Master’s-Level Cell Biology & Advanced Molecular Biology Notes
1. Definition
Myosins are ATP-dependent molecular motor proteins that interact with actin filaments to generate mechanical force and movement.
They convert the chemical energy of ATP hydrolysis into:
- Actin filament sliding
- Cellular contractility
- Cargo transport
- Membrane movement
- Cell migration
- Cytokinesis
- Muscle contraction
- Mechanical signaling
The fundamental principle is:
ATP hydrolysis → conformational change → actin interaction → mechanical work
2. Myosin and the Cytoskeleton
Myosin is primarily an actin-based molecular motor.
MOLECULAR MOTORS
┌─────────────┬─────────────┐
│ │ │
MYOSIN KINESIN DYNEIN
│ │ │
ACTIN MICROTUBULE MICROTUBULE
│ │ │
Contractility Transport TransportThus:
Myosin → Actin
is the key association to remember.
3. Major Functions of Myosin
Myosins perform both transport and force-generation functions.
Major functions
- Muscle contraction
- Cytokinesis
- Cell migration
- Vesicle transport
- Organelle movement
- Membrane remodeling
- Cell adhesion
- Mechanotransduction
- Maintenance of cortical tension
- Tissue morphogenesis
4. Basic Myosin Architecture
A typical myosin contains three major regions:
MYOSIN
┌────────────┐
│ TAIL │
│ CARGO / │
│ ASSEMBLY │
└─────┬──────┘
│
NECK
│
│
┌─────┴─────┐
│ MOTOR │
│ HEAD │
└─────┬─────┘
│
▼
ACTIN
════════════════════════════Motor head
Contains:
- ATP-binding site
- ATPase machinery
- Actin-binding interface
Neck
Acts as a mechanical lever.
Tail
Determines:
- Cargo interaction
- Membrane association
- Protein interactions
- Filament assembly
5. Myosin Motor Head
The motor head is the mechanochemical engine.
It contains binding sites for:
- ATP
- ADP
- Actin
ATP-dependent conformational changes alter the interaction between the myosin head and actin.
MYOSIN HEAD
│
┌─────────┼─────────┐
↓ ↓ ↓
ATP ADP ACTIN
binding state binding
│ │ │
└─────────┼─────────┘
↓
MECHANICAL
WORK6. Actin Polarity
Understanding actin polarity is essential for understanding myosin movement.
Actin filaments have:
- Plus/barbed end
- Minus/pointed end
− END + END
POINTED BARBED
│ │
▼ ▼
════════════════════════════════════════
F-ACTINMany conventional myosins move toward the plus/barbed end of actin.
However, myosin VI is a major exception, moving toward the minus end.
7. Myosin ATPase Cycle
The myosin cycle can be simplified into several stages.
ATP
↓
ATP binds myosin
↓
Actin dissociation
↓
ATP hydrolysis
↓
ADP + Pi
↓
Myosin cocked state
↓
Actin binding
↓
Pi release
↓
POWER STROKE
↓
ADP release
↓
Strong binding
↓
New ATP binds
↓
REPEATThis cycle is the foundation of myosin-mediated movement.
8. Step 1 — ATP Binding
ATP binds to the nucleotide-binding site of the myosin motor domain.
ATP binding decreases myosin’s affinity for actin.
Myosin–Actin
│
ATP
↓
Myosin–ATP
↓
Actin dissociationThis allows the myosin head to detach from actin.
9. Step 2 — ATP Hydrolysis
The bound ATP is hydrolyzed:
ATP → ADP + Pi
The released chemical energy produces a conformational change in the motor head.
The myosin head enters a primed/cocked state.
ATP
↓
ADP + Pi
↓
Conformational change
↓
"Cocked" myosin head10. Step 3 — Weak Actin Binding
The ADP + Pi-containing myosin head can interact with actin.
The interaction becomes stronger as the cycle progresses.
Myosin–ADP–Pi
│
↓
Actin binding
│
↓
Strong interaction11. Step 4 — Phosphate Release
Release of inorganic phosphate (Pi) is a key transition.
It is associated with:
- Stronger actin binding
- Conformational change
- Initiation of the power stroke
Myosin–ADP–Pi
↓
Pi release
↓
Conformational change
↓
POWER STROKE12. Step 5 — Power Stroke
The power stroke is the major mechanical event.
The neck region undergoes a conformational change relative to the motor domain.
This moves the actin filament relative to myosin.
