Cytoskeleton Comparison (Actin, Microtubules, Intermediate Filaments)

Core Concept

The cytoskeleton is built from three filament systems that differ in subunit, size, polarity, dynamics, and function. Actin microfilaments (~7 nm) are polar polymers of G-actin that drive cell shape, migration, cytokinesis, and muscle contraction. Microtubules (~25 nm) are hollow polar tubes of α/β-tubulin that organize the cytoplasm, form the mitotic spindle, and serve as tracks for kinesin/dynein. Intermediate filaments (~10 nm) are non-polar, tissue-specific ropes (keratin, vimentin, lamin) that provide mechanical strength. Actin and microtubules are highly dynamic (treadmilling, dynamic instability) and have motor proteins and drugs; intermediate filaments are stable, non-polar, and lack motors and drugs — a recurring contrast.

Key Components

Mechanism

1. Nucleation. Polymerization is seeded by nucleators — Arp2/3 and formins for actin, the γ-tubulin ring complex for microtubules; intermediate filaments self-assemble without a nucleator.
2. Polymerization and dynamics. Actin and tubulin add subunits at the fast-growing (+) end, hydrolyzing ATP/GTP; this creates treadmilling (actin) and dynamic instability (microtubules, with catastrophe/rescue), allowing rapid reorganization.
3. Force and transport. Myosin walks actin to generate contraction and motility; kinesin and dynein walk microtubules to move vesicles/organelles.
4. Structural support. Intermediate filaments form stable, coiled-coil networks (and the nuclear lamina) that resist mechanical stress without motors or dynamics.

Energy and Directionality

Actin and microtubule dynamics are powered by nucleotide hydrolysis: actin hydrolyzes ATP and tubulin hydrolyzes GTP after subunit addition, and the resulting nucleotide gradient drives subunit dissociation at the (−) end (treadmilling) or catastrophic depolymerization (microtubules). Motor proteins are ATPases — myosin, kinesin, and dynein convert ATP hydrolysis into directional stepping, with kinesin walking to the (+) end and dynein to the (−) end. Intermediate filaments require no nucleotide for assembly and have no motor proteins, which is why they are stable and non-polar.

Experimental Evidence

• Drug pharmacology: taxol stabilizes microtubules and blocks mitosis (anticancer); colchicine depolymerizes microtubules; cytochalasin blocks cytokinesis by disrupting actin — each drug's phenotype maps a filament's function.
• Fluorescence speckle/FRAP microscopy: showed actin treadmilling and microtubule dynamic instability in live cells (growth/shrinkage at + ends, catastrophe and rescue).
• In vitro motility assays: purified myosin/kinesin move along filaments on coverslips, proving they are ATP-driven motors with defined directionality.
• Motor-knockout/phenotype studies: dynein loss disrupts organelle positioning (dynein → − end toward centrosome); kinesin loss blocks outward transport.
• Intermediate-filament genetics: keratin mutations cause skin blistering (epidermolysis bullosa simplex), proving IFs provide mechanical integrity.

Common confusions

• "All three filaments have motors" — Only actin (myosin) and microtubules (kinesin/dynein) have motors; intermediate filaments have none.
• "All three are polar" — Intermediate filaments are non-polar.
• "Microtubules and actin both treadmill" — Microtubules show dynamic instability; actin shows treadmilling (both are dynamic, but by different mechanisms).
• "Taxol and colchicine act the same way" — Opposite: taxol stabilizes, colchicine depolymerizes microtubules (both still block mitosis by freezing dynamics).
• "Dynein and kinesin move the same direction" — Kinesin → + end, dynein → − end.

Quick review

• Actin (7 nm, polar, ATP, myosin, Arp2/3) — shape/migration/cytokinesis; drugs cytochalasin/phalloidin.
• Microtubules (25 nm, polar, GTP, kinesin/dynein, γ-TuRC) — spindle/tracks/cilia; drugs taxol/colchicine.
• IF (10 nm, non-polar, no motors/drugs) — mechanical strength (keratin, lamin).
• All driven by nucleotide hydrolysis except IFs.