Microtubules are dynamic structures within cells, serving as components of the cytoskeleton. They are present in all eukaryotic cells and contribute to various cellular processes. These hollow, rod-like structures play a role in maintaining cell shape and facilitating movements inside the cell.
The Primary Component: Tubulin
Microtubules are primarily composed of a globular protein called tubulin. This tubulin protein exists as a dimer, meaning it is made up of two distinct but related protein subunits: alpha-tubulin (α-tubulin) and beta-tubulin (β-tubulin). The alpha-tubulin and beta-tubulin monomers non-covalently bind together to form a stable unit known as an alpha-beta tubulin dimer, which is the fundamental building block of microtubules. Both alpha and beta tubulins have similar structures, each consisting of a core of beta-sheets surrounded by alpha-helices. Each tubulin monomer within the dimer can bind a guanine nucleotide; the GTP bound to alpha-tubulin is stable and non-exchangeable, while the GTP bound to beta-tubulin can be hydrolyzed and is exchangeable.
From Dimers to Dynamic Structures
Alpha-beta tubulin dimers polymerize, or link together end-to-end, to form linear strands called protofilaments. These protofilaments then associate laterally to form the hollow, cylindrical structure of a microtubule. In most eukaryotic cells, a microtubule is typically formed from 13 protofilaments arranged side-by-side around a hollow core. While 13 protofilaments are most common, some cells can form microtubules with different numbers.
Microtubules exhibit structural polarity, meaning they have a distinct “plus” end and a “minus” end. The beta-tubulin subunit is exposed at the faster-growing plus end, while the alpha-tubulin subunit is exposed at the slower-growing minus end. This polarity is essential for their function, particularly for the movement of motor proteins. Microtubules also display “dynamic instability,” a behavior where they constantly switch between phases of rapid growth (polymerization) and rapid shrinkage (depolymerization) by adding or losing tubulin dimers. This dynamic nature is fueled by the hydrolysis of GTP bound to beta-tubulin shortly after incorporation into the growing microtubule.
Essential Functions in the Cell
Microtubules contribute to maintaining cell shape and providing structural support, forming a significant part of the cell’s internal scaffolding, known as the cytoskeleton. They extend throughout the cell, helping to organize organelles and other cellular components. This framework helps establish the overall polarity and internal organization of the cell.
These structures also serve as “tracks” for intracellular transport, acting like internal roadways for moving various cellular components. Motor proteins, such as kinesin and dynein, bind to cargo and “walk” along microtubules, transporting vesicles, organelles, and macromolecules throughout the cytoplasm. Kinesins typically move towards the plus end of microtubules, while dyneins generally move towards the minus end.
Microtubules are also important for cell division, forming the spindle fibers that are responsible for separating chromosomes. During mitosis, these spindle fibers attach to chromosomes and then shorten, pulling the duplicated chromosomes to opposite ends of the dividing cell, ensuring that each new daughter cell receives a complete set of genetic material. Finally, microtubules are key to the structure and movement of cilia and flagella, which are hair-like appendages found on the surface of some cells. These structures contain a characteristic “9+2 array” of microtubules, where nine doublets surround two central microtubules, enabling cell movement or the movement of substances across cell surfaces.