A microtubule is a hollow, cylindrical polymer of tubulin that forms a major component of the cytoskeleton in eukaryotic cells. Approximately 25 nanometres in outer diameter, microtubules help organize the cell interior, provide tracks for transporting cellular material, and participate in cell division. They also form the supporting framework of eukaryotic cilia and flagella. Many microtubules continually grow and shrink, allowing their networks to be reorganized rather than functioning as a permanent scaffold. (ncbi.nlm.nih.gov)
Structure and polarity
The principal building block is a heterodimer of α-tubulin and β-tubulin, two closely related globular proteins. These dimers associate head-to-tail into longitudinal chains called protofilaments. A typical microtubule contains 13 protofilaments joined laterally around a hollow lumen. Longitudinal and lateral contacts together produce a mechanically coherent tube. (ncbi.nlm.nih.gov)
Because every dimer has the same orientation, a microtubule has structural polarity. Its plus end exposes β-tubulin, whereas its minus end exposes α-tubulin. These names describe molecular orientation and distinct assembly properties, not electrical charge. Both ends can exchange subunits under suitable conditions, although their kinetics differ and minus ends are often anchored or protected in cells. Polarity also determines the direction in which many molecular motors travel. (ncbi.nlm.nih.gov)
Thirteen protofilaments are characteristic rather than an absolute requirement. Experimental microtubules can adopt other arrangements, and specialized cellular structures include doublets and triplets in which a complete microtubule shares its wall with incomplete adjoining tubules. (pubmed.ncbi.nlm.nih.gov)
Assembly and dynamic instability
Both α-tubulin and β-tubulin bind guanosine triphosphate (GTP). The nucleotide bound to α-tubulin is relatively inaccessible and normally retained within the dimer. The nucleotide on β-tubulin is exchangeable in soluble tubulin and undergoes hydrolysis after incorporation into the microtubule lattice. This nucleotide cycle couples assembly to changes in the structure and stability of the polymer. (ncbi.nlm.nih.gov)
Growth, catastrophe, and rescue
Many microtubules display dynamic instability: an individual end switches between growth and rapid shortening even when the surrounding tubulin concentration remains approximately constant. The transition from growth to shortening is called a catastrophe; the reverse transition is a rescue. These stochastic transitions distinguish dynamic instability from a simple, continuously balanced exchange of subunits. (ncbi.nlm.nih.gov)
During growth, newly incorporated tubulin maintains a stabilizing region commonly called the GTP cap, while older regions contain predominantly GDP-bound tubulin. Loss or disruption of this protective end region favors catastrophe. During shortening, protofilaments peel outward as lateral contacts are lost. Dynamic instability allows microtubules to explore cellular space repeatedly and to remodel an array rapidly. (ncbi.nlm.nih.gov)
The GTP-cap model is a useful framework, but the cap is not simply an invariant ring of identical subunits. Cryo-electron microscopy and fluorescence experiments show that nucleotide state, lattice conformation, and regulatory proteins are interdependent. Studies using hydrolysis-deficient tubulin also demonstrate that mutations employed as cap mimics can alter lattice geometry, an important limitation when interpreting structural experiments. (pubmed.ncbi.nlm.nih.gov)
Treadmilling
Microtubules can also undergo treadmilling, in which net addition at one end is balanced by net loss at the other. A filament can then retain approximately the same length while its constituent subunits turn over. Treadmilling and dynamic instability are distinct behaviors: the former concerns directional subunit flux through a filament, whereas the latter concerns switching between growth and shortening at an individual end. (ncbi.nlm.nih.gov)
Nucleation and cellular organization
Forming a new microtubule requires nucleation, the establishment of an initial assembly capable of continued growth. Cellular nucleation commonly involves γ-tubulin and associated proteins. The γ-tubulin ring complex, or γ-TuRC, provides an organized scaffold that promotes assembly of αβ-tubulin into a microtubule. Structural studies combined with single-complex nucleation assays have connected its architecture with its activity. (ncbi.nlm.nih.gov)
A microtubule-organizing center is a cellular site that concentrates nucleation and organization activities. In many animal cells, the principal organizing center is the centrosome, from which microtubules extend into the cytoplasm. This arrangement creates a polarized network, often with minus ends concentrated centrally and plus ends directed outward. It is not a universal architecture: different cell types organize microtubules according to their transport, shape, and division requirements. (ncbi.nlm.nih.gov)
Regulation and mechanical properties
Microtubule-associated proteins regulate growth, shortening, stability, bundling, and interactions with other cellular structures. Examples include MAP2 and tau in neuronal cells. Other factors promote disassembly, either by severing existing microtubules or by accelerating loss of tubulin from their ends. Consequently, microtubules within one cell can have markedly different lifetimes and functions. (ncbi.nlm.nih.gov)
