Master’s-Level Cell Biology & Advanced Molecular Biology Notes

1. Definition

The actin cytoskeleton is a dynamic network of actin filaments (microfilaments) distributed throughout the cytoplasm.

It provides structural support and participates in:

Cell shape and polarity
Cell migration
Cell adhesion
Cytokinesis
Endocytosis
Exocytosis
Vesicular trafficking
Muscle contraction
Mechanotransduction
Cell–cell and cell–matrix interactions

Unlike a static structural scaffold, the actin cytoskeleton is a highly dynamic, regulated system that continuously undergoes polymerization, depolymerization and remodeling.

2. Organization of the Cytoskeleton

The cytoskeleton consists of three major filament systems:

Component	Approx. diameter	Major function
Actin filaments	~7 nm	Cell shape, movement, contraction
Intermediate filaments	~10 nm	Mechanical strength
Microtubules	~25 nm	Intracellular transport, mitosis

                 CYTOSKELETON
                      │
       ┌──────────────┼──────────────┐
       ↓              ↓              ↓
     ACTIN       INTERMEDIATE    MICROTUBULES
    ~7 nm          ~10 nm           ~25 nm
       │              │              │
       ↓              ↓              ↓
   Movement        Strength       Transport
   Cortex          Stability      Mitosis
   Contractility                  Polarity

3. Actin Structure

Actin exists in two major forms:

G-actin

Globular actin

A soluble monomer.

F-actin

Filamentous actin

A polymer composed of actin monomers.

G-actin + G-actin + G-actin
        ↓
     Polymerization
        ↓
F-actin filament

4. Actin Monomer

G-actin is a globular protein of approximately 42 kDa.

Each actin monomer binds a nucleotide, usually:

ATP or ADP

The nucleotide state influences actin polymerization and filament stability.

5. F-Actin

F-actin is a helical polymer formed from G-actin monomers.

A simplified representation:

       G       G       G
      / \     / \     / \
     G   G---G   G---G   G
      \ /     \ /     \ /
       G       G       G

Actin filaments are polar structures.

They have:

Plus (+) end
Minus (−) end

6. Actin Filament Polarity

The two ends behave differently.

Plus end

Usually polymerizes more rapidly.

Minus end

Usually polymerizes more slowly and depolymerizes more readily.

Minus end                         Plus end
   (−)                               (+)
    │                                 │
    ●—●—●—●—●—●—●—●—●
    │                                 │
Slower dynamics                 Faster dynamics

This polarity is crucial for:

Cell migration
Vesicle movement
Cytokinesis
Polarized cell organization

7. Actin Polymerization

Actin polymerization can be divided conceptually into three phases:

Nucleation
Elongation
Steady state

G-actin
   ↓
Nucleation
   ↓
Small actin nucleus
   ↓
Elongation
   ↓
Long F-actin
   ↓
Steady state

8. Nucleation

Nucleation is the formation of a stable initial actin oligomer.

It is the rate-limiting stage of spontaneous actin polymerization.

G-actin
 G   G
  \ /
   G
   ↓
Nucleus
   ↓
Filament growth

Cells use specialized proteins to make nucleation more efficient.

9. Actin Nucleators

Important actin nucleation systems include:

Arp2/3 complex

Produces branched actin networks.

Formins

Promote formation of linear/unbranched actin filaments.

              ACTIN NUCLEATORS

             ┌───────────────┐
             │               │
          Arp2/3          Formins
             │               │
             ↓               ↓
       Branched network   Linear filaments

10. Arp2/3 Complex

The Arp2/3 complex contains seven protein subunits, including Arp2 and Arp3.

It nucleates new actin filaments from the sides of existing filaments.

The resulting branches are typically approximately 70° from the mother filament.

Mother filament
────────────────────────
        \
         \
          \
           \
            ↓
         Branch

This is important in:

Lamellipodia
Endocytosis
Cell migration
Membrane protrusion

11. Formins

Formins promote nucleation and elongation of linear actin filaments.

