Microtubules are hollow, cylindrical protein filaments that are a key component of the cytoskeleton in eukaryotic cells. They are involved in maintaining cell shape, intracellular transport, cell division, and the movement of cilia and flagella.
Structure:
- Made of tubulin dimers:
- Each dimer consists of α-tubulin and β-tubulin subunits.
- Dimers assemble in a head-to-tail fashion to form protofilaments.
- 13 protofilaments align side-by-side to form the hollow tube.
- Microtubules are polarized:
- Plus end (+): Fast-growing
- Minus end (−): Slow-growing or anchored
Key Properties:
- Dynamic instability: Constantly grow and shrink by adding/removing tubulin dimers, especially at the plus end.
- Motor protein tracks: Used by kinesin (moves toward + end) and dynein (moves toward − end) to transport vesicles, organelles, and chromosomes.
Functions:
| Function | Description |
|---|
| Structural support | Maintain cell shape, resist compression |
| Intracellular transport | Serve as tracks for motor proteins moving cargo like vesicles and organelles |
| Cell division | Form the mitotic spindle to separate chromosomes during mitosis/meiosis |
| Cilia and flagella | Core structural element enabling motion |
| Organelle positioning | Help anchor and organize organelles like the Golgi apparatus and ER |
Specialized Microtubule Structures:
- Centrosomes – Microtubule organizing centers (MTOCs) that nucleate microtubules using γ-tubulin.
- Spindle apparatus – Microtubules that pull chromosomes apart during mitosis/meiosis.
- Axonemes – Microtubule-based core of cilia and flagella, arranged in a 9 + 2 pattern (9 doublets + 2 central microtubules).
Microtubules vs. Other Cytoskeletal Elements:
| Feature | Microtubules | Actin Filaments | Intermediate Filaments |
|---|
| Diameter | ~25 nm | ~7 nm | ~10 nm |
| Subunits | α- and β-tubulin | Actin | Various (keratin, vimentin) |
| Function | Transport, division, motility | Shape, movement, division | Strength, structure |
| Polarity | Yes | Yes | No |
| Dynamic Instability | Yes | Yes | No |
Clinical Relevance:
- Cancer treatment: Drugs like taxol and vincristine disrupt microtubule dynamics to inhibit cell division.
- Neurodegenerative diseases: Dysfunction in microtubule transport can lead to conditions like Alzheimer’s disease (involving tau protein).
- Ciliary dyskinesia: Defects in microtubule structure in cilia can cause respiratory and fertility issues.
Summary Table:
| Property | Description |
|---|
| Composition | α- and β-tubulin dimers |
| Shape | Hollow tube |
| Polarity | Plus and minus ends |
| Main Functions | Cell structure, transport, mitosis, cilia/flagella movement |
| Specialized Roles | Spindle fibers, axonemes, MTOCs |
| Dynamic Behavior | Grows/shrinks rapidly (dynamic instability) |