Chemical modification of tubulin provides another level of regulation. In particular, acetylation of α-tubulin at a site facing the lumen can increase resistance to mechanical breakage. Experiments in cells and with purified microtubules show that this modification helps long-lived microtubules withstand mechanical stress. Mechanical resilience should be distinguished from resistance to endwise depolymerization: the two describe different aspects of stability. (pubmed.ncbi.nlm.nih.gov)
Biological functions
Intracellular transport
Microtubules serve as tracks for kinesin and dynein motors. These proteins use energy from adenosine triphosphate (ATP) hydrolysis to produce movement. Many transport kinesins move toward plus ends, whereas cytoplasmic dynein moves toward minus ends. Motor proteins can carry membrane-enclosed cargo or generate movement between microtubules and other structures. (ncbi.nlm.nih.gov)
This transport system is particularly important in neurons, where material must travel over long distances between the cell body and distant processes. In an axon, predominantly aligned microtubules support outward transport by plus-end-directed kinesins and return transport by dynein. The organization of the tracks, together with motor–cargo interactions, gives intracellular movement its directionality. (ncbi.nlm.nih.gov)
Chromosome segregation
During mitosis, microtubules form the mitotic spindle, which separates duplicated chromosomes. Spindle microtubules include those connected to chromosome-associated kinetochores, those overlapping with microtubules from the opposite spindle pole, and—in many animal cells—astral microtubules extending toward the cell cortex. Their dynamics and interactions with motor proteins contribute to chromosome movement, spindle organization, and positioning. (ncbi.nlm.nih.gov)
Cilia and flagella
The core of a eukaryotic cilium or flagellum is an axoneme, an ordered assembly of microtubules and associated proteins. A common motile arrangement consists of nine outer microtubule doublets surrounding two central microtubules, known as the 9 + 2 pattern. Basal bodies anchor and organize these structures and contain nine microtubule triplets. (ncbi.nlm.nih.gov)
Axonemal dyneins generate sliding between neighboring doublets. Structural connections constrain that sliding and convert it into bending, producing ciliary beating or flagellar waves. Bacterial flagella are fundamentally different structures and do not have a microtubule-based axoneme. (ncbi.nlm.nih.gov)
Plant cell organization
In plant cells, cortical microtubules influence the organization of cellulose deposition. Live imaging of cellulose synthase complexes in the plasma membrane has shown their trajectories aligning with underlying cortical microtubules. Disrupting microtubule polymerization changes the distribution and movement patterns of these complexes, demonstrating a functional connection between the cytoskeleton and construction of the cell wall. (pubmed.ncbi.nlm.nih.gov)
Research methods and pharmacological significance
Microtubule research combines purified-protein reconstitution, live-cell fluorescence imaging, and structural methods. Fluorescently labeled tubulin enables direct observation of growth and shortening. Cryo-electron microscopy reveals the tubulin lattice and its nucleotide-dependent conformations, while total internal reflection fluorescence microscopy can follow individual microtubules near a surface. Combining these methods connects molecular structure with dynamic behavior. (ncbi.nlm.nih.gov)
Microtubule-binding compounds are important experimental tools and include drugs used in chemotherapy. Vinca alkaloids interfere with assembly and dynamics, while paclitaxel stabilizes microtubules. Both destabilizing and stabilizing interventions can disrupt cell division because spindle function requires regulated turnover, not merely the presence of assembled microtubules. (ncbi.nlm.nih.gov)
Historical development
A major conceptual advance came in 1984, when Tim Mitchison and Marc Kirschner reported that microtubules assembled in vitro could coexist as growing and shrinking populations that interconverted. Their description of dynamic instability established that a polymer population need not behave as a uniform collection of steadily growing or steadily shrinking filaments. It provided a mechanistic basis for understanding how microtubules can continually explore space and reorganize cellular structures. (pubmed.ncbi.nlm.nih.gov)
References
- Microtubules - The Cell - NCBI Bookshelfncbi.nlm.nih.gov
- The Self-Assembly and Dynamic Structure of Cytoskeletal Filaments - Molecular Biology of the Cell - NCBI Bookshelfncbi.nlm.nih.gov
- Dynamic instability of microtubule growthpubmed.ncbi.nlm.nih.gov
- How Cells Regulate Their Cytoskeletal Filaments - Molecular Biology of the Cell - NCBI Bookshelfncbi.nlm.nih.gov
- Mitosis - Molecular Biology of the Cell - NCBI Bookshelfncbi.nlm.nih.gov
- Microtubules acquire resistance from mechanical breakage through intralumenal acetylationpubmed.ncbi.nlm.nih.gov
- Molecular Motors - Molecular Biology of the Cell - NCBI Bookshelfncbi.nlm.nih.gov
- The Cytoskeleton and Cell Behavior - Molecular Biology of the Cell - NCBI Bookshelfncbi.nlm.nih.gov
- Microtubule Motors and Movements - The Cell - NCBI Bookshelfncbi.nlm.nih.gov
- Visualization of cellulose synthase demonstrates functional association with microtubulespubmed.ncbi.nlm.nih.gov
- Structural transitions in the GTP cap visualized by cryo-electron microscopy of catalytically inactive microtubulespubmed.ncbi.nlm.nih.gov