They are particularly important in:

Stress fibers
Filopodia
Cytokinetic structures
Contractile actomyosin systems

Simplified:

Formin
  ↓
Linear actin nucleation
  ↓
Processive elongation
  ↓
Long actin filament

12. Actin Treadmilling

A major concept in actin biology is treadmilling.

At steady state:

Actin is added preferentially at the plus end.
Actin is lost preferentially at the minus end.

        PLUS END
           ↑
       G-actin added
           │
●—●—●—●—●—●—●—●
           │
       G-actin lost
           ↓
        MINUS END

The filament can therefore maintain approximately constant length while its subunits move through it.

13. ATP-Actin and ADP-Actin

New actin monomers generally bind ATP.

After incorporation into the filament:

ATP → ADP + Pi

The nucleotide state influences filament stability.

Simplified:

ATP-actin
    ↓
Polymerization
    ↓
F-actin-ATP
    ↓
ATP hydrolysis
    ↓
ADP-Pi actin
    ↓
Pi release
    ↓
ADP-actin

14. Actin ATP Hydrolysis

Actin’s nucleotide hydrolysis is not primarily used like ATP hydrolysis in a motor protein.

Instead, ATP hydrolysis changes the biochemical properties and stability of actin subunits within the filament.

This helps generate differences between older and newly assembled filament regions.

15. Dynamic Instability vs Actin Treadmilling

Do not confuse actin treadmilling with the dynamic instability characteristic of microtubules.

Actin

Commonly exhibits:

Treadmilling

Microtubules

Characteristically exhibit:

Dynamic instability

ACTIN
Addition (+) → filament → loss (−)

MICROTUBULE
Growth ↔ catastrophe ↔ shrinkage ↔ rescue

16. Actin-Binding Proteins

Actin filaments are regulated by numerous actin-binding proteins (ABPs).

They control:

Nucleation
Polymerization
Depolymerization
Bundling
Branching
Severing
Capping
Cross-linking

                 ACTIN
                   │
      ┌────────────┼────────────┐
      ↓            ↓            ↓
   Nucleation    Capping      Severing
      ↓            ↓            ↓
   Branching    Stability    Remodeling

17. Profilin

Profilin is an actin-binding protein that promotes actin assembly by interacting with G-actin.

It can facilitate:

ATP-actin formation
Actin monomer delivery
Formin-mediated filament elongation

Conceptually:

G-actin
   ↓
Profilin
   ↓
Actin assembly
   ↓
F-actin

18. Thymosin β4

Thymosin β4 binds actin monomers and helps regulate the pool of available G-actin.

It can therefore act as an actin monomer-sequestering factor.

19. Capping Proteins

Capping proteins bind filament ends and regulate actin growth or shortening.

Two conceptual types:

Plus-end capping

Limits addition of new actin subunits.

Minus-end capping

Limits loss or addition at the minus end.

Cap
 ↓
●—●—●—●—●
         ↑
      Protected end

Capping allows cells to control filament length and organization.

20. Cofilin

Cofilin is a major actin-remodeling protein.

It preferentially interacts with older ADP-actin-rich filament regions.

Functions include:

Actin filament severing
Promotion of depolymerization
Recycling of actin subunits

Long filament
●—●—●—●—●—●—●
        ↓
      Cofilin
        ↓
Severing/remodeling
        ↓
Shorter filaments + monomers

21. ADF/Cofilin Family

ADF = Actin depolymerizing factor

ADF/cofilin proteins regulate actin turnover.

They are especially important during:

Cell migration
Membrane remodeling
Cytoskeletal recycling

22. Profilin vs Cofilin

Protein	Major function
Profilin	Promotes actin monomer utilization/polymerization
Cofilin	Promotes filament turnover and remodeling
Thymosin β4	Sequesters actin monomers
Capping proteins	Regulate filament ends
Arp2/3	Branched nucleation
Formins	Linear filament nucleation/elongation

23. Actin Networks

Actin filaments can be organized into different architectures.

