Motor protein cargo transport on microtubules − + cargo dynein → (−) cargo kinesin → (+)

The diagram makes three points: the three filament classes differ in diameter (7, 10, 25 nm), in architecture (helix, cable, hollow tube) and in their motor proteins (myosin for actin, none for intermediate filaments, kinesin and dynein for microtubules). The lower panel shows the canonical motor-protein cargo transport — kinesin walks cargoes toward the (+) end (cell periphery), dynein walks them toward the (−) end (cell body).

Microfilaments

Microfilaments are the thinnest of the three cytoskeletal elements, typically 7 nm in diameter. They are made of the protein actin.

Structure

• Composed of globular G-actin monomers that polymerise to form a long, double-helical chain known as F-actin (filamentous actin).
• Each filament is polar, with a "plus" (+) end that grows faster than the "minus" (−) end. This polarity is crucial for directional movement.
• Microfilaments can assemble and disassemble quickly as actin monomers are added or removed, making them a highly dynamic structure.

Roles of Microfilaments

1. Cell shape and mechanical support — a dense network of actin filaments just inside the plasma membrane forms the cell cortex, giving the cell surface shape and stiffness. Changes in the cortex drive amoeboid movement and the formation of microvilli.
2. Cell movement — polymerising actin pushes the leading edge of a migrating cell forward, creating lamellipodia and filopodia. This is essential in phagocytes moving to infection sites and in developing tissues.
3. Cytokinesis — after mitosis, a contractile ring of actin and myosin forms around the equator of the cell and tightens like a purse-string, pinching the cell into two. This physical division of the cytoplasm is driven entirely by the cytoskeleton.
4. Muscle contraction — in muscle cells, actin interacts with myosin motor proteins to slide filaments past one another and shorten the sarcomere. This is the basis of the sliding filament model.
5. Cytoplasmic streaming in plant cells — actin–myosin interactions drive bulk movement of cytoplasm around the cell, distributing organelles and nutrients.
6. Microvilli — the actin-based core of each microvillus gives it rigidity, maintaining the brush border of intestinal epithelial cells.

Intermediate Filaments

Intermediate filaments are so-named because their diameter (10 nm) is intermediate between that of microfilaments (7 nm) and microtubules (25 nm). Unlike the other two, intermediate filaments are not polar and are considerably more stable and less dynamic.

Structure

• Built from a diverse family of proteins; the specific protein depends on the tissue.
• Each monomer is a long, rod-like protein that dimerises and then assembles into tetramers. Tetramers stack end-to-end and laterally to form a cable-like filament.
• The cable-like arrangement gives intermediate filaments exceptional tensile strength — much like the strands of a rope.

Examples of Intermediate Filament Proteins

Roles of Intermediate Filaments

1. Mechanical strength — the primary role. Intermediate filaments resist tensile forces (stretching), preventing cells from being torn apart under mechanical stress.
2. Anchoring organelles — in epithelial cells, keratin filaments anchor the nucleus and other organelles in position. Lamins anchor chromatin within the nucleus.
3. Structural continuity across tissues — intermediate filaments in adjacent cells connect through desmosomes, forming a continuous mechanical network that distributes stress throughout a tissue (essential in skin and heart muscle).
4. Maintaining nuclear shape — the nuclear lamina is a mesh of intermediate filament proteins (lamins) lining the inner nuclear membrane.

Microtubules

Microtubules are the thickest of the three cytoskeletal components, with an outer diameter of about 25 nm. They are hollow tubes built from the protein tubulin.

Structure

• Formed from dimers of α-tubulin and β-tubulin. Dimers polymerise end-to-end to form protofilaments.
• Thirteen protofilaments arrange in parallel to form a hollow cylindrical tube 25 nm across.
• Microtubules are polar, with a slow-growing (−) end and a fast-growing (+) end. The (−) ends are typically anchored in microtubule organising centres (MTOCs), especially the centrosome in animal cells.
• Microtubules are highly dynamic, exhibiting "dynamic instability" — rapid growth alternating with sudden catastrophic shrinking. This allows the cytoskeleton to rebuild itself quickly.
• Some microtubules, such as those of cilia and flagella, are stabilised in the famous "9 + 2" arrangement: nine pairs of microtubules arranged in a ring around a central pair.

Roles of Microtubules

1. Cell shape and support — microtubules, often radiating out from the centrosome, provide a rigid internal scaffold in many cells.
2. Intracellular transport (motor-driven) — microtubules are the "tracks" along which motor proteins kinesin and dynein move vesicles, organelles, and chromosomes. Kinesin generally moves cargo towards the + end, while dynein moves it towards the − end. This system is essential for delivering synaptic vesicles along axons over distances of up to a metre.
3. Chromosome separation in mitosis and meiosis — the mitotic spindle is made of microtubules. Kinetochore microtubules attach to chromosomes and pull sister chromatids apart; polar microtubules push the spindle poles apart.
4. Cilia and flagella — the axoneme (internal structure) of these motile appendages consists of the "9 + 2" arrangement of microtubules. Dynein arms between adjacent microtubule doublets slide them past one another, bending the cilium or flagellum.
5. Basal bodies and centrioles — microtubules in the 9 × 3 triplet arrangement form these MTOC structures.
6. Positioning organelles — the endoplasmic reticulum and Golgi apparatus are positioned and extended along microtubules, giving each organelle its characteristic location.

Summary Table of the Three Components

Diagram: Cytoskeleton Overview

flowchart TD
    A[Cytoskeleton]
    A --> B[Microfilaments 7 nm]
    A --> C[Intermediate filaments 10 nm]
    A --> D[Microtubules 25 nm]
    B --> B1[Made of actin]
    B --> B2[Cortex and shape]
    B --> B3[Cytokinesis ring]
    B --> B4[Muscle contraction with myosin]
    C --> C1[Keratin, vimentin, desmin, lamins]
    C --> C2[Tensile strength]
    C --> C3[Anchoring organelles]
    D --> D1[Made of tubulin]
    D --> D2[Spindle fibres]
    D --> D3[Cilia and flagella 9+2]
    D --> D4[Vesicle transport via kinesin and dynein]

Cytoskeleton dynamics — the polymer view