1. The Molecular Motor Myosin
Jonathan P. Davis, Ph.D. Assistant Professor
Office/Lab Phone
Department of Physiology and Cell Biology, The Ohio State University, 400 Hamilton Hall

2. Assorted Molecular Motors/Machines
Rotors – ATP Synthase
RATCHETS – Kinesin & Dynein
CABLE CAR – DNA/RNA Proteins
PUMPS – Ion Channels
SWITCHES - Calmodulin
Ca2+
MLCK
Apo State
Ca2+ Saturated
Ca2+ Saturated + MLCK

3. What Does Myosin Look Like – The Structure is Known

4. Which Physiological Systems Utilize Myosin (Not All Shown)

5. The Four Fundamental Functions of Myosin
1) Motility:
2) Cargo Transport:
3) Cell Shape:
4) Signal Transduction:

6. Cellular Myosin Functions (Again Not All Shown)
Mermall, Post & Mooseker . “Unconventional Myosins….” Science 279(3), 1998

7. Molecular Composition of Myosin
Myosin is Composed of Multiple Protein SUBUNITS
Consider myosin II
2 Heavy Chains
4 Light Chains
Proteolytic digestion of myosin II
A) Light meromyosin (LMM)
Tail (Determines Function)
B) Heavy meromyosin (HMM)
S2 (neck region)
S1 (globular heads)
ATP binding
Actin binding

8. The Myosin Subunits Can Interact in Many Different Ways
Various Combinations of Heavy and Light Chains
Assorted Filamentous Structures

9. Myosin is a Large Family of Molecular Motors

10. Variation in Myosin Sequence Determines Structure/Function
Head or motor domain relatively constant
Great variation in neck and tail domains
The myosin head domain is the most conserved region

11. Five Fundamental Motor Properties of Myosin
Force
2) Duty Ratio
3) Velocity
4) Processivity
5) Directionality

12. Myosin Burns the Fuel Source ATP into ADP and Pi+
ADP H Pi
ADP
M + ATP M*ATP M*Pi*ADP M*ADP M
SLOW
Once through all these chemical states, myosin can “Cycle” through the reaction scheme again.

13. Actin GREATLY Accelerates Myosin’s ATPase Rate

14. A Molecular Model for the Myosin ATPase or “Cross-Bridge” Cycle

15. The Myosin Power Stroke Can Produce Force
Optical Trap Assay
Duty Ratio - % of time spent strongly bound to actin (~5% for skeletal myosin)
~10nm Single Steps
~5pNm Force
Economy – the ratio of force to the energy needed to maintain that force (smooth muscle myosin is more economical than skeletal muscle myosin)

16. The Myosin Power Stroke Can Produce Velocities of Movement Too
There are three primary factors that govern the velocity that myosin can move actin: 1) intrinsic ATPase rate; 2) the step size of the myosin and 3) the force of the load being moved.

17. ATPase Rate Correlates with Maximal Speed of Muscle Contraction

18. Both Myosin II and V Are Double Headed – But…
Conventional Myosin II
Unconventional Myosin V
Detached
Attached
~10nm Single Steps
~37nm Multiple Steps
~5pNm Force
~5pNm Force
Myosin V Moves Cargo Processively Over Long Distances
Myosin II Filament Array – Concerted Contraction

19. Different Myosins Can Move in Opposite Directions
Brush Border Microvilli
Actin Cables
Myosin I
Myosin VI
Helps to Endocytose Plasma Membrane Toward the Cell Body (- end actin Movement)
Moves Plasma Membrane Away from Cell Body (+ end actin Movement)

20. Myosin Decorated Actin Filament
Myosin Binds to Actin Stereospecifically And the Motion of the Converter Domain Change Determines Direction of the Powerstroke
Myosin Decorated Actin Filament
Actin Filament
Pointed (+)
Converter Domain
Possibly for Myosin VI and IX the Converter Domain Moves in the reverse direction
Barbed (-)
Strong Hydrophobic Myosin Binding Sites
Weak Electrostatic Myosin Binding Sites

21. Without Molecular Motors There Would Be No Movement

22. Suggested Reading
Molecular Biology of the Cell, 4th Edition, Alberts et al, Chapter 16
Functional Studies of Individual Molecules, Dantzig et al, Ann. NY Acad. Sci., 1080, 1-18, 2006.
Myosins: Tails (and heads) of Functional Diversity, Krendel & Mooseker, Physiology, 20, , 2005.
Vesicle Transport: The role of actin filaments and myosin motors, DePina & Langford, Microscopy Research and Technique, 47, , 1999.
The way things move: looking under the hood of molecular motor proteins, Vale and Milligan, Science, 288, 88-95, 2000.
Myosin at work: motor adaptations for a variety of cellular functions, O’Connell et al., BBA, 1773, , 2007.