Microtubules are one of the most important components of the cytoskeleton of a cell. They have a diameter of 25 nanometers and a length that varies from 200 nanometers to 25 micrometers. They serve as structural components within cells and are involved in many cellular processes that are vital for the survival of a cell, including mitosis, cytokinesis, and vesicular transport.
Structure of Microtubules
Microtubules are nothing but polymers of α and β tubulin dimers. In protofilaments, these tubulin dimers polymerize end to end. The protofilaments then form bundles of hollow cylindrical filaments. Typically, the protofilaments arrange themselves in an imperfect helix, wherein one turn of the helix contains 13 tubulin dimers, each of which are from a different protofilament. A striking feature that aids in microtubules function is its peculiar polarity. Tubulin polymerizes end to end with the α subunit of one tubulin dimer coming in contact with the β subunit of the next. Therefore, in a protofilament, one end will have an exposed α subunit, while there will be an exposed β subunit at the other end. These ends are designated the (−) and the (+) ends, respectively. The protofilaments bundle in a parallel manner to one another, so in a microtubule, there is one end, the (+) end, with only the β subunits exposed while the other end, the (−) end, only has α subunits exposed. The (-) end is capped, thus, leaving out only the (+) end from where elongation of the microtubule can occur.
When it comes to mitosis, this process is facilitated by a subgroup of microtubules known as astral microtubules, which are microtubules originating from the centrosome that do not connect to a kinetochore. Astral microtubules develop in the actin skeleton and interact with the cell cortex to aid in orientation of spindles during cell division. They are organized around the centrosomes into radial arrays. Astral microtubules function in tandem with specialized dynein motors, which are oriented with the light chain portion attached to the cell membrane and the dynamic portion which is attached to the microtubule. This allows for dynein contraction to pull the centrosome towards the cell membrane, thus assisting in cytokinesis in plants and animals.
Microtubules act as conveyor belts inside cells. They help to move vesicles, granules, and organelles like mitochondria, and chromosomes via special attachment proteins. Vesicles get attached to microtubule associated proteins and move along their conveyor belt. The microtubule associated proteins include kinesins and dynein which move along these structures in opposite directions. Kinesins move vesicles along towards the plus end and dynein moves towards the minus end. This is how vesicles are moved from one region to another. This is active transport and hence, requires the breakdown of ATP, though it is not yet known how the energy from ATP breakdown is converted into vectorial transport.
Also, it is microtubules that join with other proteins to form more complex structures called cilia, flagella, or centrioles. Microtubules also play a role in maintaining the cytoskeleton, that is, the basic structure of the cell. This is because, structurally, they are linear polymers of tubulin which is a globular protein present in the cytoplasm.
The functioning and formation of microtubules can be disrupted by the use of certain drugs. This is the basis of using certain drugs like colchicine which help to treat cancer. These drugs inhibit polymerization by binding itself to tubulin and preventing its addition to the (+) ends.