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Motor Proteins - Introduction Part 2. Biochemistry 4000 Dr. Ute Kothe. Myosin II. Muscle Myosin ATPase 2 heavy chains (230 kDa): N-terminal globular head + C-terminal long a -helical tail 2 essential light chains (ELC) 2 regulatory light chains (RLC)

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motor proteins introduction part 2

Motor Proteins- Introduction Part 2

Biochemistry 4000

Dr. Ute Kothe

myosin ii
Myosin II
  • Muscle Myosin
  • ATPase
  • 2 heavy chains (230 kDa): N-terminal globular head + C-terminal long a-helical tail
  • 2 essential light chains (ELC)
  • 2 regulatory light chains (RLC)
  • the a-helical tails of 2 heavy chains form a coiled-coil

Voet Fig. 35-62

actin
Actin
  • ATPase
  • Monomer called G-actin: active site in deep cleft, release of g-phosphates triggers conformational change
  • polymer called F-actin: double chain of subunits, head-to-tail orientation
  • (-) end: nucleotide binding cleft
  • (+) end: opposite end

Voet Fig. 35-67 & 35-68

microfilament treadmilling
Microfilament Treadmilling
  • Upon polymerization, F-actin hydrolyzes ATP and releases Pi
  • Actin-ADP has lower affinity for other actin subunits
  • Newly polymerized (+) end containing Actin-ATP more stable than (–) end containing Actin-ADP
  • under steady state, subunits added to (+) end move toward (-) end where they dissociate

Voet Fig. 35-80

structure of striated muscle
Structure of Striated Muscle
  • Thick filaments (purple): myosin II coiled-coil tails packed end to end
  • Thin filament (gray): F-actin + other proteins (Tropomyosin, Troponin)
  • Protein-built Z-disk and M-disk which organize and anchor the thick and thin filament

Voet Fig. 35-57

sliding filament model
Sliding Filament Model
  • Lenght of thin and thick filaments remains constant
  • Thick and thin filaments slide past each other
  • Sliding is driven by many myosin heads (thick filament) walking along F-actin (thin filament)
  • Overal results in contraction of muscle and generation of force

Voet Fig. 35-70

myosin cycle
Myosin Cycle
  • Key features:
  • ATP reduces Myosin’s affinity for actin
  • Myosin-ADP strongly binds to actin
  • Actin binding to Myosin induces phosphate release

Voet Fig. 35-71

myosin cycle8
Myosin Cycle

6. ADP release

Actin is ADP

release factor

  • ATP binding
  • Actin dissociation

2. ATP hydrolysis

Cocking of myosin

head

5. Power stroke

4. Pi release

Strong acting binding

3. Weak actin binding

Voet Fig. 35-73

myosin cycle9
Myosin cycle
  • ATP binds to Myosin and induces opening of actin binding cleft; Myosin dissociates from actin.
  • ATPase catalytic site closes and ATP is hydrolyzed inducing a conformational change into high-energy state (cocking); myosin head is moved forward & perpendicular to F-actin
  • Myosin head binds weakly to actin one monomer further towards Z-disk
  • Myosin releases phosphate causing the actin binding cleft to close; strengthens myosin-actin interaction
  • Immediate power stroke: conformational change that sweeps myosin’s C-terminal tail about 10 nm toward the Z-disk relative to the motor domain (head)
  • ADP is released; actin acts as a nucleotide exchange factor

Link to Movie on Bchm4000 webpage!

myosin ii structure
Myosin II Structure

Actin binding

cleft

Converter domain

& Lever Arm

Nucleotide

Relay Helix

Converter domain

(green)

Lever Arm

Voet Fig. 35-62

myosin versus kinesin
Myosin versus Kinesin

Valle, Science 2000

conformational changes in myosin
Conformational changes in Myosin
  • Presence or absence of g-phosphates influences position of relay helix
  • Changes in relay helix are transferred to converter domain
  • Ultimately, result in large displacement of stiff lever arm

Voet Fig. 35-74

is myosin ii a processive motor
Is Myosin II a processive motor?
  • the two myosin heads are not coordinate, cycle independently of each other
  • net muscle contraction results from uncoordinated actin-attachement and -detachement of many myosins
  • Myosin II is not processive on its own!
unconventional myosin
Unconventional Myosin
  • found in nonmuscle cells: often homodimers, some monomers
  • mostly move to (+) end of actin, but Myosin VI travels to (-) end
  • Myosin V: transports cargo via hand-over-hand mechanism, highly processive motor, large step size of net 37 nm per ATP hydrolysis (74 nm movement of one head)

Voet Fig. 35-86

comparison of myosin kinesin
Comparison of Myosin & Kinesin

Kinesin Myosin

Structure

Conformational changes

Power stroke

Step size

Processivity

comparison of myosin kinesin16
Comparison of Myosin & Kinesin

Kinesin Myosin

Structure small large

same core: ATPase domain, relay helix

Conformational changes comparable movement in relay helix

different effect on power stroke

Power stroke upon ATP binding upon Pi release

Step size (per head) 16nm (8nm net) 10nm

Processivity highly non-processive

myosin versus kinesin17
Myosin versus Kinesin

Similar structural elements (ATPase domain – blue, relay helix – green, mechanical elements – yellow)

Similar conformational changes in Motor domain

Power stroke in “different directions”

Red/light green:

ADP/Nucleotide free

Yellow, dark green:

ADP-Pi

Valle,

Science 2000

model for power strokes
Model for Power Strokes

Myosin

Kinesin

Power stroke induced by ATP binding

Step size: 8 nm

Power stroke induced by Pi release

Step size: 10 nm

Valle, Science 2000

processivity
Processivity

Kinesin & Myosin V:

Highly processive

Myosin II:

unprocessive

Net 37 nm

Step size

Net 8 nm

Step size

Valle, Science 2000

evolution of motor proteins
Evolution of Motor Proteins

Valle, Science 2000