26.1: Microtubules
Microtubules are the thickest cytoskeletal filaments with a diameter of 25 nm. In prokaryotic organisms, microtubules are commonly found in locomotory appendages like cilia and flagella. In eukaryotic cells, microtubules form specialized extensions for moving fluid over the surface, like those found in cells lining the intestine.
Microtubules have two structurally similar globular protein subunits: α and β tubulins. In the cytosol, the α and β tubulins form a heterodimer. These αβ-heterodimers enter the centrioles, a type of microtubule organizing center (MTOCs), and assemble into a filament with the help of the γ-tubulin ring complex. Each microtubule has 13 protofilaments, each with alternating α and β tubulins.
Microtubules are polar structures with plus (+) and minus (−) ends. The end originating from the MTOCs is the minus end, while the outward-facing end is the plus end. Microtubules are dynamic structures that undergo repeated polymerization and depolymerization. However, these filaments are not flexible; they cannot flex or bend when force is applied, and they will break apart if a deforming force is sufficient.
In microtubules, both α-tubulin and β-tubulin are associated with GTP. The GTP-bound-β-tubulins at the plus end are necessary for microtubule polymerization. GTP-α-tubulin cannot be hydrolyzed to GDP, however, GTP-β-tubulin may be. The hydrolysis of GTP to GDP breaks the lateral interaction between the protofilaments, allowing tubulin monomers to dissociate.
Microtubules have multiple roles within the eukaryotic cell, and they act as tracks for transporting cargo and vesicles within the cell due to their polar nature. Microtubule-associated motor proteins, such as kinesin and cytoplasmic dyneins, transport cargoes throughout the cell.
