28 Cytoskeletons Study Notes

Introduction

  • The lecture covers the topic of cytoskeletons.

  • The instructor starts with an apology for being late previously.

  • There is a focus on memorizing types and functions of cytoskeletons which are essential in cell biology.

Learning Outcomes

  • Students should be able to:

    • List the three types of cytoskeletons.

    • Understand the role of each type.

    • Describe the dynamic reorganization of actin filaments and microtubules.

    • Identify the roles of motor proteins and accessory proteins related to the first two cytoskeletons.

Overview of Cytoskeletons

Types of Cytoskeletons

  1. Actin Filaments (Microfilaments)

    • Painted red in the provided images.

    • Generally 8 nm in thickness, the thinnest among cytoskeletons.

    • Form a helical structure with globular actin as monomers.

    • Known as "cortical actin" when located at the periphery of cells.

    • Important for cell shape, movement, and muscle contraction (in collaboration with myosin).

    • Can generate a contractile ring during cell division, pinching daughter cells apart.

  2. Microtubules

    • Painted green in the provided images.

    • Diameter of 25 nm, making it the thickest among the cytoskeletons.

    • Composed of alpha and beta tubulin subunits forming a hollow cylindrical structure.

    • Function as tracks for organelle movement (cargo) and are important during cell division for forming the mitotic spindle.

    • Located primarily in the cytoplasm, with some presence in structures like cilia and flagella.

  3. Intermediate Filaments

    • Painted blue in the provided images.

    • More robust, providing mechanical strength and structural support to cells, especially under stress.

    • Connect with neighboring cells, creating a protective layer, such as in skin cells.

    • The primary protein structure in the nucleus that forms the nuclear lamina, providing additional mechanical support.

Detailed Structures and Functions

Actin Filaments

  • Actin filaments contribute to the overall shape of the cell and utilize motor proteins like myosin for movement.

  • Cytoskeleton Dynamics:

    • During cell division, actin filaments decrease, focusing on mitotic functions.

    • Utilize non-covalent bonds allowing for quick assembly and disassembly, optimizing response to cellular needs.

  • Treadmilling: Dynamic process where the length remains constant while actin monomers are added at the plus end and removed at the minus end, promoting active cellular movement and structure adaptation.

Microtubules

  • Microtubules display dynamic instability, alternating between phases of growth and shrinkage controlled by GTP cap presence.

  • Serve as tracks for kinesin and dynein motors, which facilitate cargo movement towards cell periphery (kinesin) and towards the centrosome (dynein).

  • Centrosome: Organizing center for microtubules, with structures known as centrioles aiding in organization.

  • Cilia and Flagella Structure:

    • Characterized by a “9 + 2” arrangement of microtubules, essential for cellular motility, primarily through the action of dynein.

Intermediate Filaments

  • Composed of fibrous proteins that provide tensile strength and structural stability. Examples include keratin and desmin.

  • Create a mesh network providing support and are connected through desmosomes for intercellular integrity.

  • Nuclear Lamina: A specific type of intermediate filament providing mechanical support to the nucleus, crucial for maintaining its shape and position.

Accessory Proteins

Function and Types

  • Accessory proteins regulate the organization and dynamics of actin and microtubules:

    • Tropomyosin: Stabilizes actin filaments.

    • Gelsolin: Breaks down actin filaments.

    • Formin: Aids in actin filament nucleation and elongation.

    • ARP (Actin-Related Protein) Complex: Involved in creating branched actin networks at specific angles (70 degrees).

  • Motor Proteins:

    • Myosin (for actin): Facilitates muscle contraction and cellular movements.

    • Kinesin and Dynein (for microtubules): Facilitate transport of cargo within cells, driven by ATP hydrolysis.

Conclusion

  • Understanding the cytoskeleton is crucial for grasping cell movement, shape, and division processes.

  • The dynamics of cytoskeletal components illustrate not only their component structure but also their functional importance in cellular activities.

  • Continuous reading and review of Chapter 16 is recommended for deeper understanding.