The cytoskeleton of a cell helps provide shape, strength, and an organised structure to the cell. The cytoskeleton can be compared to a transport network facilitating various types of movement in the cell. It helps with cell reproduction, the movement of organelles, the functions of muscles and intracellular transport between the organelles. It also enables the separation of daughter chromosomes to opposite poles during cell division.
Shapers and the Cytoskeleton
The cytoskeleton is made of three types of structural proteins that help it to maintain the shape of the cell and move when it needs to:
Microtubules are tubulin proteins with a diameter of around 25nm. These are largest of the cytoskeleton structures and look like filaments. They can grow to be around 50 micrometres. These are essential for movement of the cell, contribute to nucleic and cell division, and help with intracellular structure and transport. Microtubules are also present in the structure of the cilia, a sensory organelle that protrudes from the cell and flagella - another sensory organelle involved in movement which helps the cilia move liquid past the cell.
Actin microfilaments are monomers made of globular actin protein in a double helix and have a diameter of about 8nm. This makes them the thinnest of the three types of structure in the cytoskeleton. They tend to be present in all eukaryotic cells except for nematode sperms and have a variety of different roles.
These filaments help organize the cell and define its poles. They also help cells significantly with motility. Just before cell division, they also modify their structure so that the cell can become rounder. It then facilitates the development of the mitotic spindle to prepare for the separation of the daughter cells. During reproduction, the actin filaments enable the sperm head to infiltrate the egg by dismantling the filaments that would obstruct it.
Intermediate filaments are the mid-sized protein filaments in anti-parallel helices or dimers with an average diameter of around 10nm. They have a cytosol inside them. Their composition varies and can include proteins such as vimentin, keratins, and nuclear lamins. Functionally, the intermediate filaments provide strength to the cell, link up proteins, and separate cells during cytokinesis. They also contribute to cytoplasmic streaming.
There are also three types of movement proteins connected to the cytoskeleton. These motor proteins help to move vesicles and organelles.
Kinesin, fuelled by ATP, moves the microtubules filaments; dynesin, a motor protein in cells, changes energy from ATP into movement; and myosin, another type of motor protein, contributes to essential movement in muscles and other activities with the energy from ATP.
Malfunctions in the cytoskeleton are linked to some diseases. However, the ways in which this happens is not completely understood yet. The errors are believed to affect synaptic signals and organelle or vesicle trafficking. For example, in synaptic cytoskeletons, the diseases tend to be inherited. These include mental retardation, spastic paraplegias and spinocerebellar ataxias.
Reviewed by Susha Cheriyedath, MSc