Major forms include:

Branched networks
Parallel bundles
Contractile bundles
Cortical networks
Three-dimensional meshworks

ACTIN ARCHITECTURE

Branched       Parallel        Contractile
   \             |||||             ||||
────\──        |||||||          ═══════
─────\─        |||||||          ═══════

24. Lamellipodia

Lamellipodia are broad, sheet-like actin-rich protrusions at the leading edge of migrating cells.

They contain dense branched actin networks.

                 Direction of movement →
         
             ┌────────────────
            /  \ / \ / \ / \
Cell body  |  /\/\/\/\/\/\/
            \________________
                  ↑
             Lamellipodium

Arp2/3-mediated branching is particularly important.

25. Filopodia

Filopodia are thin, finger-like membrane protrusions containing parallel bundles of actin filaments.

They function in:

Environmental sensing
Cell migration
Cell adhesion
Guidance

Cell
████████
████████──────
████████────────
████████───────
       ↑
   Filopodia

Formins and actin-bundling proteins are important in filopodial formation.

26. Stress Fibers

Stress fibers are contractile actin structures found in many adherent cells.

They contain:

Actin filaments
Myosin II
Cross-linking proteins

They connect functionally with focal adhesions.

Actin ───────────────
       ↑
    Myosin II
       ↓
Contractility
       ↓
Focal adhesion

27. Actomyosin System

Actin interacts with myosin motor proteins to generate force.

Actin filament
════════════════════
     ↑       ↑
   Myosin  Myosin
     ↓       ↓
     Mechanical force

This produces:

Contraction
Cell movement
Cytokinesis
Muscle contraction

28. Myosin II

Myosin II is a major contractile motor.

It contains:

Motor heads
Neck region
Tail region

Its motor heads bind actin and use ATP hydrolysis to generate movement.

ATP
 ↓
Myosin conformational cycle
 ↓
Actin interaction
 ↓
Force generation
 ↓
Sliding

29. Actin–Myosin Sliding

The basic mechanism resembles muscle contraction.

Actin filament
════════════════════→

Myosin
   ↓
<======
   ↑
Movement

Relative sliding between actin filaments and myosin generates contractile force.

30. Muscle Contraction

In skeletal muscle, actin is associated with the thin filament.

Major components include:

Actin
Tropomyosin
Troponin

Thin filament
Actin + Tropomyosin + Troponin
          ↓
      Ca²⁺ signal
          ↓
      Myosin binding
          ↓
      Contraction

31. Cortical Actin

A dense actin network exists immediately beneath the plasma membrane.

This is called the:

cell cortex

Functions include:

Cell shape
Membrane stability
Cell polarity
Endocytosis
Exocytosis
Mechanical sensing

Plasma membrane
──────────────────
Actin cortex
══════════════════
Cytoplasm

32. Actin and Cell Shape

Actin networks allow cells to dynamically change shape.

For example:

Round cell
   ↓
Actin remodeling
   ↓
Protrusion
   ↓
Polarization
   ↓
Elongated/migrating cell

This is essential for:

Development
Immune responses
Wound healing
Cancer cell invasion

33. Actin in Cell Migration

Cell migration requires coordinated actin dynamics.

A simplified cycle:

1. Protrusion
       ↓
2. Adhesion
       ↓
3. Cell-body contraction
       ↓
4. Rear release
       ↓
5. Forward movement

Actin polymerization at the leading edge pushes the plasma membrane forward.

34. Rho Family GTPases and Actin

A major molecular regulatory system involves:

Rho
Rac
Cdc42

These small GTPases regulate different aspects of actin organization.

Simplified:

Rho
 ↓
Contractile actin structures

Rac
 ↓
Lamellipodia

Cdc42
 ↓
Filopodia/polarity

This is a simplified framework; their functions overlap and depend on cellular context.

35. Actin and Endocytosis

Actin contributes to membrane deformation during endocytosis.

