26.3: Microtubule Formation
Microtubules are dynamic structures that undergo continuous assembly and disassembly. They originate from specialized multi-protein complexes known as microtubule organizing centers or MTOCs. Within the MTOC, the point of origin of the microtubule is known as the minus end, while the end radiating outward is the plus end. Microtubules serve two primary functions — the organization of spindle complexes to separate sister chromatids during mitotic or meiotic cell division and the formation of locomotory appendages, like cilia and flagella.
MTOCs are found in both prokaryotic and eukaryotic organisms. However, some lower eukaryotes, like most fungi, lack organized MTOCs. Instead, they have organized centrosomes consisting of centrioles and the pericentriolar material. In animal cells, the structure and location of MTOCs, vary within different cell types depending on the function of the microtubules.
Microtubule Nucleation
The nucleation of microtubules occurs within the MTOCs, i.e., the centrioles, where different γ-tubulin complex proteins interact with γ-tubulin subunits to form the γ-tubulin-ring complex (γ-TRC). Nucleation is initiated when the α-tubulin subunit of the αβ-tubulin heterodimer attaches to the γ-TRC. Several intrinsic and extrinsic factors influence microtubule nucleation. Intrinsic factors like the α- and β-tubulins isotype incorporated; the concentration of free αβ-tubulin heterodimers, the post-translational modifications, and the microtubule-associated proteins (MAPs) affect the microtubule nucleation dynamics. Extrinsic factors like temperature, pH, and microtubule interfering drugs are also responsible for the rate of microtubule polymerization or depolymerization.
