The Cytoskeleton: Structure and Function in Eukaryotic Cells

The Cytoskeleton

Key Concepts

The cytoskeleton is a dynamic network of protein filaments that provides structural support, facilitates intracellular transport, and enables cellular movement in eukaryotic cells.

• Cytoskeletal proteins support the structure of cells, maintaining their shape and integrity.

• They help move materials through the cell, such as organelles and vesicles.

• Cytoskeletal proteins also allow cells to move, either by changing shape or by locomotion.

• There are three main types of cytoskeletal structures in eukaryotes: microtubules, microfilaments (actin filaments), and intermediate filaments.

Functions of the Cytoskeleton

How the Cytoskeleton Determines Cell Shape and Movement

The cytoskeleton is essential for maintaining cell shape and enabling various forms of movement.

• Shape of cells: The cytoskeleton defines the morphology of cells, such as neurons and muscle cells.

• Movement: It is involved in muscle contraction (skeletal and cardiac muscle cells), cell crawling (e.g., white blood cells), and flagellar movement (e.g., sperm cells).

• Intracellular transport: The cytoskeleton enables movement of organelles, vesicles, and chromosomes within the cell.

The Three Types of Cytoskeletal Elements

Overview and Comparison

Eukaryotic cells contain three major types of cytoskeletal filaments, each with distinct structure and function.

Type	Polymer Subunit	Diameter	Main Functions
Microtubules	α- and β-tubulin heterodimers	25 nm	Cell shape, intracellular transport, chromosome movement, cilia/flagella motility
Microfilaments (Actin filaments)	G-actin monomers	7 nm	Cell shape, muscle contraction, cell locomotion, cytokinesis
Intermediate Filaments	Various fibrous proteins (e.g., keratins, lamins)	8–12 nm	Structural support, maintenance of cell shape, anchoring organelles

Example: In epithelial cells, microfilaments form the core of microvilli, microtubules organize the cell's interior, and intermediate filaments provide mechanical strength.

Microtubules

Structure and Properties

Microtubules are hollow tubes composed of α- and β-tubulin heterodimers arranged in protofilaments. They are the largest cytoskeletal filaments and play key roles in cell structure and transport.

• Diameter: 25 nm (outer), 15 nm (inner)

• Structure: Typically 13 protofilaments form the wall of a microtubule.

• Subunits: Each protofilament is made of alternating α- and β-tubulin monomers.

• Polarity: Microtubules have a plus (+) end and a minus (–) end, which affects their dynamics and interactions.

Example: Microtubules are essential for the formation of the mitotic spindle during cell division, and for the transport of vesicles along axons in nerve cells.

Functions of Microtubules

• Intracellular transport: Motor proteins (e.g., kinesin and dynein) move cargo along microtubules.

• Cell division: Microtubules form the spindle apparatus that separates chromosomes.

• Cell motility: Cilia and flagella are composed of microtubules arranged in a characteristic "9+2" pattern.

Microtubule Assembly and Dynamics

Microtubules undergo dynamic instability, alternating between phases of growth and shrinkage.

• Polymerization: Tubulin dimers bind GTP and add to the plus end of the microtubule.

• Depolymerization: GTP hydrolysis leads to loss of the GTP cap, causing rapid shrinkage (catastrophe).

• Rescue: Regaining a GTP cap allows microtubules to resume growth.

Equation:

$\text{Microtubule growth rate} \propto [\text{Tubulin-GTP}]$

Example: In nerve cells, microtubules provide tracks for the transport of neurotransmitter vesicles from the cell body to the axon terminal.

Microtubule-Organizing Centers (MTOCs)

Microtubule assembly is initiated at specialized sites called microtubule-organizing centers (MTOCs), such as the centrosome in animal cells.

• Centrosome: Major MTOC in animal cells, contains γ-tubulin ring complexes that nucleate microtubule growth.

• Polarity: Minus ends are anchored at the MTOC, plus ends extend outward.

Microtubule-Associated Proteins (MAPs)

MAPs regulate microtubule stability, organization, and interactions with other cellular components.

• Stabilizing MAPs: Promote microtubule assembly and prevent disassembly.

• Destabilizing MAPs: Promote microtubule disassembly.

Microfilaments (Actin Filaments)

Structure and Properties

Microfilaments are thin, flexible filaments composed of actin monomers. They are involved in cell shape, movement, and muscle contraction.

• Diameter: 7 nm

• Structure: Two intertwined chains of F-actin (filamentous actin)

• Polarity: Plus (+) end and minus (–) end; monomers add preferentially to the plus end.

Example: Microfilaments form the contractile ring during cytokinesis and are abundant in the cell cortex, supporting cell shape.

Functions of Microfilaments

• Cell motility: Actin polymerization drives cell crawling (e.g., in fibroblasts and neutrophils).

• Muscle contraction: Actin interacts with myosin to produce contraction in muscle cells.

• Structural support: Microfilaments support microvilli and other cell surface projections.

Regulation of Actin Polymerization

Actin dynamics are controlled by various actin-binding proteins.

• Monomer-sequestering proteins: (e.g., thymosin β4) prevent actin polymerization by binding actin monomers.

• Polymerizing proteins: (e.g., formin) promote actin filament assembly.

• Capping, crosslinking, severing, and bundling proteins: Regulate filament length, organization, and stability.

Equation:

$\text{Actin filament growth rate} \propto [\text{Actin-ATP}]$

Example: Treatment of fibroblasts with phalloidin stabilizes actin filaments, revealing their organization in the cell cortex.

Intermediate Filaments

Structure and Properties

Intermediate filaments are rope-like fibers that provide mechanical strength to cells and tissues.

• Diameter: 8–12 nm

• Structure: Composed of fibrous protein dimers (e.g., keratins, lamins) assembled into staggered tetramers and then into filaments.

• Tissue specificity: Different cell types express distinct intermediate filament proteins.

Example: Keratin filaments in epithelial cells provide resistance to mechanical stress; nuclear lamins support the nuclear envelope.

Functions of Intermediate Filaments

• Structural support: Maintain cell shape and integrity, especially under mechanical stress.

• Cell-cell junctions: Anchor cells together at desmosomes.

• Organelle positioning: Lamins form a network underlying the nuclear envelope, supporting nuclear structure.

Summary Table: Cytoskeletal Elements

Element	Diameter	Subunit	Main Functions
Microtubules	25 nm	α/β-tubulin	Transport, cell division, motility
Microfilaments	7 nm	Actin	Shape, movement, contraction
Intermediate Filaments	8–12 nm	Various (e.g., keratin, lamin)	Strength, support, anchoring

Additional info: The cytoskeleton is highly dynamic, with continuous assembly and disassembly regulated by nucleotide hydrolysis (GTP for microtubules, ATP for actin). Intermediate filaments are more stable and less dynamic than microtubules or microfilaments.