Cytoskeleton

Introduction

The cytoskeleton is composed of protein filaments that join together and lie within the whole cytoplasm except the nucleus. It is present in all types of cells except in bacteria and archaea. It is entirely made up of similar types of proteins. It provides an adequate shape to the cell and adds mechanical resistance. They also provide cell contraction, which enables its movement. Nikolai K. Koltsov, in 1903, termed this bundle as the cytoskeleton. He proposed that these networks of tubules were responsible for the shape of the cells. The earlier cytoskeleton was estimated to be a gel-like substance that holds organelles in place. Earlier scientists believed that it was only present among eukaryotic cells, but later in 1992, it was also discovered in prokaryotes. These are extremely small and are only visible through the electron microscope.

Eukaryotic Cytoskeleton

It was believed that cytoskeletons were earlier only present in eukaryotes. This eukaryotic cytoskeleton comprises the polymerisation of specific types of proteins. They possess distinct shapes due to this particular protein organisation. The study of neurogenic disorders like Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis shows the degradation of the cytoskeleton of neurons. This proves that these structures play an essential role in the normal neurological health of an organism.

Types of Cytoskeleton

There are three types of cytoskeleton in eukaryotic cells:

• Microfilament
• Intermediate filaments
• Microtubules

Microfilaments

They are often referred to as actin filaments made up of proteins. They are the polymers of actin proteins, so-called actin filaments. They are extremely small in size, about 7nm in diameter and are present in the cytoplasm of cells. The two strands of actin join spirally, forming this structure. Sometimes the G–actin monomer joins to form a polymer that continues to form an actin filament (microfilament). It then assembles into two chains known as F– actin chains. Microfilaments perform many functions. They assist in cytokinesis and cell motility and grant it to amoeba-like single-celled organisms. They are also a component of muscle cells and along with myosin, grant them to contract. Actin and myosin are two main elements of muscle contractile elements. The Rho proteins control the actin. The microfilaments also help in intracellular transport.

Intermediate filaments

The intermediate filaments are in the middle of microfilaments and microtubules. They are about 8 to 12nm in width. They comprise keratin as the protein and also consist of vimentin. The keratin is present in hairs, nails, scales of animals, and horns. Intermediate filaments are present in the cytoplasm of cells. However, in lamins (nuclear proteins), it is present in the cell’s nucleus. There it supports the nuclear envelope surrounding the nucleus. Like actin filaments, they also help give the cells a good shape. They sometimes participate in cell-cell and cell-matrix junctions. Although all animals possess intermediate filaments, there are some exceptions, like fruit flies. They are commonly referred to as the cell’s support system or “scaffolding”. Problems in these intermediate filaments can lead to serious health conditions like Alexander disease and muscular dystrophy.

Microtubules

• The largest cytoskeletons in animal cells are the microtubules. They exist like hollow tubules about 25 nm in diameter. 
• They are composed of alpha and beta-tubulin as monomers. They bind GTP for polymerisation and possess a dynamic behaviour.
•  Centrosomes organise them in animal cells. The centrosome is the structure present in an animal cell in the middle of the cell. 
• The microtubules radiate out of these centrosomes. Microtubules can form structures resembling flagella. These flagella possess tails that push the cell in a forward direction. 
• They are also present in cilia – the thread-like appendages that can provide motility to the cells (motile cilia) and also increase their surface area (non-motile cilia). 
• They also possess great significance in the intracellular transport of molecules within the cell. They aid plant cell wall formation and are essential in forming spindle apparatus during mitotic division. This helps in the separation of sister chromatids during cell division.

Prokaryotic Cytoskeleton

Earlier it was believed that the prokaryotes completely lack cytoskeleton in their cells. However, research in the early 1990s proved its existence in the prokaryotic cells as well.

•  All the structural filaments within the prokaryotic cells are the prokaryotic cytoskeleton. 
• The proteins analogous to eukaryotes and those that are not analogous to eukaryotes are present in the prokaryotic cells.
• These structures also play an essential role in cell division, protection, and providing an adequate shape to prokaryotes.
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• The first prokaryotic cytoskeleton is FtsZ. They form filamentous Z-ring structures in the middle of the cell during cell division. These Z-ring are also highly dynamic. They are functionally similar to actin and are also homologous to tubulin. Like the tubulin, they are also bound to GTP and polymerise the other monomers of FtsZ. 
• MreB is another example – it is a bacterial protein that shows homologous behaviour towards eukaryal actin.

Functions of Cytoskeleton

The cytoskeleton possesses numerous functions. Cytoskeleton in animal cells and plant cells has its distinct functions. The first and foremost function of it is to provide the cell with a well-defined shape. It provides mobility to the cell. The filaments and tubules of its contract assemble and disassemble, due to which the cells can easily crawl forward. It makes the other cell organelles intact in their place.

Conclusion

Cytoskeletons are present in both the eukaryotic and prokaryotic cytoplasm. They are the polymers of protein units. They vary in their protein units, and their primary function is to provide the cell with a proper shape. They also help in the mobility of cells. These structures in the prokaryotes were discovered in the late 1990s. The prokaryotic cytoskeleton resembles that of eukaryotic ones.