Plasma membrane
      ↓
Membrane invagination
      ↓
Actin assembly
      ↓
Neck constriction/remodeling
      ↓
Vesicle formation

This is especially important in cells where membrane tension or cargo geometry makes actin assistance necessary.

36. Actin and Exocytosis

Actin also regulates secretory trafficking.

The actin cortex can act as:

A barrier
A trafficking platform
A structural organizer

Local actin remodeling can permit secretory vesicles to access the plasma membrane.

37. Actin and Vesicular Transport

Unlike microtubules, which often mediate long-range intracellular transport, actin is particularly important for short-range transport near the cell cortex.

Microtubules
       ↓
Long-range transport
       ↓
Cell periphery
       ↓
Actin network
       ↓
Short-range transport
       ↓
Plasma membrane

Myosin motors mediate movement along actin.

38. Actin–Myosin Transport

Different myosin proteins transport specific cargos.

Cargo
  │
Myosin
  │
Actin filament
════════════════════
       ↓
Cargo movement

This creates a functional relationship between:

actin → myosin → cargo transport

39. Actin in Cytokinesis

During animal-cell cytokinesis, actin and myosin form the:

contractile ring

        Cell
    ┌───────────┐
    │           │
    │  ═══════  │
    │  Actin +   │
    │  Myosin    │
    │  ring      │
    └───────────┘

The ring contracts and helps produce the cleavage furrow.

40. Cytokinetic Ring

Sequence:

Actin + Myosin II
       ↓
Contractile ring assembly
       ↓
Ring constriction
       ↓
Cleavage furrow
       ↓
Daughter-cell separation

This is a classic example of actomyosin-generated force.

41. Actin and Cell Adhesion

Actin interacts with cell-adhesion structures.

Cell–matrix adhesion

Actin connects functionally to integrins through focal adhesion complexes.

Cell–cell adhesion

Actin connects to cadherin-based adherens junctions.

Actin
  ↓
Adaptor proteins
  ↓
Adhesion complex
  ↓
Cadherin / Integrin
  ↓
Extracellular environment

42. Focal Adhesions

Focal adhesions connect:

extracellular matrix → integrins → intracellular proteins → actin cytoskeleton

ECM
 │
Integrin
 │
Focal adhesion proteins
 │
Actin
══════════════════

They function in:

Adhesion
Mechanotransduction
Migration
Signaling

43. Adherens Junctions

At adherens junctions:

Cadherins connect neighboring cells.

Intracellular adaptor proteins connect cadherins to actin.

Cell A                 Cell B
Actin                   Actin
  │                       │
Adaptor                 Adaptor
  │                       │
Cadherin ───────────── Cadherin
          ↑
      Cell-cell adhesion

44. Actin and Mechanotransduction

Actin networks can sense and transmit mechanical forces.

Mechanical force
      ↓
Adhesion complex
      ↓
Actin cytoskeleton
      ↓
Conformational/signaling changes
      ↓
Cellular response

This process is known as mechanotransduction.

45. Actin and Cell Polarity

Actin organization contributes to establishment and maintenance of cell polarity.

This is particularly important in:

Epithelial cells
Migrating cells
Neurons
Developing tissues

Cell polarity
      ↓
Localized signaling
      ↓
Localized actin remodeling
      ↓
Directed membrane trafficking
      ↓
Polarized cell behavior

46. Actin in Neurons

Actin is highly abundant in neuronal structures including:

Growth cones
Dendritic spines
Axonal regions
Synaptic terminals

It contributes to:

Neurite extension
Synaptic plasticity
Spine remodeling
Membrane trafficking

47. Dendritic Spines

Dendritic spines contain dynamic actin networks.

Actin remodeling contributes to changes in spine:

Size
Shape
Stability

These changes are important in synaptic plasticity.

48. Actin in the Growth Cone

The neuronal growth cone uses actin-rich structures to sense environmental cues and direct axon growth.

