They are the largest structures in the cytoskeleton at approximately 24 nanometers thick.
Microtubules are hollow microscopic tubes made of the proteins alpha and beta tubulin that are part of a cell’s cytoskeleton, a network of protein filaments that runs throughout the cell, shapes the cell, and holds its organelles in place.
They have roles in cell movement, cell division, and the transport of materials within cells.
Microtubules are filamentous intracellular structures that are responsible for various types of movements in all eukaryotic cells. Microtubules are involved in cell and nucleic division, organization of intracellular structure and intracellular transport, as well as ciliary and flagellar motility.
Microtubules are hollow cylinders formed by repetitive structures of proteins, specifically dimers of alpha and beta tubulin (also referred to as ɑ-tubulin and β-tubulin).
Dimers are complexes of two proteins. tub-tubulin and β-tubulin join together to form a dimer, and then multiple units of these dimers join together, always alternating alpha and beta, to form a chain called a protofilament. It is explained in detail below:
Microtubule “building blocks” – tubulins
All eukaryotic cells produce the protein tubulin, in the usual way. The usual way, of course, is by transcription of genes encoding tubulin to produce messenger RNA, followed by translation of mRNA by ribosomes to produce protein.
Cells maintain at least two types of tubulin, which we call alpha tubulin and beta tubulin. However, it is doubtful that the two types can be found in cells as individual proteins. Alpha and beta tubulin spontaneously bind to each other to form a functional subunit that we call a heterodimer.
A heterodimer is a protein that consists of two different gene products. The term is fully descriptive, the prefix hetero means “different”, the prefix “di” means “two” and the suffix “mer” refers to a unit, in this case a single polypeptide.
Obviously, cells don’t continue to make tubulin (or any other protein) until they run out of resources. Some processes must regulate the synthesis of tubulin. A common regulatory mechanism is feedback inhibition.
When intracellular conditions favor assembly, tubulin heterodimers assemble into linear protofilaments. The protofilaments in turn assemble into microtubules. All such assembly is subject to regulation by the cell.
Dynamic microtubule instability
Under steady-state conditions, a microtubule may appear completely stable, yet there is action that is constantly taking place.
Microtubule populations usually consist of some that are shrinking and some that are growing. A single microtubule can oscillate between the growth and shortening phases.
During growth, heterodimers attach to the end of a microtubule, and during contraction they shed as intact subunits. The same heterodimer can go out and reignite.
Cilia and flagella
Understanding the regulation of microtubule assembly and function in any organism is a difficult task. Studying microtubules in cells as complex vertebrate (eg human) cells is an almost impossible task, without some “advice” on how to proceed.
The basic mechanisms can be solved using a much less complex biological model, such as a flagellate. For example, the flagella of the photosynthetic protist Chlamydomonas are composed of microtubules, as are all flagella and cilia.
Cilia and flagella have the same basic structure. They are attached to structures known as basal bodies, which in turn are anchored to the cytoplasmic side of the plasma membrane. From the basal bodies the “backbone” of the microtubules extends, pushing the plasma membrane outward.
Ciliary and flagellar movement
One can appreciate the complexity of microtubule organelles by observing the movement of the cilia and flagella. Despite the similarities in structure, the difference in the nature of motility by flagella versus cilia is profound, as can be seen by comparing representatives of the groups Ciliophora (the ciliates) and Mastigophora (the flagellates). .
Ciliates and flagellates behave differently, living in different habitats and occupying different niches, and probably represent two different evolutionary lineages.
The main difference in function is in how they are organized. Flagella are much longer than cilia and are generally present singly or in pairs. A single flagellum can propel the cell with a whip motion.
A pair of flagella can move in sync to drag the organism through the water, similar to the breaststroke of a human swimmer. Cilia tend to cover the surface area of a cell.
Both cilia and flagella bend when microtubules slide past each other. The arrangement of the cilia allows their coordinated movement in response to signals from the cytoplasm.
A small ciliate can have hundreds of individual cilia, all beating in a coordinated fashion. How are all the slides and push-ups coordinated? How does the body “decide” in which direction to move, or how to turn, turn, or feed itself? How do you transmit the information to hundreds of cilia to bend in a certain way?
Questions of that nature are fascinating to cell biologists.
They are very difficult to tackle, because each system is so complex. However, with a genome a hundred times smaller than that of a human, a typical protist is much easier to study than a human cell.
The 4 main functions of microtubules are:
- Form an architectural framework that establishes the general polarity of the cell by influencing the organization of the nucleus, organelles, and other components of the cytoskeleton.
- Form the spindle apparatus and ensure proper segregation of duplicate chromosomes into daughter cells during cell division (i.e., cytokinesis). The spindle apparatus also regulates the assembly and placement of the actin-rich contractile ring that pinches and separates the two daughter cells.
- To form an internal transport network for the traffic of vesicles that contain essential materials for the rest of the cell. This trafficking is mediated by microtubule-associated proteins (MAPs) with motor protein activity such as kinesin and dynein.
- To form a rigid inner core that is used by microtubule-associated motor proteins to generate force and motion in mobile structures such as cilia and flagella. A nucleus of microtubules in the neural growth cone and axon also imparts stability and drives neuronal guidance and navigation.
Microtubules give structures like cilia and flagella to their structure. Cilia are small bumps on a cell. In humans, they are found in the cells that line the windpipe, where they prevent materials such as mucus and dirt from entering the lungs.
They are also found in the fallopian tubes of the female reproductive system, where they help move the egg that is released from the ovary to the uterus.
Flagella are tail-shaped appendages that allow cells to move. They are found in some bacteria, and human sperm also move through flagella. Microtubules also allow whole cells to migrate from one place to another by contracting at one end of the cell and expanding at another.
Microtubules play a key role in the formation of the mitotic spindle, also called the fusiform apparatus. This is a structure that forms during mitosis (cell division) in eukaryotic cells.
The mitotic spindle organizes and separates the chromosomes during cell division so that the chromosomes can divide into two separate daughter cells. Its components include microtubules, MTOC, and microtubule-associated proteins (MAP).
Three subgroups of microtubules assist in the mitosis process: astral, polar, and kinetochore microtubules. Astral microtubules radiate from a cell’s MTOCs to the cell membrane, holding the mitotic spindle in place.
Polar microtubules intertwine between two MTOCs and help separate chromosomes. (All microtubules are polar, these are specifically called polar microtubules).
Kinetochorphic microtubules attach to chromosomes to help separate them; chromosomes are attached to microtubules by a complex of proteins called the kinetochore.
As part of the cytoskeleton, microtubules help move organelles within a cell’s cytoplasm, which is the entire contents of the cell except its nucleus. They also help various areas of the cell communicate with each other.
However, although microtubules help cell components to move, they also provide the cell with shape and structure.
Other components of the cytoskeleton
The other two main components of the eukaryotic cytoskeleton are microfilaments and intermediate filaments. Microfilaments are smaller than microtubules at approximately 7 nm in diameter.
They aid in the division of the cytoplasm during cell division, and they also have a role in cytoplasmic transmission, which is the flow of cytosol (cellular fluid) throughout the cell.
Intermediate filaments are larger than microfilaments, but smaller than microtubules. They help shape the cell and provide structural support.