in cell biology but also help in the treatment of cancer”

By stoping microtubular polymerization we can control cancer 

Colcemid and colchicine are examples of experimental drugs that bind protein (tubulin) and cease microtubule polymerization, which in turn blocks mitosis. Vincristine and vinblastine drugs are used in cancer chemotherapy because they selectively cease rapidly dividing cells. Taxol is another helpful drug that stabilizes microtubules rather than inhibiting their arrangement. This stabilization also blocks cell division, and taxol (drug)  is used as an anticancer agent as well as an experimental tool.

Functions of microtubules

1-Microtubules give shape to the cell.

 2-Microtubules are necessary for cells which have not a cell wall. e.g. like animal cells as they did not get shape from the outer layer which is thick.

3- Like cilia and flagella structures, microtubules give help to cell movement.

4-It supports and maintain the cell.

5-Many cellular organelles (Golgi vesicles, lysosomes and mitochondria) are embedded in the cytoskeleton. It also helps in the movement of the organelle.

6- Microtubules also help the productions of vacuoles.

7-It forms cellular outgrowth such as cilia and flagella, in some cells.

      8- Spindle fibers are tubular and have a diameter of about 25nm. These are composed of Microtubules formed during mitosis and meiosis.                                        

9- The microtubules control separation of chromatids and chromosomes.

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