Lec 18 Myosin
Actin, Myosin, and Motility: Comprehensive Notes
Context
Chapter 9 of the course material covers the Cytoskeleton and Cell Motility, focusing on sections 9.8 (Actin and Myosin), 9.9 (Muscle Organization and Contraction), and 9.12 (Cellular Motility).
Related resources: glossary, flashcards, quizzes, practice questions, and animations such as “Video: Neutrophil Chasing a Bacterium” and “Video: A schematic model of the actinomyosin contractile cycle.”
Core idea: motor proteins convert chemical energy (ATP) into mechanical energy to generate force, enabling muscle contraction and various forms of cell movement.
Actin and Myosin (9.8)
Microtubules (MT) and long-distance transport
MTs drive movement of biomolecules and cellular components over long distances using molecular motors kinesin and dynein.
At MT ends, cargo often switches to microfilaments (actin filaments) for local movements, utilizing myosin motors at the actin-rich cell periphery.
This handoff coordinates long-range and local transport within the cell.
Myosin motors: broader roles
Myosins are a large family of actin-based motors involved in muscle contraction, cell movement, phagocytosis, vesicle transport, and more.
They convert chemical energy from ATP hydrolysis into mechanical energy to produce force, regardless of the context (muscle or vesicle movement).
Myosins: Diversity, Structure, and Directionality (9.8–9.9)
Myosin family and classifications
17 known classes of myosins (I–XVII); humans possess about 40 different myosin motors.
Grouped into:
Unconventional myosins (types I, III–XVII)
Conventional myosins (type II)
Most myosins move toward the plus (+) end of actin filaments, with myosin VI as a notable exception (moves toward the minus end).
Basic structural organization
Each myosin motor has at least one heavy chain consisting of:
A globular head domain that binds actin and hydrolyzes ATP to generate movement
A tail region that varies by class and determines cargo binding
Many myosins form dimers, yielding two motor domains; some, like myosin I, function as monomers.
Hand-over-hand movement and step sizes (Type V vs Kinesin)
Type V myosins are efficient motors and move in a hand-over-hand fashion along actin filaments.
Key differences from kinesin (a microtubule motor):
Myosins move along actin filaments; kinesins move along microtubules.
Step sizes: kinesin ≈ ; myosin V ≈ (reflecting the helical nature of actin filaments).
Similarities to kinesin: exert comparable forces, are highly efficient, move toward the (+) end, use ATP hydrolysis to drive conformational changes, often processive (one head remains attached while the other steps).
Conventional vs Unconventional Myosins; Type II in Muscle (9.7–9.9)
Type II myosins (best understood)
Structure: 2 heavy chains (each with a globular head, a neck, and a rod-like tail) and 4 light chains.
Roles: essential for muscle contraction, cytokinesis, cell migration, and focal adhesions.
Filament formation: myosin II can form bipolar filaments with heads oriented outward from the center; this filament arrangement is critical for pulling actin filaments during contraction.
Important note: unconventional myosins (types I, III–XVII) do not form bipolar filaments.
Historical note: skeletal muscle myosin II was the first motor protein identified.
Type II myosins in muscle organization
They are the motor proteins that drive muscle contraction, working in concert with actin to shorten sarcomeres.
Muscle Organization and Contraction (9.9)
Muscle fiber architecture
Each muscle fiber: a long, thin, multinucleated cell specialized for contraction.
Inside each fiber are numerous myofibrils (~1–2 μm in diameter) that may run the entire length of the cell.
Myofibrils are subdivided into repeating units called sarcomeres—the basic contractile unit.
Sarcomeres comprise thick (myosin II bipolar filaments) and thin (actin) filaments.
Sarcomere: definition and components
The distance from one Z line to the next defines a sarcomere.
Key structural proteins include:
Thick filaments: myosin II
Thin filaments: actin, plus associated proteins
Focus is on molecular structure and mechanism of movement rather than memorizing all bands/lines.
Thick filaments (myosin II) overview
Arrangement: staggered arrays of myosin II heads form cross-bridges with thin filaments.
Cross-bridges enable force generation by pulling on actin filaments during contraction.
Thin filaments (actin) and regulatory components
Core actin filaments are associated with regulatory proteins:
Tropomyosin: long, rod-like molecule that fits along the grooves of the F-actin helix.
Troponin complex: TnT (tropomyosin binding), TnC (calcium binding), TnI (actin binding).
