Actin Dynamics Quiz
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Questions and Answers

Thymosin-b4 sequesters G-actin, making it polymerisation-______

incapable

Actin assembly is primarily facilitated by ______

formins

Capping proteins bind to the ends of the actin filaments to regulate G-actin ______ and F-actin growth.

addition

The critical concentration for ATP-actin differs at the two ends, with values of 0.1 µM at the ______ end and 0.6 µM at the plus end.

<p>minus</p> Signup and view all the answers

F-actin ends bind to ______ proteins, providing one mechanism to regulate actin assembly.

<p>capping</p> Signup and view all the answers

The actin-binding protein with over 100 times affinity for ATP-actin than ADP-actin is called ______-thymosin.

<p>beta</p> Signup and view all the answers

In the context of actin dynamics, ______ refers to the phenomenon where filament growth occurs at one end while disassembly happens at the other.

<p>treadmilling</p> Signup and view all the answers

ERM proteins link actin filaments to the ______ membrane.

<p>plasma</p> Signup and view all the answers

G-actin monomers polymerize to form ______ filaments during actin polymerization.

<p>F-actin</p> Signup and view all the answers

The phases of actin polymerization include nucleation, ______, and steady state.

<p>elongation</p> Signup and view all the answers

Cofilin is a type of ______ protein that plays a role in actin filament turnover.

<p>actin-binding</p> Signup and view all the answers

The ______ mechanism describes the dynamic turnover of actin filaments where the growth at the plus end is balanced by disassembly at the minus end.

<p>treadmilling</p> Signup and view all the answers

Regulatory proteins involved in actin assembly include members of the Rho-family such as Cdc42 and ______.

<p>Rac</p> Signup and view all the answers

When G-actin binds ATP, it is referred to as ______ actin.

<p>ATP-bound</p> Signup and view all the answers

The polymerization of actin leads to the formation of ______ filaments.

<p>F-actin</p> Signup and view all the answers

The basic building block of actin filaments is known as ______.

<p>G-actin</p> Signup and view all the answers

Actin polymerization occurs in three phases: nucleation, ______, and steady state.

<p>elongation</p> Signup and view all the answers

The filamentous form of actin is referred to as ______.

<p>F-actin</p> Signup and view all the answers

The ______ end of the actin filament is where elongation is favored.

<p>barbed</p> Signup and view all the answers

Actin polymerization goes through phases, starting with ______ nucleation.

<p>de novo</p> Signup and view all the answers

In the process of treadmilling, there is a net addition of subunits at the barbed end and a net loss at the ______ end.

<p>pointed</p> Signup and view all the answers

The ______ proteins help regulate the concentrations of actin monomers and filaments.

<p>actin-binding</p> Signup and view all the answers

Profilin is a regulatory protein that binds G-actin in the ______ state.

<p>ATP</p> Signup and view all the answers

The mechanism where actin filaments grow at one end while simultaneously disassembling at the other is known as ______.

<p>treadmilling</p> Signup and view all the answers

The critical concentration for the barbed end is ______ than that for the pointed end.

<p>lower</p> Signup and view all the answers

Small GTPases such as rac, rho, and cdc42 are considered ______ proteins in actin assembly.

<p>regulatory</p> Signup and view all the answers

Too much or too little profilin can be detrimental to ______ formation.

<p>filament</p> Signup and view all the answers

At the ______ end of the actin filament, the critical concentration is lower, allowing for more dynamic growth.

<p>plus</p> Signup and view all the answers

Actin-binding proteins, such as ______, help regulate actin assembly by controlling G-actin concentration.

<p>thymosin-b4</p> Signup and view all the answers

The ______ end of the actin filament has a higher critical concentration and grows more slowly.

<p>minus</p> Signup and view all the answers

ATP hydrolysis converts ATP-actin into ______-actin, which is in a depolymerization state.

<p>ADP</p> Signup and view all the answers

Actin filaments exhibit a right-handed double-helical ______.

<p>polymer</p> Signup and view all the answers

During actin assembly, fragments of ______ can be used to demonstrate filament polarity.

<p>myosin</p> Signup and view all the answers

G-actin is a globular protein that polymerizes to form ______.

<p>F-actin</p> Signup and view all the answers

Actin-binding proteins regulate actin filament dynamics, including cofilin and ______.

<p>profilin</p> Signup and view all the answers

The process by which G-actin is incorporated into F-actin while subunits dissociate at the ______ end is known as treadmilling.

<p>pointed</p> Signup and view all the answers

Regulatory proteins such as Rho, Rac, and cdc42 are essential for the ______ of actin structures in response to signals.

<p>regulation</p> Signup and view all the answers

WASP is a nucleation-promoting factor that activates the ______ complex for actin polymerization.

<p>Arp2/3</p> Signup and view all the answers

Stress fibers are composed of ______, which provides mechanical support and structure to cells.

<p>F-actin</p> Signup and view all the answers

Actin depolymerizing factor (ADF) binds to ______-actin to promote disassembly of filaments.

<p>F</p> Signup and view all the answers

During cell migration, the formation of ______ at the leading edge is essential for movement.

<p>lamellipodia</p> Signup and view all the answers

The activation of the Arp2/3 complex is influenced by specific ______ that bind to it.

<p>proteins</p> Signup and view all the answers

What is the primary role of the Arp2/3 complex in actin dynamics?

<p>It nucleates new actin filaments and links them to existing ones. (C)</p> Signup and view all the answers

Which Rho-family GTPase is primarily associated with lamellipodium formation during cell movement?

<p>Rac (C)</p> Signup and view all the answers

How does cofilin contribute to actin dynamics?

