Lecture 6. Cytoskeleton

Cell Biology Overview

Lecture Title: Cytoskeletal Structure & Function



Lecturer: Dr. Ria DiakogiannakiInstitution: School of Life Sciences, University of BedfordshireEmail Contact: Eleftheria.diakogiannaki@beds.ac.uk

Learning Outcomes

By the end of this lecture, students should be able to:

  • Understand the structural features and functions of different cytoskeletal fibers.

  • Recognize the varying structural organizations that cytoskeletal fibers can adopt, and how these correspond to their specific roles in the cell.

  • Identify various motor proteins, detailing their mechanisms of movement along cytoskeletal fibers, and their importance in cellular movement and transport.

  • Provide examples of cellular functions influenced by cytoskeletal fibers, highlighting their roles in cell shape, motility, and intracellular transport.

The Cell Cytoskeleton

The cytoskeleton acts as both the skeleton and the muscle of cells, providing structural stability while also facilitating movement.

  • Eukaryotic vs. Prokaryotic Cytoskeleton: While originally believed to be unique to eukaryotic cells, recent studies have shown that prokaryotes also possess a simpler version of the cytoskeleton, suggesting a more universal role in cellular organization.

  • Composition: The cytoskeleton is composed of polymers formed from protein subunits, demonstrating a dynamic three-dimensional structure that is continuously remodeled in response to cellular activity.

Types of Cytoskeletal Fibers

Major Types:

  1. Microfilaments (Actin Filaments):

    • Diameter: ~7 nm

    • Structure: Fine, thread-like protein fibers composed of actin subunits, forming a double helix.

    • Functions: Key roles in muscle contraction, cellular motility (e.g., amoeboid movement), and cytokinesis during cell division.

  2. Microtubules:

    • Diameter: ~25 nm

    • Structure: Long, hollow cylinders made from tubulin dimers (composed of alpha and beta subunits), typically arranged in 13 parallel protofilaments.

    • Functions: Serve as tracks for intracellular transport, organizational scaffolds for cell division (mitotic spindle formation), and structural components of cilia and flagella.

  3. Intermediate Filaments:

    • Diameter: ~10 nm

    • Structure: Long, rope-like fibers composed of various protein subunits that vary depending on cell type (e.g., keratin in epithelial cells, neurofilaments in neurons).

    • Functions: Provide mechanical strength to cells, maintaining their shape and resisting shear stress; important for cell-cell junctions (e.g., desmosomes).

Roles of Cytoskeletal Filaments

  • Microfilaments: Play a crucial role in maintaining the cell's shape and rigidity, as well as in the locomotion of cells through structures like microvilli that increase surface area for absorption.

  • Microtubules: Essential for organizing the cell's internal structure, facilitating the transport of organelles and vesicles. They dynamically grow and shrink, allowing for rapid changes in the cell’s architecture.

  • Intermediate Filaments: They anchor organelles and provide structural integrity to cells, linking adjacent cells to resist mechanical stress and distribute tension across cell layers.

Dynamic Formation of Cytoskeleton

  • Dynamic Instability: Microfilaments and microtubules exhibit dynamic instability, continuously alternating between phases of growth and shrinkage, driven by the addition or removal of subunits.

  • Treadmilling: A phenomenon where there is a constant addition of subunits at one end (plus end) and loss at the opposite end (minus end), which helps maintain filament length while allowing for rapid cellular responses to environmental changes.

  • Regulation: The rates of polymerization depend on the concentration of free subunits and nucleotide hydrolysis (with GTP involved for microtubules), showing the cytoskeleton’s responsiveness to cellular signals.

Organization of the Cytoskeleton

Accessory proteins are essential in regulating cytoskeletal dynamics:

  • Nucleation: Promoting the formation of new filaments.

  • Regulation of Subunit Addition: Facilitating or inhibiting the addition of subunits.

  • Stability Control: Maintaining filament stability under mechanical stress.

  • Filament Compiling: Arranging filaments into complexes for specific functions, such as forming the contractile ring during cytokinesis.

Cytoskeletal Motor Proteins

  1. Myosin:

    • Interacts primarily with microfilaments; crucial for muscle contraction, enabling movement through ATP hydrolysis.

    • Motor Domain: Conserved across myosin types, while the tail regions vary to allow cargo specificity (e.g., organelles, vesicles).

  2. Kinesin:

    • A motor protein associated with microtubules, moving cargo toward the plus end, often involved in organelle transport and vesicle trafficking.

    • Shares structural similarities with myosin, such as a motor domain, but operates in a different direction relative to the microtubules.

  3. Dynein:

    • Moves towards the minus end of microtubules, playing a critical role in vesicle transport and facilitating the motility of cilia and flagella, essential for movement in many cell types.

Functions of the Cytoskeleton

  • Microtubules: Vital for vesicular transport processes within the cell and structural support during cell division. They form spindle fibers that separate chromosomes during mitosis.

  • Myosin and Actin Interaction: Central to the contraction of muscle fibers and amoeboid movement; essential for cell migration and shape changes.

  • Axoneme Structure in Cilia/Flagella: Dynein's action causes the bending motion, enabling propulsion; crucial in motile cells such as sperm and epithelial cells with cilia.

Summary

Students should be equipped to understand the diverse types of cytoskeletal fibers, their integral structures, and functions, as well as their roles in essential cellular processes, including movement, shape maintenance, and transport mechanisms. Key takeaways include the importance of dynamic interactions within the cytoskeleton and the specific contributions of motor proteins in cellular motility.

Directed Study Suggestions

  • Create a comprehensive comparative table detailing the structures, functions, and dynamics of microfilaments, microtubules, and intermediate filaments.

  • Illustrate various cytoskeletal organizations and their cellular locations, highlighting functional significance.

  • Diagram the movement of motor proteins along cytoskeletal fibers, emphasizing the energy dynamics involved.

  • Discuss a specific cellular process regulated by the cytoskeleton and analyze its components in detail.