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 themselves 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 shape of cells, such as neurons and cardiac muscle cells.

• Movement: It is involved in muscle contraction, migration of white blood cells, and other forms of cellular motility.

• Flagellar movement: The cytoskeleton powers the movement of flagella and cilia, enabling swimming or movement of extracellular fluids.

• Organelle and vesicle movement: Cytoskeletal elements facilitate the transport 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	Main Functions
Microtubules	α- and β-tubulin heterodimers	25 nm	Cell shape, intracellular transport, mitotic spindle, cilia/flagella movement
Microfilaments (Actin filaments)	G-actin monomers	7 nm	Cell shape, muscle contraction, cell motility, cytokinesis
Intermediate Filaments	Various fibrous proteins (e.g., keratins, lamins)	8–12 nm	Structural support, maintenance of cell shape, nuclear envelope integrity

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 division, transport, and motility.

• 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 form the mitotic spindle during cell division and serve as tracks for motor proteins like kinesin and dynein.

Functions of Microtubules

• Intracellular transport: Movement of vesicles and organelles along microtubule tracks.

• Cell division: Formation of the mitotic spindle for chromosome segregation.

• Cell motility: Cilia and flagella are composed of microtubules, enabling movement.

• Structural support: Maintenance of cell shape and organization.

Microtubule Assembly and Dynamics

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

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

• Depolymerization: GTP hydrolysis leads to instability and rapid shortening (catastrophe).

• Microtubule-Organizing Centers (MTOCs): Structures like the centrosome nucleate and anchor microtubules.

Equation:

$\text{Tubulin-GTP} \xrightarrow{polymerization} \text{Microtubule} \xrightarrow{GTP\ hydrolysis} \text{Tubulin-GDP}$

Example: In neurons, microtubules extend from the cell body into axons and dendrites, providing tracks for the transport of neurotransmitter vesicles.

Microtubule-Associated Proteins (MAPs)

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

• Stabilizing MAPs: Promote microtubule assembly and prevent disassembly.

• Motor proteins: Kinesin and dynein move cargo along microtubules.

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 (+) and minus (−) ends, with monomers preferentially added to the plus end

Example: Microfilaments form the contractile ring during cytokinesis and the core of microvilli in intestinal epithelial cells.

Functions of Microfilaments

• Cell shape: Support for the plasma membrane and cell cortex

• Cell movement: Enable cell crawling, lamellipodia, and filopodia formation

• Muscle contraction: Interaction with myosin in muscle cells

• Cytokinesis: Formation of the contractile ring during cell division

Microfilament Assembly and Regulation

Actin polymerization is regulated by various actin-binding proteins.

• Monomer-sequestering proteins: Prevent actin polymerization (e.g., thymosin β4)

• Polymerizing proteins: Promote filament growth (e.g., formin)

• Capping, crosslinking, severing, and bundling proteins: Organize actin filaments into networks or bundles

Equation:

$\text{G-actin-ATP} \xrightarrow{polymerization} \text{F-actin} \xrightarrow{ATP\ hydrolysis} \text{G-actin-ADP}$

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. 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)

Example: Lamins form a network underlying the nuclear envelope, while keratins provide strength to skin cells.

Functions of Intermediate Filaments

• Structural support: Maintain cell shape and resist mechanical stress

• Cell-cell junctions: Anchor cells together in tissues

• Nuclear envelope integrity: Lamins support the nuclear membrane

Additional info: Intermediate filaments do not exhibit the same dynamic instability as microtubules or microfilaments, making them ideal for long-term structural support.