Define the microtubules

Microtubules are one of the three main components of the cytoskeleton in eukaryotic cells, alongside microfilaments (Actin Filaments) and intermediate filaments. They are long, hollow, cylindrical structures composed of protein subunits called tubulins, and they play a crucial role in various cellular processes, including maintaining cell shape, facilitating intracellular transport, and ensuring proper chromosome segregation during cell division.

They are the largest, with diameters of 25nm compared to actin filaments and intermediate filaments. A microtubule is a dimmer of α- and β-tubulin subunits. Microtubules comprise of 13 protofilaments - alpha (α) tubulin and beta (β) tubulin which are arranged in a helical fashion to form a hollow tube. The lateral interactions of the protofilaments include the wall of the microtubule. A third form of tubulin, gamma (γ) tubulin, is essential for starting microtubule assembly and is mainly localised to the centrosome.

Microtubules within cells are organised in various ways and can form individual filaments, bundles, or networks. In some cases, mitotic spindle are the specialised structures of microtubules during cell division. Microtubules are also structural units for the formation of cilia and flagella.

Structure and Organisation of Microtubules

Microtubules are composed of tubulin dimers that form linear protofilaments, which align to create a hollow, cylindrical tube. Their inherent polarity, dynamic nature, and structural flexibility make them essential components of the cytoskeleton, enabling cells to maintain their shape, divide, and transport materials efficiently.

Hare is a comprehensive overview of the structure of microtubules, focusing on their composition, organization, and unique features.

1. Basic Composition:

At the molecular level, microtubules are primarily composed of proteins known as tubulins. The basic building blocks of microtubules are tubulin dimers, which consist of two different but closely related proteins: alpha-tubulin (α-tubulin) and beta-tubulin (β-tubulin).
  1. Alpha (α) tubulin: This protein binds to the beta-tubulin and does not exchange with the cytosolic pool.
  2. Beta (β) tubulin: This protein can bind GTP (guanosine triphosphate) and is capable of exchanging it, playing a key role in the dynamics of microtubule growth and shrinkage.
These tubulin dimers are fundamental to the formation of microtubules, as they stack together to create linear chains called protofilaments.

2. Protofilament Structure:

Microtubules are constructed from protofilaments, which are linear strands formed by the end-to-end polymerization of tubulin dimers. Each protofilament is made up of alternating alpha and beta tubulin subunits, which align in a head-to-tail manner. This arrangement gives rise to the inherent polarity of the protofilaments, with a distinct minus end (where alpha-tubulin is exposed) and a plus end (where beta-tubulin is exposed).

Typically, a microtubule consists of 13 protofilaments that are arranged side by side to form a cylindrical structure. The lateral interactions between adjacent protofilaments involve weak non-covalent bonds, such as hydrogen bonds and van der Waals forces, which help to stabilize the overall structure of the microtubule.

3. Cylindrical Configuration:

The collective arrangement of the 13 protofilaments results in a hollow, cylindrical structure that is characteristic of microtubules. The outer diameter of a microtubule is approximately 25 nanometers (nm), while the inner lumen is about 12-15 nm wide. This hollow design allows microtubules to be strong yet flexible, making them ideal for providing structural support to cells and tissues.

The microtubule's cylindrical architecture allows it to withstand compressive forces, maintaining the cell's shape and integrity. Additionally, the hollow structure serves as a track for motor proteins, which transport cellular cargo along the microtubules.

4. Polarity:

One of the most critical features of microtubules is their polarity. 
  1. Plus End: The β-tubulin is exposed at this end, and it is the primary site for the addition of new tubulin dimers, allowing rapid elongation.
  2. Minus End: The α-tubulin is exposed at this end, typically more stable and often anchored to structures such as the centrosome or microtubule-organizing centers (MTOCs).
This polarity is essential for the directional movement of motor proteins, such as kinesins and dyneins, which transport organelles, vesicles, and other cellular materials along the microtubule tracks.

5. Dynamic Instability:

Microtubules are characterized by dynamic instability, a process whereby they can rapidly grow and shrink. This is regulated by the hydrolysis of GTP bound to beta-tubulin:
  • Growth Phase: When GTP-bound tubulin dimers are added to the plus end faster than GTP is hydrolyzed, the microtubule elongates.
  • Shrinkage Phase: If GTP is hydrolyzed to GDP before new dimers are added, the microtubule destabilizes, leading to depolymerization.
This ability to rapidly reorganize is crucial for cellular processes such as cell division, where microtubules form the mitotic spindle to segregate chromosomes.

