Myosins: Actin-Based Motor Proteins

Core Concept

Myosins are a superfamily of motor proteins that use the energy of ATP hydrolysis to move along actin filaments. All myosins share a conserved head (motor) domain that binds actin and hydrolyzes ATP, converting chemical energy into a conformational "power stroke." Myosin II forms bipolar filaments that slide actin filaments (muscle contraction, cytokinesis), while the many "unconventional" myosins (myosin I, V, VI, and others) carry cargo, organize the cell cortex, and power membrane dynamics.

Key Components

• Head (motor) domain: binds actin and ATP; contains the ATPase and actin-binding sites.
• Neck (lever arm): binds light chains (calmodulin or essential/regulatory light chains); its rotation amplifies small conformational changes into a large stroke.
• Tail domain: determines function — self-association into filaments (myosin II) or cargo binding (unconventional myosins).
• Actin-binding cycle: the sequence of ATP binding, hydrolysis, phosphate release, and ADP release that drives movement.
• Myosin II: the filament-forming, contractile myosin (muscle, cytokinesis, stress fibers).
• Unconventional myosins: myosin V (processive cargo transport), myosin VI (moves toward the pointed/minus end), myosin I (membrane–cortex interactions).

Mechanism / How It Works

1. Rigor state: the myosin head is tightly bound to actin (no nucleotide).
2. ATP binding to the head lowers its affinity for actin, releasing the head.
3. ATP hydrolysis (to ADP + Pi) cocks the head into a high-energy conformation while it is detached.
4. Weak rebinding: the head re-attaches weakly to actin further along the filament.
5. Phosphate release triggers the power stroke — the lever arm swings, pulling the actin filament (relative to myosin) toward the minus end.
6. ADP release returns the head to the rigor state, tightly bound, ready for the next ATP.
7. Repeated cycles cause the head to "walk" stepwise along the filament.

Energy and Directionality

Myosin movement is directly ATP-driven: one ATP is hydrolyzed per cross-bridge cycle, and the chemical energy is transduced into mechanical work by the power stroke. Directionality is set by the motor's structure: most myosins (I, II, V) walk toward the barbed (plus) end of actin, while myosin VI walks toward the pointed (minus) end. In myosin II, because the filament is bipolar, sliding of oppositely oriented actin filaments toward each other produces contraction rather than transport.

Experimental Evidence / Technique

• In vitro motility assays: myosin-coated beads or filaments moving over actin (or vice versa) showed myosin generates force and moves directionally — directly visualizing motility.
• Optical tweezers (Finer, Simmons & Spudich): measured single myosin steps (~5–40 nm) and forces (a few pN) per ATP, resolving the elementary power stroke.
• X-ray crystallography (Rayment et al.): the myosin head structure with nucleotide analogs revealed the lever-arm and conformational changes underlying the stroke.
• Muscle fiber biochemistry: rapid kinetics (caged ATP) correlated ATP hydrolysis with cross-bridge detachment and force generation.
• Myosin mutations: mutations in myosin heads (e.g. in MYH7 β-cardiac myosin) cause cardiomyopathies, linking motor function to disease.

How it works

1. Rigor state: the myosin head is tightly bound to actin (no nucleotide).
2. ATP binding to the head lowers its affinity for actin, releasing the head.
3. ATP hydrolysis (to ADP + Pi) cocks the head into a high-energy conformation while it is detached.
4. Weak rebinding: the head re-attaches weakly to actin further along the filament.
5. Phosphate release triggers the power stroke — the lever arm swings, pulling the actin filament (relative to myosin) toward the minus end.
6. ADP release returns the head to the rigor state, tightly bound, ready for the next ATP.
7. Repeated cycles cause the head to "walk" stepwise along the filament.

Common confusions

• "Myosin moves on microtubules." No — myosins move on actin; kinesins and dyneins move on microtubules (a common mix-up).
• "The power stroke happens when ATP is hydrolyzed." Hydrolysis cocks the head; the power stroke occurs on phosphate release.
• "All myosins move toward the plus end." Myosin VI is a notable exception, moving toward the minus (pointed) end.
• "Rigor mortis is muscle over-contraction." Rigor occurs because, without ATP, myosin heads stay attached to actin (rigor state), locking filaments — it is the absence of ATP, not active contraction.
• "Only muscle cells have myosin." All eukaryotic cells express myosins (cytokinesis, transport, migration).

Quick review

• Myosin = actin motor; ATP hydrolysis drives a lever-arm power stroke.
• Cycle: attach → ATP binds → release → cock → rebind → Pi release → power stroke → ADP release.
• Myosin II contracts (muscle, cytokinesis); unconventional myosins transport cargo.
• Direction: mostly plus-end; myosin VI is minus-end.
• ATP is required for detachment as well as the stroke (hence rigor mortis without ATP).