4.10:
Microtubules
Microtubules the thickest cytoskeletal elements in cells are hollow structures that consist of paired globular proteins, alpha and beta tubulins.
These heterodimers form linear rows called protofilaments, which have structural polarity. Meaning that each array is arranged with plus and minus ends. On the plus end where beta tubulins are exposed dimers are added. In contract, on the minus side where alpha tubulins are outward facing dissociation occurs.
However, in other cases microtubules secure stability by directly binding with different proteins like microtubule-associated proteins.
In addition their polarity allows for directional movement throughout the cytoplasm as is the case with dynein and kinesin motor proteins that efficiently transport various cargoes like vesicles.
Microtubules are also key components of cilia and flagella which are specialized extensions that move fluid over the surface of stationary cells and function as propellers in other cells moving them throughout their environments.
In the end, whether they're involved in chromosomal separation during cell division, transporting vesicles in the brain, or sweeping debris out of the lungs microtubules are essential for the growth and development, organizational strength and support, and motility that cells need.
4.10:
Microtubules
There are three types of cytoskeletal structures in eukaryotic cells—microfilaments, intermediate filaments, and microtubules. With a diameter of about 25 nm, microtubules are the thickest of these fibers. Microtubules carry out a variety of functions that include cell structure and support, transport of organelles, cell motility (movement), and the separation of chromosomes during cell division.
Microtubules are hollow tubes whose walls are made up of globular tubulin proteins. Each tubulin molecule is a heterodimer, consisting of a subunit of α-tubulin and a subunit of β-tubulin. The dimers are arranged in linear rows called protofilaments. A microtubule usually consists of 13 protofilaments, arranged side by side, wrapped around the hollow core.
Because of this arrangement, microtubules are polar, meaning that they have different ends. The plus end has β-tubulin exposed, and the minus end has α-tubulin exposed. Microtubules can rapidly assemble—grow in length through polymerization of tubulin molecules—and disassemble. The two ends behave differently in this regard. The plus end is typically the fast-growing end or the end where tubulin is added, and the minus end is the slow-growing end or the end where tubulin dissociates—depending on the situation.
This process of dynamic instability, where microtubules rapidly grow and shrink, is important for functions such as the remodeling of the cytoskeleton during cell division and the extension of axons from growing neurons.
Microtubules also can be stable, often by binding to microtubule-associated proteins, which help the cell to maintain its shape. Other proteins, called motor proteins, can interact with microtubules to transport organelles in a particular direction. For example, many neurotransmitters are packaged into vesicles in the cell body of a neuron and are then transported down the axon along a “track” of microtubules, delivering the vesicles to where they are needed. Finally, microtubules can also protrude outside of the cell—making up the filamentous flagella and cilia that move to push cells (such as sperm) along, or to move fluid across their surfaces, such as in the lungs.
Suggested Reading
Brouhard, Gary J., and Luke M. Rice. “Microtubule Dynamics: An Interplay of Biochemistry and Mechanics.” Nature Reviews. Molecular Cell Biology 19, no. 7 (July 2018): 451–63. [Source]
Hashimoto, Takashi. “Microtubules in Plants.” The Arabidopsis Book / American Society of Plant Biologists 13 (April 27, 2015). [Source]