2/6 Lecture

Overview of Actin Dynamics and Myosin Motor Proteins

Actin Treadmilling and Polymerization

  • Actin treadmilling can move cellular components without the need for myosin motors or additional forces.

    • Actin polymerization generates force and motion within cells.

  • Example of Visualization:

    • Listeria monocytogenes, a bacterium that acts as an intracellular pathogen, demonstrates this concept.

    • Intracellular pathogens need to invade host cells for infection, including viruses and certain fungi.

    • Listeria hijacks the host's actin cytoskeleton to propel itself through the cytoplasm.

Listeria and Actin Hijacking

  • Act A Protein:

    • A bacterial protein that interacts with host protein Arp2/3 to nucleate actin filament formation behind the bacterium.

    • This nucleation causes rapid actin assembly, pushing the bacterium forward within the host cell.

  • Visualization of Bacteria:

    • Bacteria are shown in red, with actin filaments in green, illustrating the formation of actin tails.

  • Intercellular Movement:

    • Actin polymerization is strong enough to push bacteria through adjacent cell membranes, facilitating infection spread.

Actin-Associated Proteins

  • Overview of various actin-associated proteins affecting polymerization:

    • Capping proteins stabilize and prevent further growth of actin filaments.

    • Cofilin breaks actin filaments apart, regulating filament dynamics.

    • Myosin as a key motor protein is introduced next.

Myosin: The Motor Protein

  • Definition: Myosin is classified as a motor protein interacting with actin filaments.

  • Structure of Myosin:

    • Contains a motor region (head) that binds to actin and hydrolyzes ATP to generate movement.

    • Additional domains include:

    • Neck or Hinge Region: Connecting motor head to the coiled-coil domain.

    • Coiled-Coil Domain: Varies between myosin types, involved in filament assembly or cargo binding.

  • Muscle Myosin: A dimer (two identical heavy chains) that facilitates muscle contraction and cargo transport.

    • Myosin is also regulated by phosphorylation of myosin light chains.

Mechanism of Muscle Contraction

  • Process involves ATP binding and hydrolysis:

    • Binding States of Myosin Motor:

    1. Empty Myosin Motor: Can attach to actin without ATP.

    2. ATP Binding: Changes myosin shape, causing detachment from actin.

    3. ATP Hydrolysis: Converts ATP to ADP and phosphate, re-cocking the lever arm.

    4. Phosphate Release: Strengthens myosin-actin binding.

    5. Power Stroke: Myosin returns to a resting state, pulling actin and generating muscle contraction.

  • ATP Hydrolysis Cycle: The cycle continues energetically, allowing constant contraction and relaxation.

Biological Importance of Myosin Motors

  • Myosin motors in muscle movement exemplify the relationship between molecular movement and macroscopic muscle function.

  • Coordination of motor heads in myosin dimers promotes effective movement along actin filaments.

  • Regulation of myosin activity via phosphorylation impacts muscle function significantly.

Sarcomere Structure and Function

  • Sarcomere: The fundamental contractile unit in muscle fibers consisting of:

    • Bipolar Myosin Filaments: Myosin heads project in opposite directions.

    • Actin Filaments: Antiparallel organization facilitated by actin-associated proteins like α-actinin.

  • Stabilization of Actin Filaments: Capping Proteins:

    • CapZ (stabilizes plus end of actin filaments)

    • Tropomodulin (stabilizes minus end of actin filaments)

  • Titin Protein: Connects myosin filaments to the Z band, maintaining structural integrity during contraction and relaxation.

Non-muscle Myosins

  • Muscle Myosin (Myosin II) is studied in depth due to the abundance in muscular systems.

  • Non-muscle Myosin (e.g., Myosin V), characterized as cargo-carrying myosin, is crucial for intracellular transport along actin filaments.

Microtubules Overview

  • Microtubules: The largest cytoskeletal elements, composed of dimers of alpha and beta tubulin.

    • Structure: Hollow tubes with a diameter of approximately 25 nm, formed by 13 protofilaments.

    • Dynamic Properties: Microtubules show polymerization and depolymerization primarily at the plus end.

Microtubule Dynamics

  • GTP Cap: Necessary for microtubule stability; prevents rapid shrinkage.

    • Catastrophe occurs when the GTP cap is lost, leading to rapid depolymerization.

    • The process can be rescued by re-establishing the GTP cap.

  • Dynamic Instability: Important for cellular function and microtubule reorganization.

Microtubule Organizing Centers (MTOCs)

  • Centrosomes: The main MTOCs in animal cells, containing centrioles and associated proteins.

    • Centrioles: Consist of triplet microtubules and play a vital role in organizing microtubules.

Gamma-tubulin and Microtubule Formation

  • Gamma-tubulin Rings: Essential for nucleating microtubules from the minus end, requiring accessory proteins to form a ring structure.

    • Function in microtubule growth analogous to the role of actin-associated nucleating proteins in actin organization.

Key Concepts and Terminology to Memorize

  • Actin Polymerization and Myosin Mechanism Stages: Understand various stages of myosin with respect to ATP involvement.

  • Importance of Phosphorylation for Myosin Activation: Myosin light chain kinase (MLCK) phosphorylates light chains to activate myosin.

  • Binding interactions and structures within sarcomeres: The roles of capping proteins, titin, and organization of actin.

Concluding Remarks

  • Each component and process discussed plays an essential role in cellular structure, movement, and function. This underscores the complexity and interdependence within cellular systems.