Before
Myosin
\
\
══════════════════
ACTIN
After power stroke
Myosin
/
/
══════════════════
ACTINRepeated cycles produce continuous movement.
13. Step 6 — ADP Release
Following the power stroke, ADP is released.
The myosin head is then in a strongly actin-bound state.
A new ATP molecule binds, causing actin dissociation and beginning another cycle.
14. Complete Myosin Cycle
ATP
│
▼
Myosin + ATP
│
▼
Actin dissociation
│
▼
ATP hydrolysis
│
▼
ADP + Pi state
│
▼
Actin binding
│
▼
Pi release
│
▼
POWER STROKE
│
▼
ADP release
│
▼
Strong actin
binding
│
└──────→ New ATP15. The Lever-Arm Model
A central concept in myosin mechanics is the lever-arm mechanism.
The motor domain interacts with actin, while the neck acts as a lever.
NECK
/
/
/
MOTOR HEAD
●
│
▼
════════════════════════
ACTINChanges in the motor domain are amplified through movement of the lever arm.
16. Role of Light Chains
The neck of many myosins associates with light chains.
These can include:
- Essential light chain
- Regulatory light chain
They help stabilize and regulate the lever arm.
In myosin II, phosphorylation of the regulatory light chain is an important mechanism controlling contractility.
17. Myosin II
Myosin II is the major conventional contractile myosin.
It is essential for:
- Skeletal muscle contraction
- Smooth muscle contraction
- Non-muscle cell contractility
- Cytokinesis
- Cell migration
- Tissue morphogenesis
18. Myosin II Structure
Myosin II molecules can assemble into bipolar filaments.
Actin Myosin II Actin
→→→→→→→ ← ↑ ↑ → ←←←←←←←
═════════ ═══════════ ═════════The bipolar arrangement allows myosin II to pull actin filaments toward one another.
19. Muscle Contraction
In skeletal muscle, myosin II interacts with actin within the sarcomere.
The fundamental mechanism is the:
Sliding filament mechanism
Actin filaments slide toward the center of the sarcomere.
Z disc Z disc
│ │
│ →→→ Actin Actin ←←← │
│ \ / │
│ \ MYOSIN / │
│ ═══════ │
│ │The filaments themselves do not substantially shorten; their relative sliding reduces sarcomere length.
20. Regulation of Skeletal Muscle Myosin
Skeletal muscle contraction is regulated primarily through:
Ca²⁺ → troponin → tropomyosin → actin–myosin interaction
Action potential
↓
Ca²⁺ release
↓
Troponin C binds Ca²⁺
↓
Tropomyosin moves
↓
Myosin-binding sites exposed
↓
Actin–myosin cycling
↓
Contraction21. ATP in Muscle Contraction
ATP has several important roles.
It is required for:
- Myosin detachment from actin
- Re-cocking of the motor
- Ion pumping that restores Ca²⁺ gradients
A key concept:
ATP binding causes myosin to detach from actin.
This explains why ATP depletion produces persistent actin–myosin attachment in rigor.
22. Rigor Mortis
After death, ATP production ceases.
Without ATP:
Myosin cannot efficiently detach from actin.
Therefore persistent actin–myosin cross-bridges contribute to muscle stiffness.
ATP depletion
↓
Myosin remains actin-bound
↓
Persistent cross-bridges
↓
Muscle stiffness23. Non-Muscle Myosin II
Myosin II is also essential in non-muscle cells.
It generates contractile forces in:
- Stress fibers
- Cell cortex
- Adhesion structures
- Cytokinetic rings
Thus:
Myosin II ≠ muscle-specific protein
24. Myosin II and Cytokinesis
During cytokinesis, actin and myosin II form a contractile ring.
CELL
┌───────────────┐
│ │
│ ○ │
│ ║║ │
│ ║ ║ │
│ ║║ │
└───────────────┘
↑
Actomyosin ringMyosin II contracts the actin network.
Actin + Myosin II
↓
Contractile ring
↓
Furrow ingression
↓
Two daughter cells25. Myosin II and Cell Migration
Cell migration requires coordinated actin polymerization and actomyosin contractility.
DIRECTION
→
┌─────────────────────┐
│ Leading edge │
│ Actin polymerization
│ →→→ │
│ │
│ Myosin II │
│ contraction │
│ ↓↓↓ │
└─────────────────────┘Myosin II helps generate contractile forces that move the cell body and regulate adhesion.
26. Myosin V
Myosin V is a processive actin-based cargo transporter.