Axon
───────→
          \  |  /
           \ | /
         Growth cone
        actin-rich

Filopodia and lamellipodia-like structures explore the extracellular environment.

49. Actin Cross-Linking Proteins

Cross-linking proteins organize individual actin filaments into networks or bundles.

Examples include:

α-actinin
Filamin
Fimbrin
Fascin

Different proteins generate different architectures.

Individual filaments
│ │ │ │ │ │
↓ ↓ ↓ ↓ ↓ ↓
Cross-linking
↓
Bundle/network

50. Bundles vs Networks

Parallel bundle

Filaments arranged in parallel.

Common in:

Filopodia

Contractile bundle

Antiparallel actin filaments with myosin.

Common in:

Stress fibers
Contractile structures

Branched network

Dense branched architecture.

Common in:

Lamellipodia

51. Major Actin Structures

Structure	Organization	Major function
Lamellipodium	Branched network	Cell migration
Filopodium	Parallel bundles	Sensing/protrusion
Stress fiber	Contractile bundles	Tension/adhesion
Cortex	Dense meshwork	Cell shape
Contractile ring	Actomyosin ring	Cytokinesis
Microvilli	Parallel bundles	Surface-area expansion

52. Microvilli

Microvilli contain parallel actin bundles.

They increase the surface area of cells such as intestinal epithelial cells.

              Microvilli
            ↑ ↑ ↑ ↑ ↑ ↑
            │ │ │ │ │ │
────────────┴─┴─┴─┴─┴─┴────
             Cell

Actin filaments provide structural support.

53. Actin and Membrane Curvature

Actin polymerization can generate mechanical forces against the plasma membrane.

Actin polymerization
        ↓
Mechanical force
        ↓
Membrane deformation
        ↓
Protrusion

Conversely, membrane curvature can also influence actin assembly.

Thus, actin and membrane geometry form a bidirectional feedback system.

54. Actin–Membrane Feedback

A sophisticated concept:

Membrane signal
      ↓
Actin assembly
      ↓
Force generation
      ↓
Membrane deformation
      ↓
New signaling environment
      ↓
Further actin remodeling

This feedback is important in migration, endocytosis and cell morphogenesis.

55. Regulation by Rho GTPases

RhoA

Promotes:

Stress fibers
Actomyosin contractility
Contractile structures

Rac1

Promotes:

Lamellipodia
Branched actin networks

Cdc42

Promotes:

Filopodia
Polarity signaling

          RHO FAMILY
              │
     ┌────────┼────────┐
     ↓        ↓        ↓
   RhoA      Rac1     Cdc42
     ↓        ↓        ↓
Contractile  Lamelli-  Filopodia/
structures   podia     polarity

56. Actin Regulation by Phosphoinositides

Phosphoinositides in the plasma membrane can recruit actin-regulatory proteins.

For example, PIP2 participates in regulation of several actin-binding proteins and membrane-associated signaling processes.

PIP2-rich membrane
        ↓
Actin regulators
        ↓
Local actin assembly
        ↓
Membrane remodeling

57. Actin and Membrane Trafficking

Actin interacts with the trafficking systems discussed previously.

Rab
 ↓
Vesicle targeting
 ↓
Actin-associated machinery
 ↓
Myosin
 ↓
Short-range transport
 ↓
SNARE-mediated fusion

Therefore, actin is an important component of the broader membrane-trafficking network.

58. Actin and SNARE-Dependent Exocytosis

At the cell cortex:

Secretory vesicle
       ↓
Rab
       ↓
Actin/myosin-dependent positioning
       ↓
Tethering
       ↓
SNARE complex
       ↓
Fusion

Actin can therefore influence where and when secretory vesicles reach the plasma membrane, while SNAREs execute fusion.