6. Associated Proteins:

Microtubules interact with various proteins, which modulate their stability and function:
  • Microtubule-Organizing Centers (MTOCs):
    • Microtubules are typically nucleated and organized from specific cellular structures known as microtubule-organizing centers (MTOCs). The most well-studied MTOC is the centrosome, which is found in many animal cells. The centrosome contains a pair of centrioles surrounded by a protein matrix known as the pericentriolar material (PCM), which is rich in gamma (γ) tubulin.
    • Gamma-tubulin is crucial for initiating microtubule assembly. It forms a ring complex called the gamma-tubulin ring complex (γ-TuRC), which serves as a nucleation site for the formation of new microtubules. The gamma (γ) tubulin ring complex helps to stabilize the minus end of newly formed microtubules, facilitating their growth from the plus end.
  • Motor Proteins:
    • Motor proteins such as kinesins (which transport materials toward the plus end) and dyneins (which transport materials toward the minus end) utilize microtubules as tracks for intracellular transport.

Microtubule Dynamics: Assembly and Disassembly

The alternating phases of microtubule assembly (known as polymerization) and disassembly (known as depolymerization) are collectively referred to as dynamic instability, was first described by Timothy Mitchison and Marc Kirschner in 1984. This term describes the inherent ability of microtubules to rapidly switch between growth and shrinkage, which is critical for their roles in cellular processes like mitosis, intracellular transport, structural reorganization and formation of specialized structures like cilia and flagella. 


Understanding both assembly and disassembly mechanisms is key to appreciating how microtubules support a broad range of cellular functions.

Assembly (Polymerization) of Microtubules

Microtubule assembly is a finely tuned, multi-step process that begins with nucleation and progresses through elongation and stabilization.

1. Nucleation

The first and most critical step in microtubule polymerization is nucleation, which refers to the formation of a stable microtubule "seed" or nucleus. This step is typically the rate-limiting stage of microtubule formation because the initial assembly of tubulin dimers is energetically unfavorable.
  • Tubulin Dimers: Microtubules are built from α-tubulin and β-tubulin dimers, which polymerize head-to-tail to form linear protofilaments. 13 protofilaments align side by side, forming the hollow cylindrical structure of a microtubule.
  • Microtubule-Organizing Centers (MTOCs): In most cells, microtubule nucleation occurs at MTOCs, such as the centrosome in animal cells. At these sites, the gamma (γ) tubulin ring complex (γ-TuRC) provides a platform for nucleation. The γ-TuRC forms a template that stabilizes the initial assembly of tubulin dimers, facilitating the formation of the microtubule's minus end.
  • γ-Tubulin Ring Complex: The γ-TuRC is a multi-protein complex that provides structural support for nucleation, preventing premature disassembly of the tubulin seed or nucleus. It caps the minus end of the microtubule, allowing growth to occur primarily at the plus end.

2. Elongation

Once nucleation is achieved, the microtubule enters the elongation phase, during which tubulin dimers rapidly add to the growing plus end. The elongation process is driven by GTP-bound tubulin dimers, which preferentially polymerize at the plus end due to the high energy state of GTP-bound tubulin.
  • GTP-Bound Tubulin: Tubulin dimers bind to GTP before being incorporated into the microtubule. The binding of GTP to the β-tubulin subunit increases the affinity between tubulin dimers, driving the polymerization process. As long as GTP-bound tubulin is available, the microtubule will continue to grow.
  • GTP Cap: At the growing plus end, a GTP cap forms, stabilizing the microtubule and preventing disassembly. The GTP cap is made up of tubulin dimers that are still GTP-bound, and it helps maintain the structural integrity of the microtubule. As tubulin dimers move away from the plus end and become part of the microtubule lattice, the GTP is hydrolyzed to GDP, rendering the GDP-bound tubulin less stable.
  • Dynamic Growth: The growth rate of microtubules can vary depending on the concentration of free tubulin dimers and cellular conditions. The plus end is highly dynamic, allowing for rapid extension into the cytoplasm where microtubules can perform functions like vesicle transport and spindle formation during mitosis.