It transports cargo such as:
- Vesicles
- Organelles
- Protein complexes
It generally moves toward the actin plus end.
CARGO
│
MYOSIN V
│
▼
════════════════════════════
→ → → → → → → → → → → → →
ACTIN PLUS END27. Myosin V Walking
Myosin V contains two motor heads that can coordinate stepping.
STEP 1
Head A ●──────
Head B
STEP 2
Head A
Head B ●──────
STEP 3
Head A ●──────
Head BThis enables processive movement along actin.
28. Myosin VI
Myosin VI is an important directional exception.
Unlike most characterized myosins, it generally moves toward the actin minus/pointed end.
It participates in:
- Endocytosis
- Vesicle trafficking
- Membrane organization
- Cell polarity
+ BARBED END − POINTED END
│ │
│ MYOSIN VI │
│ ←←←←←←←←← │
════════════════════════════════════29. Myosin I
Myosin I proteins are generally involved in:
- Membrane–actin interactions
- Endocytosis
- Membrane remodeling
- Cortical actin organization
They often contain membrane-associated regions rather than forming conventional bipolar contractile filaments like myosin II.
30. Myosin and Membrane Remodeling
Actomyosin networks can generate forces on membranes.
Actin network
══════════════════
↑
Myosin
↑
│
Membrane
╭──────────╮
│ │
╰──────────╯This contributes to:
- Endocytosis
- Exocytosis
- Membrane tension
- Cell shape changes
- Membrane trafficking
31. Myosin and Mechanotransduction
Myosin-generated tension can be transmitted through actin to adhesion complexes.
Myosin activity
↓
Actin tension
↓
Adhesion complex
↓
Mechanical signal
↓
Biochemical signalingThus, myosin converts ATP energy into mechanical signals that cells can sense.
32. Myosin and Cell Junctions
Actomyosin networks are important at:
- Adherens junctions
- Cell–cell contacts
- Cell–matrix adhesions
Myosin-generated tension can influence junctional organization and tissue architecture.
33. Myosin and Tissue Morphogenesis
During development, actomyosin contractility drives:
- Cell shape changes
- Tissue folding
- Cell intercalation
- Convergent extension
- Junction remodeling
- Epithelial remodeling
Myosin activation
↓
Actomyosin contraction
↓
Cell shape change
↓
Tissue deformation
↓
Morphogenesis34. Regulation of Myosin II
Non-muscle myosin II is strongly regulated by phosphorylation.
A simplified pathway is:
Signal
↓
RhoA
↓
ROCK
↓
Myosin regulatory light-chain phosphorylation
↓
Myosin II activation
↓
Actomyosin contractilityAnother important regulatory enzyme is myosin light-chain kinase (MLCK).
35. Rho–ROCK–Myosin Pathway
This pathway is highly important in cell biology.
RhoA-GTP
↓
ROCK activation
↓
Myosin light-chain phosphorylation
↓
Myosin II activity
↓
Actin contractilityIt contributes to:
- Cell migration
- Adhesion
- Cytokinesis
- Tissue morphogenesis
- Cell polarity
36. Ca²⁺ and Myosin
Calcium regulates different myosin systems through different mechanisms.
For example:
Skeletal muscle
Ca²⁺ → troponin C → tropomyosin movement → actin–myosin interaction
Smooth muscle
Ca²⁺ → calmodulin → MLCK → myosin light-chain phosphorylation → contraction
Ca²⁺
│
┌─────────┴─────────┐
↓ ↓
Skeletal Smooth
muscle muscle
↓ ↓
Troponin Calmodulin
↓ ↓
Tropomyosin MLCK
↓ ↓
Contraction Myosin-P37. Myosin and Vesicular Transport
Certain myosins transport cargo over short distances along actin.
This is especially important near the plasma membrane, where actin networks become dense.
Microtubule transport
↓
Cell periphery
↓
Actin network
↓
Myosin-dependent
short-range transport
↓
Target membraneThis illustrates cooperation between kinesins/dynein and myosins.
38. Integrated Cytoskeletal Transport
Long-distance transport:
Kinesin/Dynein → microtubules
Short-range cortical transport:
Myosin → actin
CARGO
│
↓
Kinesin / Dynein
│
↓
════════════════════════════════
MICROTUBULE
│
↓
Cell cortex
│
↓
MYOSIN
│
↓
════════════════════════════════
ACTIN
│
↓
TARGET SITEThis is an important advanced concept.
