02 — Myosin and Muscle Contraction

Prerequisite Concepts

• Actin filament structure and polarity (Topic 01)
• ATP hydrolysis and free energy
• Basic protein domain architecture (motor domain, lever arm, tail)

Core Explanation

Myosin II Motor Domain and Mechanochemical Cycle

Myosin II, the motor protein of muscle, is a hexamer: two heavy chains (each with a globular motor domain, a neck/lever arm region, and a coiled-coil tail) and two pairs of light chains (essential and regulatory light chains that stabilize the lever arm).

The actin-activated myosin ATPase cycle proceeds through four states:

Key sequence:

1. ATP binds to the myosin motor domain → myosin dissociates from actin.
2. ATP is hydrolyzed (ADP-Pi remains bound); the lever arm cocks into the pre-power-stroke conformation (recovery stroke).
3. Myosin-ADP-Pi rebinds actin weakly; Pi release triggers the power stroke — the lever arm swings, pulling the actin filament ~5–10 nm toward the center of the sarcomere.
4. ADP is released; myosin remains tightly bound to actin (rigor state) until a new ATP binds.

Critical nuance: The power stroke is NOT the ATP hydrolysis step. ATP hydrolysis primes the motor (the recovery stroke, or "cocking"). The power stroke is driven by Pi release — it is the stored conformational energy from hydrolysis, released upon actin rebinding and Pi dissociation, that produces mechanical work.

The Sarcomere: Structural Organization

The sarcomere (from Z disc to Z disc, ~2–3 μm at rest) is the contractile unit of striated muscle:

• Z disc: Anchors the barbed ends of actin thin filaments; composed of α-actinin and CapZ.
• I band: Actin thin filaments only (no myosin overlap); appears light in electron micrographs.
• A band: The length of the myosin thick filaments; appears dark. Contains the zone of actin–myosin overlap.
• H zone: Myosin thick filaments only (no actin overlap); the central region of the A band.
• M line: Cross-links myosin thick filaments at their center.
• Titin: A giant elastic protein (~3 MDa) that spans from the Z disc to the M line, providing passive elasticity, maintaining sarcomere alignment, and preventing overstretch.

Sliding Filament Mechanism

Proposed independently by Huxley and Huxley in 1954, the sliding filament mechanism states: during contraction, actin thin filaments and myosin thick filaments slide past each other — neither filament shortens. The sarcomere shortens, the I band and H zone narrow, but the A band length remains constant because thick filament length does not change.

The cyclical attachment, power stroke, and detachment of myosin heads on actin filaments generates sliding force. Because myosin heads in a thick filament are oriented in opposite directions on each half of the sarcomere, the power stroke pulls actin filaments from both sides toward the center, producing sarcomere shortening.

Excitation–Contraction Coupling (Overview)

1. Motor neuron action potential → acetylcholine release at the neuromuscular junction.
2. Muscle membrane depolarization → T-tubule depolarization → DHPR (dihydropyridine receptor) conformational change.
3. DHPR couples to the ryanodine receptor (RyR) on the sarcoplasmic reticulum → Ca²⁺ release into the cytoplasm.
4. Ca²⁺ binds troponin C → tropomyosin shifts on actin → myosin-binding sites are exposed.
5. Cross-bridge cycling proceeds as long as Ca²⁺ and ATP are present.
6. Ca²⁺ reuptake into SR by SERCA pumps → relaxation.

Nonmuscle Myosins

Myosin II in Cytokinesis

Nonmuscle myosin II assembles into bipolar mini-filaments at the cytokinetic furrow. Actin filaments — nucleated by formins and organized by anillin — are pulled by myosin II in a purse-string mechanism. RhoA GTPase activates ROCK (Rho-kinase), which phosphorylates myosin regulatory light chain (and inhibits myosin phosphatase), increasing myosin II motor activity at the furrow.

Myosin V: Processive Vesicle Transport

Myosin V is a processive motor — it takes many steps along an actin filament before dissociating, unlike muscle myosin II (which works in ensembles). The long lever arm of myosin V (6 IQ motifs vs 2 in myosin II) gives it a step size of ~36 nm, matching the helical repeat of actin so it walks nearly straight. Myosin V transports vesicles, melanosomes, and mRNA particles. Mutations cause Griscelli syndrome (pigmentation and neurological defects).

Inverse Relationship: Speed vs Duty Ratio

Muscle myosin II has a low duty ratio (spends most of its cycle detached from actin) — it relies on many motors acting together. Myosin V has a high duty ratio (>70%) — it spends most of its cycle strongly bound, enabling single-motor processive transport. This trade-off reflects evolutionary specialization for ensemble force production vs single-motor cargo transport.

Disease Connections

• Familial hypertrophic cardiomyopathy (HCM): Mutations in β-cardiac myosin heavy chain (MYH7), cardiac actin, troponin, and tropomyosin — often increase myosin power output, causing hypercontractility.
• Griscelli syndrome: Myosin V mutations → defective melanosome transport (silver-gray hair) and neurological deficits.
• Nemaline myopathy: Mutations in skeletal α-actin and nebulin cause muscle weakness.

Common Misconceptions and Exam Traps

• Wrong: "The sarcomere is the muscle cell." Correct: The sarcomere is the contractile unit within a myofibril; a muscle fiber (cell) contains thousands of myofibrils, each with thousands of sarcomeres in series.
• Wrong: "ATP hydrolysis is the power stroke." Correct: Hydrolysis cocks the lever arm (recovery stroke). The power stroke occurs upon Pi release when myosin is bound to actin.
• Wrong: "Thick filaments shorten during contraction." Correct: The sliding filament mechanism: thick filament length is constant; thin filaments slide inward.
• Wrong: "Rigor mortis is caused by continuous muscle contraction." Correct: Rigor mortis results from ATP depletion after death — without ATP to detach myosin from actin, muscles lock in the rigor state, regardless of Ca²⁺ levels.