Cytoskeleton – Definition, Structure, Components, Functions

The cytoskeleton is an elaborate network of associated protein filaments present in the cytoplasm of all cells. In 1903 Nikolai K.Koltsov coined the term cytoskeleton. It is situated between the membrane of the cell and the cell nucleus.

What is Cytoskeleton?

The cytoskeleton is a complex structure that interlinks with the protein of the cell present in the cytoplasm of Prokaryote, Eukaryote, Bacteria, and archaeans. The cytoskeleton is a network of fibers that interlinks all cell organelles and helps the cell to work properly. In eukaryote cells cytoskeleton is a mess of filaments and motor proteins that help in cell motion. Cytoskeletons have the prime function to keep the cell intact and in shape. Cytoskeletons also help in transport. The cytoskeleton is composed of three components. These three components are capable of growth or disassembly dependent on the requirements of the cell.

Cytoskeleton Structure

The cytoskeleton is composed of three types those are:

1. Microtubule
2. Intermediate Filament
3. Microfilament

Microtubules 

These are minute, hollow, and round tubes. Their diameter of about 24 nm. They are hollow and the inner space of microtubules is known as the Lumen. They are formed from soluble tubulin dimers. Soluble tubulin is a heterodimer of alpha-tubulin and beta-tubulin.

One microtubule is made up of 10 to 15 protofilaments(In mammalian cells 13 protofilaments form a microtubule )These are very dynamic structures, which reveal that they can change quickly. They keep rapidly growing due to polymerization or undergo rapid shrinking due to depolymerization. Microtubules play a vital role in cell movement, intracellular transport, and cell division.

Cell movement 

• Basal bodies are a special group of microtubules that forms protrusions from the cell surface and causes cell movement.
• If protrusions are short and numerous, they are called “Cilia”.
• If protrusions are long or fewer, they are called “Flagella“.

Intracellular transport 

• Microtubules transport organelles and membrane-bound vesicles.
• This process is driven by motor proteins like dynein. These Motor proteins attach the transport vesicles to microtubules and actin filaments and cause intracellular transport.
• Microtubules contain 2 ends, plus (+) and minus (-) ends. The plus (+) end of microtubules is at the periphery of cells, and the minus (-) end is attached to the centrosome.
• The microtubule utilizes motor proteins like kinesins (present at the positive end) and dyneins (present at the negative end) for transporting organelles and vesicles in opposite directions in the cytoplasm.

Cell division 

• Microtubule plays important role in the cell cycle – they arrange cellular components and divide them in two.
• The microtubule plays a vital role in cell division (in both meiosis and mitosis) and is the chief component of mitotic spindles, which separates the chromosomes during cell division.

Intermediate Filaments

These filaments are about 10 nm in diameter and they provide tensile strength to the cell. They help in the formation of neurofilaments and keratins. There are five types of  Intermediate filaments-

1. Type I and II- Keratins: These keratins exist in acidic and basic forms respectively. Keratin’s intermediate filaments plays important role in forming junctions. These junctions can attach cells together or attach cells and matrice together.
2. Type III- Vimentin and Desmin: Vimentin is a type of structural protein and it is found in white blood cells (WBCs), smooth muscle cells, and fibroblasts. Vimentin plays a vital role in holding organelles in the cytoplasm. whereas desmin is a structural protein present in skeletal, cardiac, and smooth muscles.
3. Type IV- These are of 3 types  NF-L, NF-M, and NF-H (Here NF denotes Neuro Filaments, and L – light, M – Medium, and H – heavy). These are protein polymers abundantly present in the cytoplasm of neurons.
4. Type V- Lamins: Lamins are important architectural proteins present inside the nuclear membrane of eukaryotic cells. They help in mechanical stability and also plays important role in binding proteins and chromatin.

The cytoskeleton is also composed of some motor proteins–

Kinesins 

These proteins move through the microtubules carrying the cellular components. They drag the organelles through the cell membrane.

Dyneins

These drag the cell organelles toward the nucleus.

Myosin

These interlink with actin protein and are responsible for muscle contractions. They also perform exocytosis, endocytosis, cytokinesis

 Microfilaments

• These are thread-like protein filaments with a diameter of 3-5 nm. They are specifically present in muscle cells.
• Microfilaments are composed of subunits of actin protein.
• Actin protein in these filaments is responsible for muscle contraction.
• Actin exists in two different forms: globular actin (G-actin) and fibrous actin (G-actin)

G-actin 

• Globular actin (G-actin) is a single polypeptide chain and its molecular weight is about 42 kilo Daltons. 
• It has a roughly globular arrangement.
• The globular arrangement of molecules is balanced due to the high-affinity calcium-binding site in the G-actin monomer.  
• G-actin also contains the binding site of ATP per monomer.

F-actin 

• A Group of G-actin monomers forms a filamentous F-actin polymer. 
• The F-actin filaments contain two helical assemblages of G-actin.  
• These G-actin assemblages are coiled around each other, with 13.5 subunits for every turn. 
• F-actin filaments are identified by different types of immunofluorescence staining techniques.

Microfilaments are responsible for cellular movements like gliding, contraction, cytokinesis

Functions of Cytoskeleton 

1. It provides motility and mechanical support.
2. It maintains the shape of the cell.  
3. It organizes the organelles.  
4. It helps in the transport of molecules.
5. Cell-cell signaling
6. Cell division.

FAQs on Cytoskeleton

Question 1:  What is Cytoskeleton?

Answer:

The cytoskeleton is a meshy network of associated protein filaments present in cytoplasm of all cells  It is situated in between the membrane of the cell and the cell nucleus. 

Question 2:  Which type of intermediate filaments are present in the cardiac cells and cytoplasm of neurons?

Answer:

Desmin is a structural protein present in skeletal, cardiac, and smooth muscles. Desmin is Type-III intermediate filaments.

Type IV– These are protein polymers abundantly present in the cytoplasm of neurons. These are of 3 types  NF-L, NF-M, and NF-H (Here NF denotes Neuro Filaments, and L – light, M – Medium, H – heavy ). 

Question 3: What are the main components of the cytoskeleton?

Answer:

 It is composed of three key components :

1. Microtubules
2. Microfilaments and
3. Intermediate filaments

Question 4: Genetic defect in the formation of which cytoskeletal structures affect?  

Answer:

Microfilaments are specifically found in muscle cells microfilaments are composed of subunits of actin protein. Actin protein in these filaments is responsible for muscle contraction. Hence, Genetic defect in the formation of these cytoskeletal structures affects muscle contraction.

Question 5: Function of the cytoskeleton?

Answer:

Cytoskeleton performs the following functions

• It provides motility and mechanical support. 
• It maintains the shape of the cell   
• It organizes the organelles   
• It helps in the transport