39. Myosin as a Mechanochemical Converter
The molecular principle can be summarized:
CHEMICAL ENERGY
│
ATP
↓
ATP hydrolysis
↓
Motor conformational
change
↓
Lever-arm movement
↓
Actin movement
↓
MECHANICAL WORK40. Myosin Classes
Myosins are classified into many classes.
Important examples:
| Myosin | Major function |
|---|---|
| Myosin I | Membrane–actin interaction, endocytosis |
| Myosin II | Contractility |
| Myosin V | Processive cargo transport |
| Myosin VI | Minus-end-directed transport |
| Myosin X | Filopodia-associated functions |
| Myosin XV | Specialized cellular functions, including stereocilia biology |
The exact functions depend on cell type and organism.
41. Myosin I vs II vs V vs VI
| Feature | Myosin I | Myosin II | Myosin V | Myosin VI |
|---|---|---|---|---|
| Major role | Membrane/actin | Contractility | Cargo transport | Cargo/membrane trafficking |
| Bipolar filament | Generally no | Yes | No | No |
| Processive transport | Variable | Generally non-processive | High | High |
| Typical direction | Usually plus-end | Usually plus-end | Plus-end | Minus-end |
| Muscle contraction | No | Yes | No | No |
| Endocytosis | Yes | Indirect/variable | Some contexts | Yes |
42. Processive vs Non-Processive Myosins
Non-processive motors
Perform individual mechanical cycles and frequently detach.
Example:
Myosin II
Processive motors
Remain attached through multiple cycles.
Examples:
Myosin V
Myosin VI
NON-PROCESSIVE
Bind → stroke → detach
↓
Repeat
PROCESSIVE
Bind → step → step → step → step43. Duty Ratio
The duty ratio is the fraction of the mechanochemical cycle during which a motor is strongly attached to actin.
A high duty ratio favors sustained attachment and processive transport.
This is particularly relevant to cargo transporters such as myosin V.
44. Myosin and ATP Consumption
ATP consumption depends on:
- Motor number
- ATPase cycle rate
- Mechanical load
- Cargo
- Regulatory state
- Cellular activity
Thus, cytoskeletal transport represents a significant component of cellular energy expenditure.
45. Load-Dependent Motor Behavior
Motor activity changes under mechanical load.
Cargo load
↓
Mechanical resistance
↓
Motor kinetics
↓
Step frequency / detachment
↓
Transport behaviorThis is important in understanding intracellular transport under physiological conditions.
46. Myosin and Mechanosensing
Myosin-generated force can affect protein conformation and signaling.
For example:
ATP
↓
Myosin contraction
↓
Actin tension
↓
Mechanical force
↓
Mechanosensitive proteins
↓
Signal transductionThus, myosin participates in mechanobiology.
47. Myosin and Disease
Because myosins participate in muscle, hearing, vision, cell migration and intracellular transport, mutations can cause diverse diseases.
Examples include disorders involving:
- Skeletal muscle
- Cardiac muscle
- Hearing
- Cytoskeletal organization
- Intracellular trafficking
Certain myosin mutations are associated with inherited cardiomyopathies and other tissue-specific disorders.
48. Myosin in Hearing
Specialized myosins are important in the structure and function of auditory hair cells.
They participate in:
- Stereocilia organization
- Membrane trafficking
- Mechanotransduction
Thus, myosin dysfunction can contribute to hereditary hearing disorders.
49. Myosin and Cancer Biology
Altered actomyosin contractility can influence:
- Cell migration
- Invasion
- Adhesion
- Tissue mechanics
- Metastasis
Therefore, abnormal regulation of myosin II and associated pathways can contribute to cancer progression.
50. Myosin and Cell Mechanics
A useful systems-level model is:
ATP
↓
Myosin activity
↓
Actin contractility
↓
Cellular tension
↓
Cell shape
↓
Cell migration / adhesion
↓
Tissue architectureThis connects molecular biochemistry to tissue-level biology.
51. Master-Level Integration
Myosin should not be viewed merely as a “muscle protein.”
