motor proteins

• motor proteins responsible for movement of organelles and nucleic acids along polarized filaments (actin and microtubules)

  • no motor proteins exist on intermediate filaments
  • also produce force within netowrk of filaments which causes cell shape changes and locomotion
• 3 families of motor proteins: myosin, dynein, and kinesin
• myosins move toward barbed end of microfilaments
• dyneins move toward minus end of microtubules
• kinesins move toward plus end of microtubules (subset move toward minus end)
• movement is dependent on energy hydrolysis and change

  • hydrolysis step creates a conformational change in the protein that swings the protein back and forth
  • some only have a single one (back and forth)
• motor proteins are enzymes that convert energy stored in ATP into motion that produces force along polymers (e.g. microtubules and microfilaments)

  • if motor is anchored, polymer moves
  • if polymer is anchored, motor (and attached cargo) moves
  • if both are anchored, molecules stretch but nothing moves (energy lost as heat)
• actin motors: myosins

  • produces force on actin filaments (myosin "head")
  • myosins have one or two heads and tails which vary in length (unique)
  • tails anchor myosin to actin filaments or membranes
• myosin heads: two peptides - heavy (850 aa) and light chain (200 aa)

  • heavy chain contains ATP and actin binding site (catalytic domain)
  • light chain (1-6) wraps around alpha helix formed by heavy chain
• myosin actin ATPase cycle:

  • chemical energy is used to produce motion
  • myosin is wither free of or attached to actin
• free myosin:

  • myosin binds ATP in <1m sec which induces conformation change
  • myosin catalyzes hydrolysis of ATP (reversible)
  • M-ATP --> <-- M-ADP + Pi
  • hydrolysis of ATP provides energy for second conformational change (myosin is ready for producing movement)
  • inorganic phosphate is lost slowly and myosin reverts to intial form
• affinity is dependent on what it binds to
• myosin can bind to ATP and can phosphorylate it into ADP plus phosphate

  • eventually releases ADP phosphate
  • then its ready to bind to actin