Myosin

Cards (23)

  • Conventional myosin (type 2) are involved in muscle contraction, splitting the cell during cell division, and cell migration and motility
  • Parts of conventional myosin?
    1. Motor (head)
    • Binds to the actin filament
    • binds and hydrolyzes ATP
    • 2. Neck (lever arm)
    • Moves during power stroke
    • 3. Tail
    • Intertwining of the two heavy chains
    • Allows the formation of filaments of myosin
    *S1 fragment includes the head and neck
  • All myosins except for type VI move towards the + (barbed) end of actin
  • All machinery required for motor activity is contained in the S1 fragment
  • Myosin II tails allow the protein to form filaments. In the myosin filament, tails point towards the center and heads point towards the outside.

    Filaments are:
    • Bipolar: reversal of direction at the filament's center (i.e., heads are orientated in different directions)
    • Thick, since they are composed of the myosin
  • Skeletal muscle organization:
    Skeletal muscles are usually anchored to bones
    1. Muscle fibres: the cell unit of skeletal muscles that is composed of hundreds of myofibrils
    2. Myofibrils: contain both actin and myosin and are composed of repeating contractile units called sarcomeres
    3. Sarcomeres: The unit of contraction that has a characteristic banding pattern
  • Sarcomere organization:
    • A-band: Dark staining pattern due to overlap of thick and thin filaments; includes H-zone. Every thick filament is surrounded by 6 thin filaments.
    • H-zone: only contains thick filaments
    • I-band (light staining): only contains thin filaments
    • Z-line: contains proteins important for sarcomere's structure and stability
    • M-line: dark staining in center of sarcomere, contains anchoring proteins.
  • Sarcomere visualization
    A) M-line
    B) A-band
    C) I-band
    D) I-band
    E) Z
    F) Z
  • Which zones decrease in length during contraction?
    I-band and H-zone
  • Which zones do not decrease in length during contraction?
    A-band; A-band is the length of myosin filament which doesn't change in length but pulls on actin filaments to shorten the sarcomere.
  • Parts of myosin II...
    A) S1 fragment
    B) head
    C) neck
    D) light
    E) light
    F) tail
  • Molecular basis for contraction:
    1. Myosin II heads in thick filaments binds to the six surrounding thin filaments
    2. Myosin II is non-processive: each head is only in contact with actin for a fraction of the time
    3. Myosin heads are not synchronized so the myosin heads act at different times, thus allowing smooth continuous movement
    4. Single myosin power stroke moves an actin filament ⁓10nm
    5. Neck acts a lever to move actin a large distance
  • Actin myosin contraction cycle:
    1. ATP binds to myosin head causing myosin to dissociate from actin
    2. ATP hydrolysis → ADP and inorganic phosphate remain bound to myosin
    3. Energized myosin binds to actin
    4. Release of phosphate triggers conformational change: power stroke: actin moves towards the center of the sarcomere
    5. ADP is released, actin is still attached
  • What is a neuromuscular junction?
    Point of contraction between motor neuron and muscle fiber; site of transmission of the nerve impulse. Stimulated by neurotransmitters
  • Motor fibers within a motor unit are stimulated simultaneously by a single motor neuron
  • How does the nerve impulse cause contraction (excitation-contraction coupling)?
    • transverse tubules (T tubules) are membrane folds that propagate an impulse to the interior of the muscle cell
    • Sarcoplasmic reticulum is special smooth ER in muscle cells; stores Ca2+ in lumen (pumped from cytosol)
    • Arrival of action potential at the SR opens Ca2+ channels, releasing Ca2+ into the cytoplasm
  • Excitation-contraction coupling muscle cell diagram:
    A) Neuromuscular junction
    B) Transverse tubules
    C) Myofibril
    D) SR
  • What does Ca2+ control in excitation-contraction coupling?
    Controls the interactions between myosin heads and actin.
    • In the absence of Ca2+: troponin controls position of tropomyosin which blocks myosin-binding sites on actin
    • In the presence of Ca2+: Ca2+ binds to troponin which moves tropomyosin, exposing the myosin-binding site on actin
  • Ca2+ role in excitation-contraction coupling:
    A) Actin
    B) Troponin
    C) tropomyosin
    D) Ca2+
    E) presence of Ca2+
    F) Absence of Ca2+
    G) troponin
    H) troponin
  • Thin filaments contain:
    • actin
    • tropomyosin- rod shaped
    • troponin- globular
  • Unconventional myosin (type V):
    • moves processively along actin filaments and with hand-over-hand movement
    • long necks acts as swinging arms
    • can take very large steps (36nm or 13 subunits of G-actin)
    • Move towards + end
    • can associate with vesicles and organelles
    Act similarly to kinesin's role in microtubules
  • Transport by Unconventional Myosins:
    Some vesicles contain both microtubule motors and actin filament motors
    • Movement over long distances occurs mostlyon microtubules
    • Local movement in the outskirts of the cell: actin filaments
  • Practice Q
    A) treadmilling