Regulatory role: one troponin complex associates with each tropomyosin; together they act as a calcium-sensitive switch that activates contractions in striated muscle.
Sarcomere-associated regulatory proteins
Tropomodulin caps the minus (−) ends of actin filaments to maintain stability.
CapZ caps the plus (+) ends and anchors the thin filament to the Z line.
Nebulin stabilizes and binds the thin filament to the Z line.
α-actinin cross-links the Z line to the actin filament, aligning thin filaments in parallel arrays.
Myomesin bundles myosin molecules.
Titin connects thick filaments to the Z lines and helps maintain proper alignment during contraction.
Additional proteins are associated with the sarcomere and contribute to organization and function.
Concept: the sliding filament model
Myosin heads walk toward the plus end of actin filaments, moving the thin filaments toward the center of the sarcomere.
As thin filaments slide past thick filaments, the sarcomere shortens without a change in the length of the filaments themselves.
This coordinated sliding underlies muscle contraction; contraction requires ATP and Ca²⁺.
The cross-bridge cycle (core mechanism for contraction)
Cross-bridges are formed by interactions between myosin head domains (on thick filaments) and actin (on thin filaments).
For contraction to occur, cross-bridges must repeatedly form and dissociate.
The cycle is not strictly a single continuous motion (not a single processive step); it involves discrete binding and release events.
The actinomyosin contractile cycle (overview of steps)
No ATP (rigor): tight actin-myosin binding with no turnover.
Step 1: ATP binds to myosin, causing dissociation from actin.
Step 2: ATP is hydrolyzed to ADP + Pi, energizing and cocking the myosin head.
Step 3: Energized myosin binds to actin to form a cross-bridge.
Step 4: Pi is released, triggering the power stroke that moves actin toward the center of the sarcomere.
Step 5: ADP is released; myosin remains bound to actin until another ATP binds, resetting the cycle.
Important note: This is not a continuously unbroken run; individual cycles occur as the filament is cycled by many myosin heads.
Quantitative aspect of the power stroke
The power stroke results in approximately of movement per cycle.
Calcium regulation of contraction
Neuronal signaling causes a rapid rise in intracellular Ca²⁺ in muscle cells.
Mechanism:
A nerve impulse opens voltage-gated Ca²⁺ channels in the T-tubules.
This stimulates Ca²⁺ release channels on the sarcoplasmic reticulum, increasing cytoplasmic Ca²⁺.
Ca²⁺ triggers contraction by enabling the actin-myosin interaction (via troponin-tropomyosin regulation).
Contraction ends when Ca²⁺ is actively pumped back into the sarcoplasmic reticulum by Ca²⁺-transporting pumps (P2-type ATPases).
Calcium and the troponin-tropomyosin switch
Troponin is a complex of three subunits: TnT, TnC, and TnI.
Binding of Ca²⁺ to TnC changes troponin conformation, causing tropomyosin to shift away from myosin-binding sites on actin.
Exposure of myosin-binding sites enables cross-bridge formation and contraction.
Summary of non-muscle actin-myosin roles (9.12)
Actin and myosin are found in nearly all eukaryotic cells and are essential for non-muscle motility.
Roles include:
Cytokinesis
Phagocytosis
Vesicle transport
Platelet activation
Movement of membrane proteins
Cytoplasmic streaming
Axonal growth
Changes in cell shape
Cell locomotion
Actin-Based Motility and Cell Crawling in Non-Muscle Cells (9.12)
General process of cell crawling
Four-step model:
1) Extension of a protrusion at the leading edge (lamellipodia and filopodia).
2) Attachment of the protrusion to the substrate (via focal adhesions).
3) Generation of tension (contractile activity) to pull the cell forward.
4) Release of attachments at the trailing edge and retraction.Once a signal is received, cells can migrate toward the cue (directed motility).
Leading-edge structures
Lamellipodium: a thin, broad sheet of cytoplasm with a loose, branched actin network pushing the membrane forward.
Filopodium: thin, finger-like protrusions that extend ahead of the lamellipodium.
Stress fibers: organized bundles in cells that adhere tightly to the substratum; associated with focal adhesions (sites of contact).
Cell cortex and trailing edge (rear) retraction involve coordinated cytoskeletal remodeling.
Lamellipodia architecture and actin dynamics
The Arp2/3 complex nucleates new actin branches on the sides of existing filaments, creating a dendritic (tree-like) network.
Activation of Arp2/3 is driven by proteins in the WASP family, including WASP and WAVE.