It is a large family of mechanochemical proteins involved in:
MYOSIN
│
┌─────────┼──────────┐
↓ ↓ ↓
Transport Force Remodeling
│ generation │
↓ ↓ ↓
Vesicles Contractility Membranes
Organelles Cytokinesis Cell shape
│ │ │
└─────────┼────────────┘
↓
CELLULAR ORGANIZATION52. Integrated Comparison: Myosin, Kinesin and Dynein
| Feature | Myosin | Kinesin | Dynein |
|---|---|---|---|
| Track | Actin | Microtubule | Microtubule |
| Energy | ATP | ATP | ATP |
| Typical role | Contractility/transport | Transport/spindle | Transport/spindle/cilia |
| Typical direction | Usually actin + | Usually MT + | MT − |
| Major example | Myosin II | Kinesin-1 | Cytoplasmic dynein |
| Muscle contraction | Yes | No | No |
| Cytokinesis | Yes | Yes, different functions | Yes, different functions |
| Ciliary beating | No | No | Yes, axonemal dynein |
| Long-distance axonal transport | Limited | Anterograde | Retrograde |
53. Important Examination Points
Remember these five facts:
1. Myosin is an actin-based motor.
2. ATP binding causes myosin to detach from actin.
3. Pi release is associated with the power stroke.
4. Myosin II produces contractility and forms bipolar filaments.
5. Myosin V is a highly processive cargo transporter, whereas myosin VI generally moves toward the actin minus end.
54. Short Note for Master’s Examination
Myosin
Myosins are ATP-dependent actin-based molecular motors that convert chemical energy into mechanical work. A typical myosin consists of an N-terminal motor head containing ATPase and actin-binding sites, a neck or lever arm associated with light chains, and a tail responsible for cargo interaction, membrane association or filament assembly.
The myosin mechanochemical cycle involves ATP binding, actin dissociation, ATP hydrolysis, actin rebinding, phosphate release, the power stroke and ADP release. The power stroke results from conformational changes in the motor and lever-arm regions.
Myosin II forms bipolar filaments and is responsible for muscle contraction and non-muscle actomyosin contractility, including cytokinesis, migration and tissue morphogenesis. Myosin V is a highly processive cargo transporter moving along actin, whereas myosin VI is an important minus-end-directed myosin involved in membrane trafficking and endocytosis.
Myosin activity is regulated by calcium-dependent mechanisms, phosphorylation, Rho–ROCK signaling and other cellular pathways. Myosins therefore function not only in movement but also in intracellular transport, membrane remodeling, mechanotransduction and tissue organization.
55. Viva Questions
Q1. What is myosin?
An ATP-dependent molecular motor that interacts with actin.
Q2. What cytoskeletal filament does myosin move on?
Actin.
Q3. What is the energy source?
ATP hydrolysis.
Q4. What happens when ATP binds myosin?
Myosin’s affinity for actin decreases, allowing detachment.
Q5. What is the power stroke?
A conformational change in the myosin motor/lever-arm system that produces mechanical movement.
Q6. What is myosin II?
A major contractile myosin that forms bipolar filaments.
Q7. What is myosin V?
A processive actin-based cargo transporter.
Q8. Which myosin moves toward the actin minus end?
Myosin VI.
Q9. What is the role of myosin in cytokinesis?
Myosin II contracts the actomyosin ring, producing cleavage-furrow ingression.
Q10. What is the role of myosin in skeletal muscle?
It generates force through cyclic interactions with actin.
Q11. What regulates skeletal muscle contraction?
Ca²⁺-dependent regulation of troponin–tropomyosin.
Q12. What regulates smooth muscle myosin?
Ca²⁺–calmodulin activation of MLCK and phosphorylation of myosin regulatory light chain.
Q13. What is the Rho–ROCK–myosin pathway?
A signaling pathway that promotes myosin II activity and actomyosin contractility.
Q14. What is rigor mortis mechanistically?
Persistent actin–myosin attachment following ATP depletion.
Q15. What is the central function of myosin?
Conversion of ATP chemical energy into actin-dependent mechanical work.
56. One-Minute Revision Diagram
MYOSIN
│
ATP-dependent motor
│
↓
ACTIN
│
┌─────────────────┼─────────────────┐
↓ ↓ ↓
MYOSIN I MYOSIN II MYOSIN V
│ │ │
Membrane/actin Contractility Transport
interactions Bipolar filaments Processive
│ │ │
Endocytosis Muscle contraction Vesicles
Cytokinesis Organelles
│
↓
ATPase CYCLE
│
ATP → detachment → hydrolysis
│
Actin binding
│
Pi release
│
POWER STROKE
│
ADP release
│
Repeat
│
↓
MECHANICAL WORKCore memory rule
MYOSIN → ACTIN → ATP → POWER STROKE → FORCE/MOVEMENT
Myosin II → contractility
Myosin V → processive cargo transport
Myosin VI → minus-end-directed transport
Myosin + actin → muscle contraction, cytokinesis, migration, membrane remodeling and mechanotransduction