Art of the Cell Study Notes

Art of the Cell - John Liebler Study Notes

Major Structural Elements of the Cytoskeleton

• Function:

  • Microfilaments:

  • Essential components of muscle fibrils.

  • Microtubules:

  • Serve as structural elements of cilia and flagella.

Chapter 14: Cellular Movement: Motility and Contractility

• Key Concepts:

  • Cellular motility involves various forms of movement:

  • Movement of a cell or organism through the environment.

  • Movement of the environment past or through a cell.

  • Movement of components within the cell.

  • Contractility:

  • Describes shortening of muscle cells; a specialized form of motility.

Two Eukaryotic Motility Systems

1. Microtubule-based motility:

   • Fast axonal transport in neurons.

   • The sliding of microtubules in cilia and flagella.

2. Microfilament-based motility:

   • Muscle contraction.

• Mechanism:

  • Microtubules (MTs) and microfilaments (MFs) provide a scaffold for motor proteins, producing motion at a molecular level.

Molecular Motors: Common Features

• Molecular motors couple ATP hydrolysis to changes in shape and attachment.

• Undergo cycles of:

  • ATP hydrolysis.

  • ADP release.

  • Acquisition of new ATP.

• Share common structural features.

• Capable of moving along a cytoskeletal filament for significant distances.

Selected Motor Proteins of Eukaryotic Cells

Microtubule (MT)-Associated Motors:

• Dyneins:

  • Cytoplasmic dyneins: Moves cargo toward minus ends of microtubules.

  • Axonemal dyneins: Activate microtubule sliding in flagella and cilia.

• Kinesins:

  • Kinesin-1: Dimer; moves cargo toward plus ends of MTs (axon transport).

  • Kinesin-3, Kinesin-5, Kinesin-6, Kinesin-13, Kinesin-14: Various roles in cargo transport and cell division.

• Note: Kinesin-13 does not possess motor function; it fosters depolymerization of microtubules.

Microfilament (MF)-Associated Motors:

• Myosins:

  • Myosin I: Motion of membranes along MFs; involved in endocytosis.

  • Myosin II: Slips MFs in muscle and contractile events (cytokinesis, cell migration).

  • Myosin V: Vesicle positioning and trafficking.

  • Myosin VI: Moves toward minus ends of MFs.

Microtubule-Based Movement Inside Cells: Kinesins and Dyneins

• Function:

  • Microtubules provide tracks for transport of organelles and vesicles.

  • Traffic direction:

    • Toward minus ends: Considered “inbound.”

    • Toward plus ends: Considered “outbound.”

• Motor Proteins:

  • Kinesins and dyneins move along microtubules, providing force for movement.

Fast Axonal Transport

• Process:

  • Transport of proteins from cell body to nerve ending via fast axonal transport.

  • Involves packaging proteins into vesicles for transport.

  • Visual observation of organelles moving along filaments in axoplasm.

Classic Kinesins

• Kinesin I:

  • Involved in ATP-dependent transport toward the plus ends (anterograde axonal transport).

Kinesin Movement Along MTs

• Mechanism:

  • Kinesin movement resembles “walking”:

  • Two globular head domains take turns as the front foot.

  • Processive movement - can move long distances along a microtubule before detaching by releasing bound ADP and acquiring new ATP.

Dyneins

• Types:

  • Two types of cytoplasmic dyneins identified.

  • Associate with dynactin, a protein complex that links dynein to cargo.

  • Axonemal dyneins include seven types.

Microtubule Motors and the Endomembrane System

• Role:

  • Membrane extensions of the endoplasmic reticulum (ER) move along MTs.

  • Vesicles to and from the Golgi complex are carried by MT motors on microtubule tracks.

Examples of Motor Protein Mutations

• Scenario:

  • Imagine a mutation disrupts ATP-binding site of kinesin, reducing its ATP hydrolysis efficiency.

  • Question: How would this mutation likely affect cellular processes?

