Microtubule
Cytoskeletal biopolymers providing structure and intracellular transport.
Microtubules are biopolymers of tubulin that form part of the cytoskeleton, providing structure and shape to eukaryotic cells. They are long, hollow cylinders made up of polymerized α- and β-tubulin dimers, typically composed of 13 protofilaments. These dimers polymerize end-to-end into linear protofilaments that associate laterally to form the hollow tube, which can reach lengths of up to 50 micrometres, with an outer diameter of 23 to 27 nm and an inner diameter between 11 and 15 nm. Microtubules have a distinct polarity critical for their function: the end with exposed β-subunits is the rapidly growing (+) end, while the opposite (−) end has exposed α-subunits. The lateral association of protofilaments creates a pseudo-helical structure, most commonly a "13-3" architecture where the 13th tubulin dimer interacts with the next with a vertical offset of three monomers. Alternative architectures, such as 11-3 or 14-3, occur less frequently. Two types of lateral interactions exist between protofilaments: the A-type lattice, where α and β subunits interact across adjacent protofilaments, and the B-type lattice, where like subunits interact; the B-type is the primary arrangement, though a seam exists where tubulin subunits interface differently. Microtubules are nucleated and organized by microtubule-organizing centres, including centrosomes in animal cells, basal bodies of cilia and flagella, and spindle pole bodies in fungi. They serve as platforms for intracellular transport driven by motor proteins like dynein and kinesin, and are involved in moving secretory vesicles, organelles, and macromolecular assemblies. During cell division, they form the mitotic spindles that pull chromosomes apart. Microtubule dynamics are regulated by various binding proteins, including severing proteins like katanin. Historically, tubulin-mediated processes such as cell locomotion were observed by early microscopists like Van Leeuwenhoek in 1677, though the fibrous nature of flagella was confirmed later with electron microscopy and biochemical studies. In vitro assays for motor proteins use fluorescently tagged microtubules and video-enhanced microscopy to track movement, often analyzed with kymographs. Notably, an actin-like protein in *Bacillus thuringiensis* forms a microtubule-like nanotubule involved in plasmid segregation, and other bacterial microtubules have a ring of fiv
- field
- Cell biology, biochemistry
- known_for
- Cytoskeletal component, intracellular transport, mitotic spindle formation
- structure
- Hollow cylinder, ~25 nm outer diameter, 13 protofilaments typical
- components
- α- and β-tubulin dimers
- motor_proteins
- Dynein and kinesin
Lore & Background
Microtubules are long, hollow biopolymers composed of tubulin, a key component of the eukaryotic cytoskeleton that provides cellular structure and shape. They can reach lengths of up to 50 micrometers, with an outer diameter ranging from 23 to 27 nanometers and an inner diameter between 11 and 15 nanometers. These structures form through the polymerization of alpha- and beta-tubulin dimers, which assemble end-to-end into linear protofilaments; these protofilaments then associate laterally to create a hollow tube. The most common microtubule arrangement consists of 13 protofilaments. Microtubules exhibit a distinct polarity: one end, the plus end, exposes beta-tubulin subunits, while the minus end exposes alpha-tubulin subunits, with elongation occurring more rapidly at the plus end. The lateral association of protofilaments creates a pseudo-helical structure, and the most frequent architecture is the "13-3" type, though other configurations like 11-3 or 16-4 exist. Two lattice types, A-type and B-type, describe how subunits interact laterally, with the B-type lattice being the primary arrangement in most microtubules, though a seam often exists where the pattern shifts. Microtubules are nucleated and organized by microtubule-organizing centers, such as the centrosome in animal cells, basal bodies in cilia and flagella, or spindle pole bodies in fungi. They serve as platforms for intracellular transport, with motor proteins like dynein and kinesin moving along their surface, and are essential for cell division, forming the mitotic spindle. In bacteria, a microtubule-like structure called a nanotubule, composed of a ring of five protofilaments, has been observed.
Reader's Guide
Microtubules are fundamental to eukaryotic cell biology, serving as structural elements and dynamic tracks for motor proteins. They are nucleated by microtubule-organizing centers such as centrosomes, basal bodies, or spindle pole bodies. Their distinct polarity—with a plus end (β-subunits exposed) and minus end (α-subunits exposed)—directs the movement of motor proteins like kinesin and dynein, which carry organelles, vesicles, and macromolecular assemblies. Microtubules also form the core of cilia and flagella and are essential for chromosome segregation during mitosis and meiosis via the mitotic spindle. Beyond eukaryotes, some bacteria like Bacillus thuringiensis produce microtubule-like nanotubules involved in plasmid segregation, and Prosthecobacter species contain bacterial microtubules with five protofilaments. The study of microtubule dynamics and associated proteins continues to illuminate mechanisms of cell division, motility, and intracellular organization.
Did You Know?
- Microtubules can be as long as 50 micrometres, with an outer diameter of 23 to 27 nm and an inner diameter of 11 to 15 nm.
- The most common form of a microtubule consists of 13 protofilaments arranged in a hollow tube.
- Bacterial microtubules in Prosthecobacter comprise only five protofilaments, unlike the 13 in eukaryotes.
- Microtubule motor proteins dynein and kinesin can be studied using fluorescent tagging and video-enhanced microscopy.
Frequently Asked Questions
What is a Microtubule?
Microtubules are hollow cylindrical filaments that form a core part of the eukaryotic cytoskeleton. They are assembled from repeating α- and β-tubulin dimers and typically measure around 25 nanometers in outer diameter.
What is Microtubule's structure?
Each microtubule is a long, hollow tube built from 13 parallel protofilaments that run the full length of the cylinder. Those protofilaments are simply head-to-tail chains of α- and β-tubulin dimers, giving the whole assembly its characteristic tubular geometry.
What are Microtubule's main roles in the cell?
Microtubules act as a structural scaffold, form the internal axoneme of cilia and flagella, and serve as tracks for intracellular cargo transport. They are also the principal building blocks of the mitotic spindle that separates chromosomes during division.
How do motor proteins interact with Microtubule?
Kinesin and dynein are the two principal motor proteins that walk along microtubule surfaces. Kinesin typically hauls vesicles and organelles toward the cell periphery, while dynein moves cargo back toward the center, using the microtubule as a molecular rail.
Why is Microtubule critical for cell division?
During mitosis, microtubules polymerize into the bipolar spindle that physically captures and pulls sister chromatids to opposite poles. Without this structure, chromosomes cannot be segregated accurately, so microtubules are indispensable for faithful cell reproduction.
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