Microtubule Structure
Microtubules are composed of tubulin protein subunits that come in two similar forms: alpha-tubulin and beta-tubulin. These subunits assemble together to form a tubular structure with a diameter of about 25 nanometers.
Microtubules are organized into a hollow cylindrical structure made up of parallel filaments. Each filament is a linear chain of tubulin dimers. Typically, microtubules are composed of 13 filaments parallelly arranged in a circular pattern, forming a tubular structure with a hollow core.
Microtubules are highly dynamic structures that can be assembled and disassembled more quickly at one end than the other. It allows them to grow and contract rapidly, which enables cells to remodel their cytoskeleton in response to cellular signals and perform various essential functions.
Microtubule proteins are very conserved in evolution, and microtubule proteins from different organisms maintain their original functions even after interchanging.
Microtubule Function
Structural Support: Microtubules provide structural support to the cell, helping to maintain cell shape and rigidity. They are particularly important in cells that require a high degree of organization and stability, such as nerve cells and epithelial cells.
Intracellular Transport: Microtubules serve as tracks for the movement of organelles, vesicles, and other cellular components within the cell. Motor proteins, such as dynein and kinesin, move along microtubules, using ATP hydrolysis to transport cargo to specific destinations in the cell. For example, microtubules play a crucial role in the transport of vesicles from the cell body to the axon terminals in nerve cells. They also guide vesicles from the endoplasmic reticulum to the Golgi apparatus and from the Golgi apparatus to the plasma membrane.
Cell Division: During cell division, microtubules form the mitotic spindle, which is essential for the separation of chromosomes. Microtubules emanate from structures called centrosomes and attach to the chromosomes, facilitating their movement to opposite poles of the dividing cell.
Cilia and Flagella: Microtubules are a key component of cilia and flagella, cellular appendages involved in movement. In cilia and flagella, microtubules are organized in a specific arrangement known as the 9+2 pattern, where nine outer microtubule doublets surround a central pair of microtubules. The bending and movement of cilia and flagella rely on the coordinated sliding of microtubule doublets. The motor proteins in the "9+2" cylinder are connected to adjacent microtubule dimers. These proteins consume energy from ATP to move along the microtubule, causing the adjacent microtubule dimer to slide, and this movement occurs only on one side of the cylinder. This eventually results in a wave-like motion of the flagellum and a paddling motion of the cilia.