14-The Cytoskeleton and Cell Junctions: Structure, Function, and Types

The Cytoskeleton

Overview

The cytoskeleton is a dynamic network of protein fibers that extends throughout the cytoplasm of eukaryotic cells. It is essential for maintaining cell shape, organizing cellular components, and facilitating various cellular activities.

• Mechanical support: Provides structural integrity and anchorage for organelles.

• Motility: Enables movement of the cell and its internal components.

• Cell division: Plays a critical role in mitosis and cytokinesis.

• Biochemical regulation: May regulate biochemical activities within the cell.

Main Components of the Cytoskeleton

The cytoskeleton is composed of three major types of protein filaments, each with distinct structure and function:

• Microtubules (Thickest)

• Microfilaments (Actin filaments; Thinnest)

• Intermediate filaments (Intermediate thickness)

Microtubules

Structure and Composition

Microtubules are cylindrical, hollow tubes with a diameter of 20–25 nm. They are composed of subunits of the protein tubulin, which exist as α-tubulin and β-tubulin dimers.

• Form a rigid internal skeleton and act as tracks for organelle and vesicle movement.

• Grow out from the centrosome in animal cells, which contains a pair of centrioles.

• Involved in the formation of spindle fibers during mitosis.

• Arranged in geometric patterns in cilia and flagella for locomotion.

Equation:

$\text{Microtubule} = \text{Polymer of } (\alpha\text{-tubulin} + \beta\text{-tubulin})$

Roles

• Maintain cell shape

• Facilitate intracellular transport (motor proteins such as kinesin and dynein)

• Form the mitotic spindle during cell division

• Enable movement in cilia and flagella

Actin/Microfilaments

Structure and Composition

Microfilaments are solid rod-like fibers, 3–7 nm in diameter, composed mainly of the globular protein actin arranged in a twisted double chain. Actin is the most abundant cellular protein.

• Form a 3D network just inside the plasma membrane, supporting cell shape.

• Interact with myosin for muscle contraction.

• Enable cellular movements such as gliding, contraction, and cytokinesis.

Equation:

$\text{Microfilament} = \text{Polymer of actin monomers}$

Roles

• Help the cell or parts of the cell to move

• Stabilize cell shape

• Facilitate muscle contraction

Intermediate Filaments

Structure and Composition

Intermediate filaments are fibrous proteins about 10 nm in diameter, supercoiled to form thick cables. They may consist of several types of proteins, such as keratin, depending on the cell type.

• Reinforce cell shape and anchor organelles

• Form a cage around the nucleus

• More permanent fixtures compared to microtubules and microfilaments

• Abundant in skin cells

Roles

• Stabilize cell structure

• Resist tension

Comparison Table: Structure and Function of Cytoskeletal Elements

Property	Microtubules	Microfilaments (Actin)	Intermediate Filaments
Structure	Hollow tubes of tubulin	Solid rods of actin	Fibrous proteins coiled into cables
Diameter	20–25 nm	7 nm	8–12 nm
Main function	Cell motility, chromosome movement, organelle movement	Cell shape, muscle contraction, cytoplasmic streaming	Cell shape, anchorage of nucleus and organelles
Protein subunit	α- and β-tubulin	Actin	Keratin, vimentin, etc.

Major Functions of the Cytoskeleton

Locomotion

The cytoskeleton enables cell movement through specialized structures and motor proteins.

• Microtubules have a plus end (growing) and minus end (inactive).

• Motor proteins (kinesin and dynein) use ATP to move along microtubules.

• Kinesin moves toward the plus end; dynein moves toward the minus end.

Cilia and Flagella

Cilia and flagella are locomotor appendages composed of microtubules arranged in a "9+2" pattern.

• Propel unicellular eukaryotes through water.

• Sweep mucus in trachea; move eggs in oviducts.

• Cilia: short, numerous; Flagella: long, few.

• Both have a cylindrical array of 9 doublet microtubules and 2 central singlets.

Cell Division

The cytoskeleton is essential for mitosis and cytokinesis.

• Centrosome: Organizing center for microtubules; contains a pair of centrioles.

• Centrioles: Nine bundles of microtubules arranged in a ring.

• Spindle fibers: Microtubules that separate chromosomes during mitosis.

• Centrosome signals cytokinesis and cell cycle progression.

Cell Junctions

Overview

Cell junctions are specialized structures that connect cells to each other and to the extracellular matrix, forming tissues and regulating communication and barrier functions.

• Bind cells together to form tissues

• Prevent leakage in cell barriers

• Allow communication between cells

Types of Cell Junctions

• Tight junctions

• Adherens junctions

• Gap junctions

• Desmosomes

• Plasmodesmata (plants)

Tight Junctions

Tight junctions seal adjacent cells together, preventing the passage of molecules and ions through the space between cells.

• Control what substances are allowed through tissue.

• Block movement of membrane proteins between apical and basolateral surfaces.

• Example: Skin cells are made watertight by tight junctions.

Adherens Junctions

Adherens junctions provide strong mechanical attachments between adjacent cells.

• Hold cardiac muscle and epithelial cells together.

• Made up of cadherins (transmembrane proteins) and catenins (connected to microfilaments).

Gap Junctions

Gap junctions are intercellular channels (1.5–2 nm diameter) that permit the free passage of ions and small molecules between cells.

• Constructed from 4–6 copies of connexin proteins.

• Essential for communication in tissues such as heart muscle and embryos.

Desmosomes

Desmosomes are anchoring junctions that connect intermediate filaments of adjacent cells, providing mechanical strength.

• Important in tissues subject to stress, such as skin and heart muscle.

Plasmodesmata (Plants)

Plasmodesmata are channels that pass through plant cell walls, connecting the cytoplasm of adjacent cells.

• Allow movement of ions, small molecules, and some macromolecules between plant cells.

Summary Table: Cell Junctions

Junction Type	Main Function	Key Proteins	Example/Application
Tight Junctions	Seal cells, prevent leakage	Claudins, occludins	Skin, intestinal epithelium
Adherens Junctions	Mechanical attachment	Cadherins, catenins	Cardiac muscle, epithelial tissue
Gap Junctions	Cell communication	Connexins	Heart muscle, embryos
Desmosomes	Mechanical strength	Desmogleins, desmocollins	Skin, heart muscle
Plasmodesmata	Intercellular transport (plants)	Various proteins	Plant tissues

Key Terms

• Cytoskeleton: Network of protein fibers providing structure and movement in cells.

• Microtubule: Tubulin-based hollow cylinder involved in transport and division.

• Microfilament: Actin-based solid rod involved in movement and shape.

• Intermediate filament: Fibrous protein cable providing mechanical strength.

• Centrosome: Organelle organizing microtubules.

• Centrioles: Microtubule-based structures within centrosomes.

• Spindle fibers: Microtubules separating chromosomes during mitosis.

• Cell junctions: Structures connecting cells and regulating tissue function.

Additional info: The cytoskeleton is also involved in intracellular transport, spatial organization of organelles, and signal transduction pathways.