Microtubules: Structure & Function

Microtubules are small polymerized tubular structures made up of tubulin proteins. Microtubules, microfilaments, and intermediate filaments are the important parts of the cytoskeleton. Microtubule is an important component of cell structure in plant and animal cells. It plays a major role in the functions of a variety of processes including cell division, cell movement, maintaining cell shape, etc.

These are rigid, hollow rods made up of α-tubulin and β-tubulin proteins that create protofilaments for polar microtubule formation. There are three types of microtubules involved in mitosis, namely astral, polar, and kinetochore. Kinesin and dynein are microtubule-associated proteins that make the internal transport network of cells.

What are Microtubules?

Microtubules are small polymerized tubular structures made up of tubulin proteins. Microtubules stick to specific centers called microtubule organizing centers (MOC). In animal cells, the central centrosome is a major microtubule organizing center. Microtubules are also found in the cilia and flagella of prokaryotic cells and human sperm cells. Microtubules undergo processes like treadmilling and dynamic instability, where tubulin molecules are added or lost, contributing to their dynamic nature. Microtubules have functions ranging from maintaining cell shape, cell movement using cilia or flagella, and the formation of mitotic spindle cell division.

Structure of Microtubules

The structure of microtubule is described below:

Size

Microtubules are rigid, hollow rods about 25 nm in diameter, and 200 nm-25 m long. These rods are composed of tubulin proteins.

Composition

The microtubule is composed of two similar globular protein subunits: α-tubulin and β-tubulin. α-tubulin form pairs or heterodimers and β-tubulin form pairs. Many of these pairs form a chain called protofilament. 13 protofilaments fold in a cylindrical shape to form a microtubule. These parts are placed on the microtubule, so that each one of them is facing the same way and creates 13 parallel protofilaments. This results in a polar microtubule which means α-tubulin at one end and β-tubulin on the opposite end.

Properties of Microtubules

Microtubules are polar with a positive (+) charge at one end, and the opposite ends have negative (-) charges that grow more slowly. Protofilaments are tiny parts of microtubules that line up in a row with the positive end always showing β-subunits and the negative end showing α-subunits. The pairs of tubulins can depolymerize or break apart. Both α- and β-tubulin bind to GTP for polymerization or depolymerization. The GTP bound to β-tubulin hydrolyzes to GDP in polymerization. The change weakens the binding affinity of tubulin and results in depolymerization and the dynamic nature of microtubules.

Treadmilling

Microtubules go through a process known as treadmilling. In this process, tubulin molecules connected to GDP are continually lost from the negative end and replaced by tubulin with GTP at the positive end of the same microtubule.

Dynamic instability

When new tubulin bound to GTP adds up faster than hydrolyzation of GTP and then is changed into GDP, the microtubule holds onto a GTP cap at its plus end, causing it to grow. On the other hand, if the growth rate slows down, the GTP-bound tubulin at the plus end is replaced by GDP. This results in a rapid depolarization of the microtubule. This back-and-forth process of growth and shrinkage in microtubules is known as dynamic instability.

Functions of Microtubules

The functions of microtubules are given below:

• Maintenance of Cell Shape: Microtubules microfilaments, and intermediate filaments play an important role in cell shape maintenance and cell stability. This filamentous framework decides the cell polarity.
• Cell Division: During mitotic cell division, microtubules form the mitotic spindle or spindle apparatus. These spindle separates chromosomes and distributes them uniformly to daughter cells. The spindle apparatus makes a contractile ring and separates daughter cells during cytokinesis.
• Cell Movement: Microtubules play an important role in the movement of prokaryotic cells. The typical ‘9+2’ structure of microtubules has nine doublet microtubules surrounding a central pair of singlet microtubules. This arrangement is found in motile cilia and eukaryotic flagella. The microtubules block the entry of mucus and dirt into the lungs in trachea cells. In a woman’s reproductive system, fallopian tubes have microtubules that transfer eggs from the ovary into the uterus.
• Intracellular Transport and Communication: Microtubules are an integral part of the cell’s internal structure, helping to transport organelles in its cytoplasm.
• Internal Transport Network: Microtubules facilitate the transmission of various cell components. This cell transport network involves the movement of materials among the cells and between them. It is made up of microtubule-associated proteins: kinesin and dynein.

Types and Functions of Microtubules Associated with Mitosis

There are three types of microtubules associated with mitosis: Astral, polar, and kinetochore.

Type	Function
Astral	It extends from the MTOCs of the cell to the cell membrane keeping the mitotic spindle in place.
Polar	It connects two MTOCs and helps to separate the chromosomes.
Kinetochore	It attaches to the chromosomes and helps to break them apart.

Conclusion – Microtubules

Microtubules are an essential part of the cell structures along with the microfilaments and intermediate filaments. These are rigid, hollow rods made up of α-tubulin and β-tubulin proteins that create protofilaments for polar microtubule formation. It plays a major role in cell division, cell movement, maintaining cell shape, etc.

FAQs on Microtubules

1. What is Microtubule?

A microtubule is a small polymerized tubular structure made up of tubulin proteins. It is an essential part of the cell structures along with the microfilaments and intermediate filaments.

2. What are the Functions of Microtubules?

Microtubules play a major role in cell division, cell movement, maintaining cell shape, and intracellular transport and communication.

3. Which Proteins make Microtubules?

The microtubule is composed of two similar globular protein subunits: α-tubulin and β-tubulin. The α-tubulin form pairs or heterodimers and β-tubulin form pairs.

4. What are the Types of Microtubules?

Three types of microtubules are present. These microtubules are associated with mitosis and are astral, polar, and kinetochore.

5. What is the Structure of a Microtubule?

Microtubules are rigid, hollow rods about 25 nm in diameter, and 200 nm-25 m long. These rods are composed of two similar globular protein subunits: α-tubulin and β-tubulin.