Motor Proteins

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  1. What do motor proteins?
    convert chemical energy into mechanical energy and transport cellular cargo along cytoskeletal filaments; move unidirectionally, undergoing a series of steps that make a mechanical cycle, which is coupled to a chemical cycle (ATP)
  2. Kinesins- brief description
    move along microtubules; family of KRPs
  3. Kinesins: Structure
    • -          tetramer constructed from two identical heavy and two identical light chains (wwrap around each other to form a single, common stalk)
    • -          contains globular heads connected to a neck, a rodlike stalk, and a fan-shaped tail
    • motor portions, but not tails, have related amino acid sequences
  4. Kinesins: Function
    • -          globular head: binds a microtubule and acts as an ATP-hydrolyzing, force-generating engine
    • -          tail: binds cargo to be hauled
    • -          move along microtubules toward their plus end;
    • o   In an axon, it transports vesicles and other cargo toward the synaptic terminal
    • move vesicles and organelles in an outward direction toward the cell’s plasma membrane
  5. Kinesins: Method of Movement
    - rate
    - mechanism
    - head
    • -          rate of movement depends on ATP concentration
    • -          moves in hand-over-hand mechanism and a processive (far distances without falling off) manner
    • -          at least one head is attached to microtubules at all times
  6. Kinesins: Method of Movement
    - head movement
    - conformation
    • -          when one head binds to the microtubule, the resulting conformational changes in the adjacent neck region of the motor protein cause the other head to move forward toward the next binding site of the protofilament, finding the binding site randomly
    • freely soluble kinesins adopt a folded, self-inhibited conformation and require interaction with cargo and a microtubule to become activated
  7. Kinesin: Types
    • Types
    • -          kinesin-14: moves in an opposite direction, towards the minus end
    • o   possible due to differences in the neck region
    • -          kinesin-1: moves towards the plus region
    • kinesin-13: incapable of movement; bind to either end of a microtubule and bring about depolymerization rather than moving along its length
  8. Dyneins- brief description
    Move along microtubules; absent in higher plants, which have minus-end directed kinesins
  9. Dyneins- structures
    • -          huge protein composed of two identical heavy chains and a variety of intermediate and light chains
    • -          each heavy chain consists of a large globular head with an elongated projection
    • dynein molecule domain consists of a number of distinct modules organized in the shape of a wheel
  10. Dyneins- function of individual parts
    • -          dynein head acts as a force-generating engine
    • -          each stalk contains the microtubule binding site situated at its tip
    • -          the stem (or tail) binds the intermediate and light chains
  11. Dyneins- overall functions
    • -          acts as a force-generating agent in positioning the spindle and moving chromosomes during mitosis
    • -          as a minus-end directed microtubular motor with a role in positioning the centrosome and Golgi complex and moving organelles, vesicles, and particles through the cytoplasm
    • moves endosomes, lysosomes, RNA, etc.
  12. Dyneins- method of movement
    • -          move processively toward the minus end
    • does not interact directly with membrane-bounded cargo but requires an intervening adaptor—dynactin, which regulates dynein activity and helps bind the motor protein the microtubule, increasing processivity
  13. Ciliary dynein: structure
    • -          consists of three heavy chains and a number of intermediate and lght chains
    • each dynein heavy chain is composed of a long stem, a wheel-shaped head, and a stalk
  14. myosin
    • Move along microfilaments
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Motor Proteins
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