Cytoskeleton

Cytoskeleton Overview

Lesson Objectives

  • Describe the components and function of the cytoskeleton.

  • Describe the molecular structure of actin microfilaments.

  • Summarize how microfilaments change the shape of the cell.

  • Predict the effects of changes to actin or actin-affiliated protein structure.

Role of the Cytoskeleton

  • The cytoskeleton plays a crucial role in:

    1. Adopting a variety of shapes in cells

    2. Carrying out coordinated directed movements

    3. Dividing during cell division

    4. Organizing intracellular space

  • Definition: Cytoskeleton is defined as a dynamic and complex network of protein filaments that extend throughout the cell.

Types of Filaments

  1. Microfilaments

    • Building Block: Actin

    • Purpose: Membrane motility and structure; facilitates movement and transport.

    • Location: Cytoplasm of all cells, particularly near the membrane.

  2. Microtubules

    • Building Block: Tubulin

    • Purpose: Cargo transport; facilitates cell cycle division.

    • Location: Cytoplasm of all cells.

  3. Intermediate Filaments

    • Building Block: Varies (depends on cell type)

    • Purpose: Provides structural support to cells.

    • Location: Intra- or extracellularly.

Microfilaments and Phagocytosis

  • Microfilaments are essential in phagocytosis due to various factors:

    • The actin gene is present in every cell of the human body.

    • Actin is more flexible than microtubules or intermediate filaments.

    • Actin is encoded by nearly every eukaryote.

    • Myosin only binds to actin, making it crucial for motor functions in cellular processes.

Molecular Structure of Microfilaments

  • Monomer: Globular actin (G-actin)

  • Filament: Filamentous actin (F-actin)

  • They are involved in various cellular processes related to cell shape and intracellular transport.

  • Growth Dynamics: G-actin builds faster at the (+) end than at the (-) end.

Assembly of Microfilaments

  • Three Steps in Assembly:

    1. Nucleation Phase:

    • This is the rate-limiting step and is facilitated by seed proteins such as formin or Arp2/3.

    1. Elongation Phase:

    • Rapid expansion of filament assembly occurs once oligomers of 2-3 subunits form.

    1. Steady-State Phase:

    • Assembly continues until the concentration of G-actin/F-actin reaches equilibrium.

Critical Concentration

  • Actin exists in a dynamic equilibrium between globular and filamentous forms.

  • This equilibrium is referred to as the critical concentration.

  • Factors affecting critical concentration:

    • Increased cation concentration or ADP can decrease the critical concentration of actin filaments.

Treadmilling Phenomenon

  • Definition: Treadmilling occurs when there is asymmetry in the rate of elongation at the two ends (poles) of a filament, coupled with a symmetric rate of dissociation at both poles.

  • Treadmilling is a dynamic process that balances addition and removal of actin monomers, maintaining filament length while allowing for flexibility in cellular processes.

Actin-Binding Proteins

  • Different proteins play specific roles in regulating the dynamics of actin:

    • Polymerization Regulators:

    • Profilin (positive regulator)

    • Thymosin b4 (negative regulator)

    • Length Regulators:

    • Cofilin

    • Nucleation and Branching:

    • Arp2/3 complex

    • Cross-linking Proteins:

    • Filamin

    • Motor Proteins:

    • Myosin

    • Stability (Capping):

    • Capping proteins regulate the growth/shrinkage of filaments.

Enhancing Actin Building

  • Profilin and cofilin enhance polymerization rates of actin filaments.

  • Thymosin and capping proteins can suppress or halt actin building.

Impact of Toxins on Actin Dynamics

  • Toxin Example: Paraquat is a widely used herbicide that binds to G-actin.

    • It facilitates polymerization into filaments irreversibly.

    • These changing filaments alter the shape and ultimately the function of cells.

    • The low concentration of G-actin within the cell causes it to respond by increasing actin expression (Wang et al., 2023).

Actin in Cell Motility

  • Actin filaments (either unbranched via formin or branched via Arp2/3) contribute to the mechanical forces that push or pull the plasma membrane and intracellular vesicles.

  • Actin filaments are also involved in endocytosis.

  • Illustrative Figures:

    • Effects of actin on cell structures are detailed in Figures 17-17, 17-4, and 17-19 in the MCB textbook.

Movement of Amoebae

  • Conceptual diagram required:

  • Discuss how pseudopodia are formed and how actin filaments are involved in this process of cell movement.

Cross-linking of Actin Filaments

  • Actin filaments often bundle together due to the action of cross-linking proteins, which increases their mechanical strength.

  • Adapter proteins frequently connect microfilaments to the cell membrane, contributing to overall cell stability.

Myosin Structure and Function

  • Composition: Each myosin molecule consists of 1-2 actin-binding heads, light chain "necks," and heavy chain tails.

  • ATPase Activity: The head of the myosin is where the ATPase activity occurs, essential for actin-myosin interactions during muscle contraction.

  • Power Stroke Mechanism:

    1. During a power stroke, ADP is replaced with ATP in the myosin head, causing myosin to release from actin.

    2. Hydrolysis of the ATP molecule leads to a conformational change in the neck region of myosin.

    3. Myosin rebinds to a new actin residue, returning to its original position.

  • Myosin heads operate in an alternating "hand over hand" pattern to create movement along actin filaments.

Implications for Rigor Mortis

  • Rigor Mortis Mechanism:

  • ATP depletion during rigor mortis leads to muscle cells becoming incapable of detaching myosin heads from actin filaments, thus freezing muscles in their current contracted shape.

  • This phenomenon illustrates the crucial role of ATP in muscle function and contraction.

Myosin as a Carrier Protein

  • Myosin can serve a dual function as both a motor and a carrier protein.

  • If myosin's tail is attached to cargo, it can effectively transport this cargo along actin trackways using the hand-over-hand mechanism.

Conclusion: Lesson Objectives Review

  • Review and recap:

    • Components and functions of the cytoskeleton

    • Molecular structure of actin microfilaments

    • Summary of the role of microfilaments in altering cell shape

    • Predictions regarding structural changes in actin or actin-affiliated proteins and their potential effects on cellular function and dynamics.