3. Stabilization

Microtubules are inherently unstable structures and require stabilization to function effectively in cells. Several microtubule-associated proteins (MAPs) help stabilize microtubules by binding to their surfaces, reducing the likelihood of depolymerization.
  • Microtubule-Associated Proteins (MAPs): Proteins such as tau, MAP2, and MAP4 play key roles in stabilizing microtubules. By binding along the microtubule lattice, they reduce the chances of depolymerization and help organize the microtubules into stable structures that are crucial for specialized functions like axon formation in neurons.
  • Post-Translational Modifications: Tubulin subunits can also undergo post-translational modifications such as acetylation, detyrosination, and polyglutamylation. These modifications can enhance the stability of microtubules or alter their interactions with other cellular proteins.

The overall balance between nucleation, elongation, and stabilization allows microtubules to grow and form complex intracellular networks that perform a wide range of functions, from cargo transport to cell division.

Disassembly (Depolymerization) of Microtubules

Microtubule disassembly is a rapid process that allows cells to reorganize their cytoskeleton, especially during events such as mitosis, cell migration, or in response to stress. This process involves the loss of tubulin dimers from the microtubule ends and is initiated when the stabilizing GTP cap is lost.

1. Catastrophe

The process of microtubule disassembly often begins with a phase known as catastrophe, where the microtubule transitions from a state of growth to rapid shrinkage. This happens when the GTP cap at the plus end is lost, exposing less stable, GDP-bound tubulin.
  • GTP Hydrolysis: As tubulin dimers are incorporated into the growing microtubule, the GTP bound to the β-tubulin subunit is hydrolyzed to GDP. The presence of GDP-bound tubulin destabilizes the microtubule because GDP-bound tubulin has a lower affinity for neighboring dimers. Once the GTP cap is lost, the microtubule becomes highly susceptible to depolymerization.
  • Rapid Shrinkage: During catastrophe, microtubules undergo rapid disassembly, losing tubulin dimers from the plus end. The rate of shrinkage can be much faster than the rate of growth, allowing the microtubule to be quickly dismantled when necessary, such as during mitotic spindle breakdown after cell division.

2. Rescue

Following a catastrophe, microtubules can sometimes switch back to a growing phase, a process known as rescue. Rescue occurs when GTP-bound tubulin dimers are reincorporated into the microtubule, re-establishing the GTP cap and halting further disassembly.
  • Stabilizing Proteins: Proteins such as Xenopus microtubule-associated protein 215 (XMAP215) help promote microtubule rescue by enhancing the addition of GTP-tubulin to the shrinking microtubule. This regulation ensures that microtubules can recover from disassembly and return to their functional roles.
  • Dynamic Instability: The alternating phases of growth, catastrophe, and rescue are characteristic of dynamic instability, a hallmark of microtubule behavior. This property enables microtubules to explore the intracellular space rapidly and adapt to changing cellular needs.

3. Severing

Microtubules can also be disassembled through severing, where specialized proteins break the microtubule into smaller fragments. This process creates new plus and minus ends, both of which can either grow or shrink depending on cellular conditions and the availability of free tubulin dimers.
  • Katanin and Spastin: Two major microtubule-severing proteins are katanin and spastin. These proteins cut through microtubules by destabilizing the interactions between tubulin dimers in the microtubule lattice. Severing is an important regulatory mechanism that allows cells to rapidly remodel their microtubule network during processes such as mitosis, neuronal growth, or cellular stress responses.
  • Fragmentation and Redistribution: Severing generates new microtubule fragments that can either disassemble completely or serve as new seeds for polymerization. In neurons, for example, severing by spastin plays a critical role in branching by allowing new microtubule segments to form and extend into different regions of the cell.

Types of Microtubules

During mitosis and other cellular processes, microtubules are classified into different types based on their structure and function, including kinetochore, astral, polar, cytoplasmic, and axonemal microtubules.

01. Kinetochore Microtubules

These are the microtubules that connect to a special structure on the chromosomes called the kinetochore. The kinetochore forms at the centromere, which is the central part of a chromosome where the two sister chromatids (identical copies of the chromosome) are joined. Their primary function is to pull chromosomes apart toward opposite poles of the dividing cell during mitosis and meiosis.

Kinetochore microtubules pull the sister chromatids apart during anaphase (the stage of mitosis where the chromatids separate). They ensure that each new cell gets the correct number of chromosomes.

02. Polar (or Interpolar) Microtubules

These microtubules stretch from one side of the cell to the other, connecting the two centrosomes (the structures that organize microtubules). They do not connect to the chromosomes directly.

Polar microtubules push against each other as they grow, which helps to separate the centrosomes and create tension in the mitotic spindle. This action is important for keeping the structure of the spindle stable and ensuring that the chromosomes are aligned correctly in the middle of the cell during metaphase.