  • Possible outcomes:
    A) Increased neurotransmitter release.
    B) Decreased neurotransmitter release and impaired signal transmission.
    C) Disruption of microtubule formation, causing cell division failure.
    D) No significant impact.

• This indicates the critical importance of ATP in kinesin functioning and vesicular transport.

Comparison of Kinesin and Dynein

• Functions:

  • Kinesin moves cargo toward plus end of microtubules.

  • Dynein transports cargo toward the minus end of microtubules.

• Key questions regarding their roles help elucidate their functional differences.

Microtubule-Based Motility: Cilia and Flagella

1. Overview:

   • Microtubules are vital for movements of cilia and flagella.

   • Both share a common structural basis.

2. Cilia:

   • Length: approximately 2–10 μm, numerous on ciliated cells.

3. Flagella:

   • Propagate bending motion to generate force.

Cilia and Flagella Mechanics

• Mechanics of Movement:

  • Movement observed as beating strokes:

    • Recovery stroke and power stroke.

Doublet Sliding

• Mechanism:

  • Microtubule doublets as structural units lead to cilia and flagella bending.

  • Adjacent doublets slide relative to one another, producing bending; overall length remains unchanged.

Role of Dynein in Axonemal Bending

• Force Generation:

  • Dynein arms provide driving force for axonemal bending.

  • Experimental removal of dynein leads to loss of ciliary movement.

Comparison between Cilia and Flagella

• Similarities:

  • Both generate force parallel to the cell surface.

  • Both have dynein arms essential for their function.

  • Both used in mammalian hearing.

  • Convert membrane potential into motion.

Evidence Supporting Doublet Sliding

• Experimental observations provide direct support for doublet sliding hypotheses:

  • Dynein binds to microtubules in vitro.

  • Removal of dynein results in both a loss of movement and loss of ATP hydrolysis.

Intraflagellar Transport (IFT)

• Process:

  • Adds components to growing flagella and cilia.

  • Tubulin subunits shuttled to growing flagellum tip via plus- and minus-end-directed motor proteins.

Microfilament-Based Movement Inside Cells: Myosins

• Functionality of Myosins:

  • ATP-dependent motors of a large superfamily.

  • Interact with and exert force on actin microfilaments.

  • Currently 24 known classes of myosins; most move toward the plus end, except myosin VI.

Myosin Functions

• Roles:

  • Wide range of cellular events, such as:

  • Muscle contraction.

  • Cell movement.

  • Phagocytosis.

Kinesin versus Myosin

• Both groups have globular domains that walk along protein filaments via ATP hydrolysis to change shape.

• Differences:

  • Kinesins typically operate alone or in small numbers; myosins work in large arrays.

Muscle Cells: Microfilament-Based Motility

• Overview:

  • Mechanical work in muscle contraction mediated by intracellular filaments.

  • Types of muscle: skeletal, cardiac, and smooth muscle.

Structure of Skeletal Muscle Cells

• Composition:

  • Consist of parallel muscle fibers connected to bone via tendons.

  • Each fiber: multinucleate, long, and specialized.

Myofibrils and Sarcomeres

• Each muscle fiber contains myofibrils divided into sarcomeres.

• Sarcomeres:

  • Bundles of thin filaments (actin, troponin, tropomyosin) and thick filaments (myosin).

Striated Muscle Appearance

• Appearance:

  • Aligned filaments cause dark (A bands) and light (I bands) striations.

Thick and Thin Filaments

• Thick Filaments:

  • Comprised of hundreds of myosin molecules oriented oppositely.

  • Myosin heads form cross-bridges with nearby thin filaments.

• Thin Filaments:

  • Composed of F-actin intertwined with tropomyosin and troponin.

  • Troponin complex (TnT, TnC, TnI) acts as a calcium-sensitive switch for muscle contraction.

Organization of Muscle Filament Proteins

• Actin in thin filaments oriented at Z lines.

• Myosin II moves toward plus ends; thick filaments approach Z lines during contraction.

• Structural proteins contribute to muscle cell architecture.