59. Actin vs Microtubules

Feature	Actin	Microtubules
Diameter	~7 nm	~25 nm
Monomer	Actin	Tubulin
Motor	Myosin	Kinesin/dynein
Major ATP/GTP	ATP-actin	GTP-tubulin
Polarity	Yes	Yes
Typical role	Cortex, movement, contraction	Long-range transport, mitosis
Major dynamic behavior	Treadmilling	Dynamic instability
Cytokinesis	Contractile ring	Spindle

60. Actin vs Intermediate Filaments

Feature	Actin	Intermediate filaments
Diameter	~7 nm	~10 nm
Major role	Movement/dynamics	Mechanical strength
Polarity	Polar	Generally nonpolar
Motors	Myosin	No conventional motor
Major function	Remodeling	Structural resilience

61. Actin and Disease

Abnormal actin regulation is associated with:

Cancer invasion
Metastasis
Cardiovascular disease
Neurological disorders
Immunological dysfunction
Developmental abnormalities
Muscle diseases

Cancer cells, for example, frequently remodel actin to increase migration and invasion.

Altered actin regulation
       ↓
Cell polarity/motility changes
       ↓
Increased migration
       ↓
Potential invasion/metastasis

62. Actin and Pathogens

Some pathogens manipulate host actin.

They can induce:

Actin polymerization
Cytoskeletal rearrangement
Cellular invasion
Intracellular movement

This demonstrates that actin is also an important interface between host cells and pathogens.

63. Integrated Actin Regulatory Network

                  EXTRACELLULAR SIGNAL
                         │
                         ↓
                   Rho GTPases
                         │
            ┌────────────┼────────────┐
            ↓            ↓            ↓
          RhoA          Rac1        Cdc42
            │            │            │
            ↓            ↓            ↓
       Contractility  Lamellipodia  Filopodia
            │            │            │
            └────────────┼────────────┘
                         ↓
                  ACTIN REMODELING
                         │
          ┌──────────────┼──────────────┐
          ↓              ↓              ↓
       Migration      Adhesion      Endocytosis
          │              │              │
          └──────────────┼──────────────┘
                         ↓
                  Cellular behavior

64. Master-Level Concept: Actin Is a Dynamic Material

The actin cytoskeleton should not be considered merely a collection of rigid rods.

It behaves as a dynamic, viscoelastic network whose:

Filament density
Connectivity
Cross-linking
Polymerization
Contractility
Attachment to membranes

can change rapidly.

This allows cells to adapt mechanically and structurally to their environment.

65. Master-Level Concept: Polymerization Generates Force

Actin polymerization can itself generate mechanical force.

G-actin
  ↓
Polymerization
  ↓
F-actin elongation
  ↓
Membrane pushing
  ↓
Cell protrusion

This principle is fundamental to:

Lamellipodia formation
Cell migration
Phagocytosis
Intracellular pathogen movement

66. Master-Level Concept: Actomyosin Generates Contractile Force

Polymerization is not the only mechanism by which actin generates force.

The actin–myosin system produces active contractility.

Actin
  +
Myosin II
  ↓
ATP hydrolysis
  ↓
Filament sliding
  ↓
Contractile force

Thus, two major mechanical strategies are:

Actin polymerization → protrusive force

Actomyosin interaction → contractile force

67. Master-Level Concept: Actin Homeostasis

Cells must maintain an appropriate balance between:

G-actin ↔ F-actin

G-actin
   ⇅
F-actin

This is controlled by:

Profilin
Thymosin β4
Cofilin
Capping proteins
Nucleators
Severing proteins
Actin-depolymerizing factors

Disruption of this balance can profoundly alter cellular morphology.

68. High-Yield Examination Summary

Actin exists as:

G-actin → monomer

F-actin → filament

Actin filaments are:

Polar

Major nucleators:

Arp2/3 → branched networks

Formins → linear filaments

Major regulatory proteins:

Profilin → promotes monomer utilization

Cofilin → turnover/severing

Capping proteins → regulate filament ends

Major motor:

Myosin

Major structures:

Lamellipodia, filopodia, stress fibers, cortex, contractile ring, microvilli

Major signaling regulators:

Rho, Rac, Cdc42

69. Examination Short Note

Actin Cytoskeleton

The actin cytoskeleton is a highly dynamic network of approximately 7-nm actin filaments that plays essential roles in cell shape, migration, adhesion, intracellular trafficking, endocytosis, exocytosis, cytokinesis and mechanotransduction. Actin exists as soluble G-actin and polymerized F-actin. F-actin is a polar filament with distinct plus and minus ends and undergoes regulated polymerization, depolymerization and treadmilling.

