Eukaryotic Test 3 Lecture 5

  1. Sliding Filament Model of Muscle Contraction (5)
    All skeletal muscles operate by shortening

    Shortening of individual sarcomeres result from the sliding of filaments one over another

    Sliding of thin filaments toward the center result in increased overlap of filaments, and decreased width of I and H bands
  2. Thin Filaments (7)
    Actin: Actin filaments of each sarcomere are aligned w/barbed end linked to the Z line

    Tropomyosin: Elongated molecule fitting securely into grooves within the thin filament. Each tropomyosin molecule is associated w/ 7 actin subunits

    Troponin: Globular protein complex composed of 3 subunits, contacting both actin and tropomyosin
  3. Thick Filaments (8)
    Each thick filament is composed of several hundred myosin II molecules together w/small amount of other proteins. Polarity of thick filaments is reversed at center of the sarcomere

    • Titin:
    • 38,000 AAs, 3.5 million Daltons.
    • Originate at M line and extend along myosin filament, continuing past A band and terminating at Z line
  4. Molecular Basis of Contraction (9)
    Each myosin head extends outward & binds tightly to thin filament, forming cross-bridges

    While bound to actin filament, myosin head undergoes conformational change which moves thin actin filament ~10nm toward center of sarcomere

    Muscle myosin is non-processive motor. It remains in contact w/ thin filament for only a small fraction of the overall cycle
  5. Energetics of Filament Sliding (11)
    1. Molecule of ATP binds to myosin head, inducing disassociation of cross-bridge from actin filament

    2. ATP is hydrolyzed. ADP & P remain bound to active site while energy is being absorbed by protein

    3. Energized myosin attaches the actin molecule

    4. bound P is released triggering large conformational change, shiftin actin filament toward center of sarcomere

    5. bound ADP is released
  6. Excitation-Contraction Coupling (12)
    Process in which arrival of nerve impulse at the muscle PM is converted to the shortening of the sarcomeres

    Sarcoplasmic Reticulum: Membranous sleeve around the myofibril

    About 80% of integral protein of the SR (sarcoplasmic Reticulum) membrane consists of Ca2+ ATPase molecules
  7. Excitation-Contraction Coupling and Calcium Levels (13)
    Relaxed State: Ca2+ level w/in cytoplasm of muscle cells are very low, below required threshold for contaction

    At arrival of action potential by way of T tuules, Ca2+ channels in SR are opened & CA2+ diffuses into myofibrils, triggering muscle contraction

    After stimulation CA2+ channels close. Ca2+ is pumped out of cytosol into the lumen of SR

    When Ca2+ increase, the interaction between calcium and troponin leads to a movement of tropomyosin from position b to a, exposing the binding site on thin filament to the myosin head
  8. Nonmuscle Motility (15)
    1. Critical components are present in less ordered, labile, transient arrangements

    2. They are typically restricted to a thin cortex just beneath the PM
  9. Actin-Binding Proteins (16)
    1. Nucleating Proteins: Arp2/3 complex, after activation functions as a template to which actin monomers can be added

    2. Monomer-Sequestering Proteins (Thymosins): Bind to ATP monomers & prevent them from polymerizing

    3. End-blocking (Capping) Proteins: Binds to one or the other end of the filaments, forming a cap

    4. Monomer-Polymerizing Proteins (Profilin): Binds to the same site as thymosin but promotes growth of actin filaments

    5. Actin Filament-Depolymerizing Proteins (Cofilin Family): Bind to acting-ADP subunits & the pointed end of actin filaments

    6. Cross-linking Proteins: Cross-link 2 or more separated actin filaments to loose networks.

    7. Filament-Severing Proteins: Bind to side of existing filament and break it in 2

    8. Membrane-Binding Proteins: Link membranes to actin filaments
  10. Cell Locomotion (21)
    • 1. Protruding a part of the cell in moving direction
    • 2. Form temporary sites of anchorage at lower surface of protrusion
    • 3. Move forward
    • 4. Breaking rear contacts w/ the substratum

    Crawling cells have a broadened frontal end and a narrow tail

    Leading edge is extended out from the cell as a broad, flattened, veil-like protrustion called a lamellipodium
  11. Formation of lamellipodium (23)
    • 1. Stimulus is received at one of the cell
    • 2. Activation of the Arp2/3 complexes by member of WASP family
    • 3. Activated Arp2/3 complexes serve as nucleating sites for actin filaments
    • 4. Arp2/3 complexes bind to the sides of new actin filaments
    • 5. New actin filaments are nucleated which form as branches and the growth of barbed ends of older filaments are blocked by addition of capping protein
    • 6. Addition of acitn subunits to barbed ends of more recently formed filaments pushes membranes of lamellipodium outward. Old capped filaments undergo disassembly from pointed ends
Card Set
Eukaryotic Test 3 Lecture 5
Powerpoint 27 (11.03.10)