03. Astral Microtubules

These microtubules extend out from the centrosomes toward the edges of the cell, forming a sort of star shape (hence the name "astral"). They extend from the centrosomes to the cell membrane, helping to position the spindle apparatus and define the axis of cell division.

Astral Microtubules help position the spindle apparatus (the structure made of microtubules that organizes the chromosomes) inside the cell. They connect with the cell's outer layer (called the cortex) and help pull the spindle poles apart. This action defines the direction in which the cell will divide, ensuring that division occurs symmetrically and that the daughter cells are of equal size.

04. Cytoplasmic microtubules

Cytoplasmic microtubules are present in non-dividing cells (during interphase) and are involved in maintaining the cell's shape and internal organization. They create a flexible network that helps intracellular transport inside the cell. They act like tracks for motor proteins, such as kinesins and dyneins, which move organelles, vesicles, and other parts of the cell around. This transport system is important for moving materials throughout the cell, organizing the organelles, and making sure different parts of the cell can communicate effectively. Cytoplasmic microtubules also play a role in cell polarity, which is important for processes like cell movement and development.

05. Axonemal Microtubules

Axonemal microtubules are specialized microtubules found in cilia and flagella, organized in a "9 + 2" arrangement. They facilitate cell motility through coordinated movement and provide structural support. Dynein motor proteins enable sliding along these microtubules, essential for processes like sperm locomotion and fluid movement across epithelial surfaces. For example, cilia in the respiratory tract help to clear mucus, while flagella enable sperm cells to swim toward the egg during fertilization.

Why Are Microtubules Important?

Microtubules are important because they play essential roles in maintaining the structure, organization, and functionality of eukaryotic cells. Here are the key reasons why are microtubules important:

1. Cell Structure and Support

Microtubules form part of the cytoskeleton, which provides mechanical strength to cells. They help maintain cell shape, resist compression, and organize the cell's internal components. Without microtubules, cells would lose their structural integrity and become more vulnerable to physical stress.

2. Intracellular Transport

Microtubules serve as "tracks" for the movement of organelles, proteins, and other materials within the cell. Motor proteins, such as kinesins and dyneins, move along microtubules to transport cellular cargo. This transport system is especially important in neurons, where materials need to travel long distances, like moving neurotransmitters from the cell body to the synapse.

3. Cell Division

Microtubules are critical in cell division, particularly during mitosis and meiosis. They form the mitotic spindle, a structure that pulls chromosomes apart and ensures they are equally distributed to the two daughter cells. This is essential for proper cell reproduction and the prevention of genetic disorders.

4. Cell Movement

In specialized structures such as cilia and flagella, microtubules enable cell movement. Cilia and flagella are extensions of the cell membrane that beat rhythmically to move the cell or move fluid around the cell. Microtubules inside these structures provide the framework and enable their motion.

5. Dynamic Cellular Remodeling

Microtubules are highly dynamic, meaning they can grow and shrink quickly in response to cellular signals. This property, known as dynamic instability, allows cells to rapidly reorganize their internal structures. This is particularly important during processes such as cell migration, wound healing, and the response to changes in the environment.

6. Intracellular Signaling and Organization

Microtubules also help organize the cell's internal compartments by positioning organelles such as the Golgi apparatus, endoplasmic reticulum, and mitochondria. This organization is essential for efficient cell function and for the coordination of various cellular activities.

7. Medical Significance

Because microtubules are so crucial to cell division, they are often targeted in cancer therapies. Drugs such as taxanes and vinca alkaloids disrupt microtubule dynamics, preventing cancer cells from dividing properly. This makes microtubules a key target in the treatment of rapidly dividing cancer cells.


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SAQ

1 Fill in the blanks: 
a) The basic unit of microtubules is …………………

b) α tubulin occurs at.............end, and β tubulin is at ..............of microtubules

c) GTP-GTP tubulin cap stabilises and promotes the .......................

d) The plus end-directed motor protein of microtubules is ………….

e) The hydrolysis of GTP from β-tubulin causes …………. of microtubules.

f) The cellular function of γ-tubulins is. …………………….

g) Colchicine binds to ……………

Answers:
a) α β tubulin heterodimers
b) plus, minus
c) polymerisation of microtubules
d) kinesin
e) Depolymerisation 
f) nucleate the growth of MTs 
g) free tubulin

SAQ 2

SAQ 3 

TERMINAL QUESTIONS

6. Draw the labelled diagram of microtubules.




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