Actin nucleation is regulated by proteins such as the Arp2/3 complex, which generates branched networks, and formins, which promote linear filament assembly. Actin dynamics are controlled by numerous actin-binding proteins, including profilin, cofilin, capping proteins and cross-linking proteins. Actin interacts with myosin motors to generate mechanical force and contractility.

Different actin architectures perform specialized functions. Branched actin networks form lamellipodia, parallel bundles form filopodia and microvilli, contractile actomyosin bundles form stress fibers, and actin–myosin II forms the cytokinetic contractile ring. Rho-family GTPases, particularly RhoA, Rac1 and Cdc42, provide major upstream regulatory control.

The actin cytoskeleton also interacts with membrane-trafficking machinery, including Rab GTPases, myosin motors and SNARE-dependent exocytosis. Its dynamic nature allows cells to rapidly modify their architecture in response to biochemical and mechanical signals.

70. Viva Questions

Q1. What is the diameter of an actin filament?
Approximately 7 nm.

Q2. What is G-actin?
Globular, soluble actin monomer.

Q3. What is F-actin?
Filamentous polymerized actin.

Q4. Is an actin filament polar?
Yes; it has plus and minus ends.

Q5. What is treadmilling?
Addition of actin subunits preferentially at one end with loss at the other, allowing subunit flux through a filament.

Q6. Which complex produces branched actin networks?
Arp2/3 complex.

Q7. Which proteins promote linear actin filament formation?
Formins.

Q8. What is the function of cofilin?
Actin filament remodeling, severing and depolymerization.

Q9. Which motor protein interacts with actin?
Myosin.

Q10. What forms the contractile ring during cytokinesis?
Actin and myosin II.

Q11. Which Rho-family GTPase promotes lamellipodia?
Rac1.

Q12. Which promotes stress fibers and contractility?
RhoA.

Q13. Which is associated with filopodia formation and polarity?
Cdc42.

Q14. What is a lamellipodium?
A broad, actin-rich protrusion containing a branched actin network.

Q15. What is a filopodium?
A thin membrane protrusion containing parallel actin bundles.

71. One-Minute Revision Diagram

                         ACTIN
                           │
             ┌─────────────┴─────────────┐
             ↓                           ↓
          G-ACTIN                     F-ACTIN
          monomer                     filament
             │                           │
             │                      Polar (+ / −)
             │                           │
             ↓                           ↓
       Polymerization              Treadmilling
             │                           │
             └─────────────┬─────────────┘
                           ↓
                    ACTIN REMODELING
                           │
          ┌────────────────┼────────────────┐
          ↓                ↓                ↓
      Arp2/3             Formin           Cofilin
          ↓                ↓                ↓
     Branched           Linear          Turnover
       network          filaments
          │                │                │
          └────────────────┼────────────────┘
                           ↓
                      CELLULAR FORCE
                           │
             ┌─────────────┴─────────────┐
             ↓                           ↓
       Polymerization                Myosin
          force                    contractility
             │                           │
             ↓                           ↓
       Protrusion                   Contraction
             │                           │
             └─────────────┬─────────────┘
                           ↓
            Migration / Adhesion /
       Endocytosis / Cytokinesis /
          Mechanotransduction

Core memory rule

Arp2/3 = branches
Formin = linear filaments
Profilin = actin assembly
Cofilin = actin turnover
Myosin = force
Rac = lamellipodia
Cdc42 = filopodia/polarity
RhoA = contractility/stress fibers