The Cytoskeleton: Structure and Function in Eukaryotic Cells

The Cytoskeleton

Introduction

The cytoskeleton is a dynamic network of protein filaments that provides structural support, organization, and movement within eukaryotic cells. It consists of three main types of filaments: microtubules, microfilaments (actin filaments), and intermediate filaments. Each type has distinct structural properties and cellular functions.

Microtubules

Structure and Composition

• Microtubules are hollow tubes composed of repeating units of alpha and beta tubulin proteins.

• They have a diameter of approximately 25 nm.

• Microtubule organizing centers (MTOCs) such as centrioles and basal bodies nucleate and anchor microtubules.

Functions

• Provide structural support and maintain cell shape.

• Serve as tracks for intracellular transport of organelles and vesicles.

• Form the mitotic spindle during cell division.

• Basal bodies anchor flagella and cilia, which are involved in cell movement.

Associated Proteins and Movement

• Microtubule-associated proteins (MAPs) regulate microtubule stability and organization.

• Motor proteins such as kinesin and dynein "walk" along microtubules, transporting vesicles and organelles.

Example

• Centrioles are MTOCs that organize microtubules during cell division.

Microfilaments (Actin Filaments)

Structure and Composition

• Microfilaments are thin, flexible fibers composed of actin subunits.

• They have a diameter of approximately 7 nm.

Functions

• Located directly under the plasma membrane, providing structural support and maintaining cell shape.

• Enable cell movement through "crawling motion" (amoeboid movement).

• Play a role in muscle contraction (actin and myosin interaction).

Associated Proteins

• Myosin interacts with actin filaments to produce movement, especially in muscle cells.

Example

• Actin filaments form the contractile ring during cytokinesis.

Intermediate Filaments

Structure and Composition

• Intermediate filaments are rope-like fibers with a diameter between microtubules and microfilaments.

• Composed of various proteins, including keratin in epithelial cells.

Functions

• Provide mechanical strength to cells and tissues.

• Maintain cell integrity under stress.

Example

• Keratin forms hair, nails, and the outer layer of skin by filling dead cells.

Comparison of Cytoskeletal Elements

Type	Diameter	Main Protein	Functions	Examples
Microtubules	~25 nm	Alpha and beta tubulin	Cell shape, organelle movement, mitosis, cilia/flagella	Centrioles, spindle fibers
Microfilaments	~7 nm	Actin	Cell shape, movement, muscle contraction	Contractile ring, muscle fibers
Intermediate Filaments	~8-12 nm	Keratin, vimentin, etc.	Mechanical strength, cell integrity	Keratin in hair/nails/skin

Key Terms and Definitions

• Microtubule Organizing Center (MTOC): A cellular structure from which microtubules originate, such as centrioles and basal bodies.

• Actin: A globular protein that polymerizes to form microfilaments.

• Keratin: A family of fibrous proteins forming intermediate filaments in epithelial cells.

• MAPs (Microtubule-Associated Proteins): Proteins that interact with microtubules to regulate their stability and function.

Formulas and Equations

• Polymerization of actin filaments: $n \; \text{G-actin} \rightarrow \text{F-actin}$

• Microtubule assembly: $\alpha\text{-tubulin} + \beta\text{-tubulin} \rightarrow \text{Microtubule dimer}$

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