The Molecular Motor Myosin Jonathan P. Davis, Ph.D. Assistant Professor Office/Lab Phone 247-2559 Email: davis.812@osu.edu Department of Physiology and Cell Biology, The Ohio State University, 400 Hamilton Hall

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

What Does Myosin Look Like – The Structure is Known

Which Physiological Systems Utilize Myosin (Not All Shown)

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

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

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

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

Myosin is a Large Family of Molecular Motors

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

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

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.

Actin GREATLY Accelerates Myosin’s ATPase Rate

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

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)

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.

ATPase Rate Correlates with Maximal Speed of Muscle Contraction

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

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)

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

Without Molecular Motors There Would Be No Movement

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, 239-251, 2005. Vesicle Transport: The role of actin filaments and myosin motors, DePina & Langford, Microscopy Research and Technique, 47, 93-106, 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, 615-630, 2007.