As new actin branches form and elongate toward the (+) end, older filaments are severed and disassembled at the base of the lamellipodium, enabling actin recycling for continued protrusion.
Directional migration cues
Chemotaxis: directional movement toward diffusible chemical cues.
Chemoattractants guide cells toward higher concentrations; chemorepellents drive movement away from lower concentrations.
Binding of chemoattractants/repellents to G protein-coupled receptors (GPCRs) on the cell surface triggers signaling cascades that reorganize the cytoskeleton to favor protrusion in the desired direction.
Chemotaxis example and visualization
Video resources illustrate neutrophil chasing bacteria and the actinomyosin contractile cycle in action.
Connections, Implications, and Key Takeaways
Core concepts to master
The cytoskeleton comprises microtubules and microfilaments (actin) as primary tracks for motor proteins (kinesin/dynein on MT; myosin on actin).
Myosin motors come in many classes with diverse roles; most move toward the plus end of actin, with myosin II driving muscle contraction via bipolar filaments.
Muscle contraction is driven by the sliding of thin (actin) filaments past thick (myosin II) filaments within sarcomeres, powered by ATP hydrolysis and regulated by Ca²⁺ through troponin-tropomyosin.
Non-muscle actin-myosin systems enable cell motility, shape changes, cytokinesis, vesicle transport, and chemotaxis; these processes rely on regulated actin branching (Arp2/3) and focal adhesions.
Key numerical values and concepts to remember
Actin filament step or structural dimensions:
Power stroke movement per cycle: .
Myosin V step size along actin: (larger than kinesin’s 8 nm step) due to the actin helix geometry.
Kinesin step size along MT: .
Sarcomere length is defined by Z line spacing; sarcomere is the fundamental contractile unit.
Thin filaments and thick filaments are organized within sarcomeres, with actin and myosin arranged to enable coordinated contraction.
Practical implications and connections
Understanding motor protein directionality, step size, and processivity explains how cells coordinate long-range transport with local remodeling at the periphery.
Calcium signaling links neuronal stimuli to rapid muscle contraction and, in non-muscle cells, regulates cytoskeletal rearrangements via troponin-like mechanisms (in muscle) and calcium-sensitive cytosolic signaling in other cell types.
The Arp2/3 complex and WASP/WAVE family proteins illustrate how signaling cues translate into physical structures (branched actin networks) that drive protrusions and migration.
Ethical, philosophical, or practical considerations (contextual)
A deeper understanding of cytoskeletal dynamics informs medical insights into muscular diseases, immune cell function, and wound healing.
Therapeutic targeting of motor proteins or actin dynamics must consider widespread roles across cell types to minimize unintended systemic effects.
End-of-section objectives (as stated)
Understand the structure and function of myosin.
Know the levels of organization of muscles.
Be clear on the structures and proteins in the sarcomere.
Understand the function of actin and myosin in muscle contraction.
Comprehend how calcium is involved.
Grasp how cells move, including chemotaxis and directional migration.
Distinguish between chemoattractants and chemorepellents.
Quick Reference: Key Terms and Concepts
Myosin: A family of actin-based motor proteins; classes I–XVII; conventional (type II) vs unconventional.
Myosin II: Conventional motor in muscle; forms bipolar filaments; drives contraction.
Actin (F-actin): Filamentous actin; thin filaments in muscle.
Tropomyosin: Rod-like protein that sits in actin grooves; regulates access to myosin-binding sites.
Troponin: Complex with TnT, TnC, TnI; calcium-sensitive switch for contraction.
Ca²⁺: Central regulator of muscle contraction; rise triggers contraction; pumped back to SR to stop contraction.
Sarcomere: Repeating contractile unit within a myofibril; Z line to Z line.
Thick filament: Myosin II filaments.
Thin filament: Actin filaments.
Arp2/3: Actin nucleator that creates branched networks; activated by WASP/WAVE.
Lamellipodium: Broad, sheet-like leading-edge extension driven by branched actin.
Filopodium: Thin, finger-like protrusion at the leading edge.
Focal adhesions: Contact sites linking actin cytoskeleton to the extracellular matrix.
Chemotaxis: Directed cell movement toward/away from chemical cues via GPCR signaling.
End of Notes
This set consolidates the content from the transcript into a comprehensive, exam-ready reference, arranged by topic with explicit details, definitions, mechanisms, and numerical values expressed in LaTeX where appropriate.