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, with an outer diameter of 23 to 27 nm and an inner diameter between 11 and 15 nm; microtubules can reach lengths up to 50 micrometers. The structure has a distinct polarity: one end, the plus end, exposes only β-subunits, while the minus end exposes only α-subunits, and elongation occurs more rapidly at the plus end. The lateral association of protofilaments creates a pseudo-helical arrangement, with the most common architecture being a "13-3" pattern, though variants with different protofilament numbers (e.g., 11, 12, 14, 15, or 16) have been observed. Within the microtubule wall, lateral interactions between subunits primarily follow a B-type lattice, though a seam with an A-type lattice is present.
Microtubules play important roles in cellular processes including maintaining cell structure, forming the internal structure of cilia and flagella, providing platforms for intracellular transport, and serving as main constituents of mitotic spindles during cell division. They are nucleated and organized by microtubule-organizing centers, such as centrosomes in animal cells, basal bodies in cilia and flagella, or spindle pole bodies in fungi. Motor proteins like dynein and kinesin move along the microtubule surface to transport secretory vesicles, organelles, and macromolecular assemblies. Other microtubule-binding proteins include severing proteins like katanin and regulators of dynamics. Historically, microtubule-mediated processes were observed by early microscopists like Van Leeuwenhoek in 1677, but the fibrous nature of flagella was only clarified two centuries later with improved light microscopes and confirmed in the 20th century by electron microscopy. In bacteria, an actin-like protein in *Bacillus thuringiensis* forms a microtubule-like nanotubule involved in plasmid segregation, and other bacterial microtubules have a ring of five protofilaments.
- 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 dimer of the globular proteins alpha- and beta-tubulin. These dimers polymerize end-to-end to form linear protofilaments, which then associate laterally to create a hollow tube. The most common microtubule arrangement consists of thirteen protofilaments bundled in parallel, though microtubules with fewer or more protofilaments exist in various species. The resulting cylinder has an outer diameter of 23 to 27 nanometers and an inner diameter (the lumen) of 11 to 15 nanometers, and can reach lengths of up to 50 micrometers. Microtubules exhibit a distinct polarity: one end, the plus end, exposes beta-tubulin subunits, while the opposite minus end exposes alpha-tubulin subunits. This polarity is critical for function, as elongation occurs more rapidly at the plus end. The protofilaments associate laterally in a pseudo-helical structure, typically with a vertical offset of three tubulin monomers per turn, forming a B-type lattice as the primary arrangement, though a seam of A-type lattice is often present. Microtubules form part of the eukaryotic cytoskeleton, providing structure and shape. They are nucleated and organized by microtubule-organizing centers, such as centrosomes in animal cells, basal bodies in cilia and flagella, or spindle pole bodies in fungi. They serve as tracks for intracellular transport, with motor proteins like dynein and kinesin moving along their surface to carry vesicles, organelles, and macromolecular assemblies. Microtubules also constitute the internal structure of cilia and flagella, and are the main components of mitotic spindles, which separate chromosomes during cell division. Additionally, an actin-like protein in the bacterium *Bacillus thuringiensis* forms a microtubule-like nanotubule involved in plasmid segregation, and other bacterial microtubules have a ring of five protofilaments.
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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