Scaffolding That Rebuilds Itself
The cytoskeleton is not a static frame. It is three filament systems in continuous assembly and disassembly, and that dynamism is the point, a static scaffold could not divide a cell or crawl it across a surface.
| Filament | Diameter | Subunit | Main roles |
|---|---|---|---|
| Microfilaments | ~7 nm | Actin | Shape, movement, cytokinesis |
| Intermediate filaments | ~10 nm | Various | Mechanical strength |
| Microtubules | ~25 nm | Tubulin | Transport, cilia, mitotic spindle |
Microfilaments are actin polymers, the thinnest of the three. They form the cortex just beneath the membrane, drive cell crawling, pinch the cell in two during cytokinesis, and, with myosin, produce muscle contraction.
Intermediate filaments are the only one of the three that is not dynamic. They are rope-like, mechanically strong, and built from tissue-specific proteins: keratins in epithelia, lamins lining the nuclear envelope. Their job is purely to resist mechanical stress, and the genetic diseases affecting them produce fragile tissues.
Microtubules are hollow tubes of tubulin, the widest and stiffest. They radiate from the centrosome and act as the cell's roads: kinesin walks toward the plus end, generally outward, and dynein toward the minus end, generally inward. They also build cilia and flagella in the 9+2 arrangement, nine doublets around a central pair, and form the spindle that separates chromosomes.
Microtubules are so central to mitosis that they are a major drug target. Colchicine blocks their polymerisation and taxol blocks their disassembly; both arrest dividing cells, which is why taxol is a chemotherapy agent.
Motor proteins take discrete steps: kinesin advances about 8 nm per ATP hydrolysed, so moving a vesicle 1 µm costs roughly 125 steps and 125 ATP.
Common pitfall: thinking a drug that stabilises microtubules would help a dividing cell. It does the opposite. Mitosis needs microtubules to shorten as well as grow, so freezing them in place is just as lethal as preventing their assembly, which is exactly how taxol works.