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 allow cells to move, enabling processes like migration and division.

• 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 facilitates the movement of organelles and vesicles 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	Structure	Main Functions
Microtubules	α- and β-tubulin heterodimers	25 nm	Hollow tubes	Cell shape, intracellular transport, mitotic spindle, cilia/flagella movement
Microfilaments (Actin filaments)	G-actin monomers	7 nm	Two intertwined strands	Cell shape, muscle contraction, cell motility, cytokinesis
Intermediate Filaments	Various fibrous proteins (e.g., keratins, lamins)	8–12 nm	Rope-like fibers	Mechanical strength, nuclear envelope support, cell-cell junctions

Microtubules

Structure and Properties

Microtubules are cylindrical polymers composed of α- and β-tubulin heterodimers. They are the largest cytoskeletal filaments and play key roles in cell structure and transport.

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

• Structure: Hollow tubes made of 13 protofilaments arranged in a circle

• Subunits: α-tubulin and β-tubulin form heterodimers

• Polarity: Microtubules have a plus (+) end and a minus (−) end, which affects their dynamics and assembly

Functions of Microtubules

• Intracellular transport: Serve as tracks for motor proteins (e.g., kinesin, dynein) to move vesicles and organelles

• Cell division: Form the mitotic spindle for chromosome segregation

• Cell motility: Form the core of cilia and flagella for movement

• Structural support: Maintain cell shape and polarity

Microtubule Assembly and Dynamics

Microtubules exhibit dynamic instability, rapidly growing and shrinking by the addition or loss of tubulin dimers.

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

• Depolymerization: GTP hydrolysis leads to loss of stability and rapid shrinkage (catastrophe)

• Microtubule-organizing centers (MTOCs): Sites such as the centrosome where microtubule nucleation begins

Equation for microtubule growth:

$\text{Rate of polymerization} = k_{on}[\text{Tubulin-GTP}] - k_{off}[\text{Tubulin-GDP}]$

Microtubule-Associated Proteins (MAPs)

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

• Examples include tau proteins (stabilize microtubules in neurons) and motor proteins (kinesin, dynein)

Microfilaments (Actin Filaments)

Structure and Properties

Microfilaments are the thinnest cytoskeletal filaments, composed of actin monomers (G-actin) that polymerize into double-stranded helical filaments (F-actin).

• Diameter: 7 nm

• Structure: Two intertwined strands of F-actin

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

Functions of Microfilaments

• Cell shape: Support the cell cortex and microvilli

• Cell movement: Enable cell crawling, muscle contraction, and cytokinesis

• Intracellular transport: Involved in vesicle movement and organelle positioning

Actin Polymerization and Regulation

• Actin monomers bind ATP and polymerize to form filaments

• Regulatory proteins control filament assembly, disassembly, and organization (e.g., formin, profilin, cofilin)

• Actin-binding proteins can cap, crosslink, sever, bundle, or anchor filaments

Equation for actin polymerization:

$\text{Rate of actin polymerization} = k_{on}[\text{Actin-ATP}] - k_{off}[\text{Actin-ADP}]$

Intermediate Filaments

Structure and Properties

Intermediate filaments are rope-like fibers that provide mechanical strength to cells and tissues. They are more stable than microtubules or microfilaments and are composed of various proteins depending on cell type.

• Diameter: 8–12 nm

• Structure: Fibrous dimers assemble into staggered tetramers and then into rope-like filaments

• Tissue specificity: Different types of intermediate filament proteins are found in different tissues (e.g., keratins in epithelial cells, lamins in the nuclear envelope)

Functions of Intermediate Filaments

• Mechanical strength: Provide tensile strength and resistance to mechanical stress

• Structural support: Support the nuclear envelope and maintain cell shape

• Cell-cell junctions: Anchor cells together at desmosomes and hemidesmosomes

Summary Table: Cytoskeletal Elements

Element	Subunit	Diameter	Structure	Main Functions
Microtubules	α/β-tubulin	25 nm	Hollow tubes	Transport, cell division, motility
Microfilaments	Actin	7 nm	Double helix	Shape, movement, contraction
Intermediate Filaments	Various (e.g., keratin, lamin)	8–12 nm	Rope-like fibers	Strength, support, junctions

Example: In neurons, microtubules provide tracks for axonal transport, actin filaments support growth cones, and intermediate filaments (neurofilaments) maintain axon integrity.

Additional info: The notes have been expanded to include definitions, equations, and examples for clarity and completeness.