<p>By disassembling actin filaments through the binding to ADP-actin. (D)</p> Signup and view all the answers

What aspect of actin dynamics is influenced by profilin?

<p>It enhances the exchange of ADP for ATP on G-actin. (A)</p> Signup and view all the answers

In the dendritic nucleation model, which proteins play a critical role in coordinating actin filament growth?

<p>Scar/WAVE and Arp2/3 complex (D)</p> Signup and view all the answers

What is a primary function of the actin cytoskeleton in cells?

<p>Cell motility (C)</p> Signup and view all the answers

Which class of actin is primarily found in non-muscle cells?

<p>Class 1 - Non-muscle b, g and smooth muscle g-actin (C)</p> Signup and view all the answers

What specific role does b-actin play in non-muscle cells?

<p>Facilitating membrane ruffles (B)</p> Signup and view all the answers

Which of the following structures are generated through actin assembly?

<p>Lamellipodia (B)</p> Signup and view all the answers

What is the composition of microvilli with respect to actin?

<p>Contain bundles of actin filaments stabilized by cross-linking proteins (B)</p> Signup and view all the answers

What happens to b-actin levels in relation to cell movement?

<p>Increases alongside increased cell movement (B)</p> Signup and view all the answers

Which of the following is NOT a transient actin structure?

<p>Microvilli (C)</p> Signup and view all the answers

How does actin contribute to cell polarity?

<p>By organizing cell components at distinct regions (D)</p> Signup and view all the answers

What characterizes the dynamics of actin assembly at the plus and minus ends of the filament?

<p>The plus end has a lower critical concentration than the minus end. (A)</p> Signup and view all the answers

Which proteins are primarily responsible for cross-linking actin filaments?

<p>Villin, fimbrin, and espin (D)</p> Signup and view all the answers

What is the result of ATP hydrolysis on ATP-actin during polymerization?

<p>It shifts ATP-actin to a depolymerization state known as ADP-actin. (B)</p> Signup and view all the answers

Which mechanism allows for the dynamic turnover of actin filaments?

<p>Treadmilling (A)</p> Signup and view all the answers

What effect do small GTPases like Rac, Rho, and Cdc42 have on actin assembly?

<p>They alter the dynamics of actin assembly in response to cell signals. (A)</p> Signup and view all the answers

Which statement correctly describes the formation of new actin filaments?

<p>De novo nucleation, uncapping, and existing filament elongation contribute to filament formation. (C)</p> Signup and view all the answers

What distinguishes the plus end from the minus end of an actin filament?

<p>The kinetics of growth differ at each end. (C)</p> Signup and view all the answers

What occurs at both ends of actin filaments during assembly?

<p>Subunits can be added and dissociated. (B)</p> Signup and view all the answers

What is the primary function of capping proteins in actin dynamics?

<p>They inhibit elongation at the barbed-end. (A)</p> Signup and view all the answers

How does thymosin-b4 affect G-actin?

<p>It sequesters G-actin, making it polymerization-inactive. (A)</p> Signup and view all the answers

Which characteristic of actin binding proteins indicates their functional diversity?

<p>Over 50 types of actin-binding proteins are known. (D)</p> Signup and view all the answers

What happens when the barbed end of an actin filament becomes uncapped?

<p>A high affinity actin binding site becomes available. (D)</p> Signup and view all the answers

Which protein plays a critical role in forming a cross-linked actin network at the cell cortex?

<p>Filamin (B)</p> Signup and view all the answers

What distinguishes the critical concentration of ATP-actin at the two ends of the filament?

<p>It is lower at the plus end. (C)</p> Signup and view all the answers

Which GTPase is primarily associated with the formation of lamellipodia?

<p>Rac (C)</p> Signup and view all the answers

What effect does C3 transferase have on Rho?

<p>It inactivates Rho. (A)</p> Signup and view all the answers

What type of proteins are considered key regulators of actin assembly and dynamics?

<p>Actin-binding proteins (C)</p> Signup and view all the answers

What role does the Arp2/3 complex play in actin dynamics?

<p>Nucleating new actin filaments. (A)</p> Signup and view all the answers

What effect does profilin have on G-actin?

<p>It binds to G-actin and promotes its assembly into filaments. (B)</p> Signup and view all the answers

Which of the following is a characteristic of the Arp2/3 complex?

<p>It activates through nucleation promoting factors. (C)</p> Signup and view all the answers

Which actin-binding protein is known for enhancing actin depolymerization?

<p>Cofilin (D)</p> Signup and view all the answers

Which cell surface receptors are known to signal for Arp2/3 activation?

<p>Receptor tyrosine kinases (A)</p> Signup and view all the answers

What is the main effect of the small GTPases Rho, Rac, and Cdc42 on actin mechanisms?

<p>They regulate actin assembly through signaling. (C)</p> Signup and view all the answers

What is one of the primary methods by which pathogens hijack the actin machinery?

<p>By promoting rapid filament assembly. (A)</p> Signup and view all the answers

Which characteristic of actin filaments contributes to their dynamic nature during processes like cell migration?

<p>Treadmilling enables continual turnover of actin. (C)</p> Signup and view all the answers

What is the primary function of the Arp2/3 complex in relation to existing actin filaments?

<p>It binds laterally to promote branching. (B)</p> Signup and view all the answers

What role does profilin play in the actin polymerization process?

<p>It reduces the critical concentration of ATP-actin. (C)</p> Signup and view all the answers

Which of the following statements about actin polymerization is true?

<p>ATP hydrolysis results in the formation of ADP-actin. (A)</p> Signup and view all the answers

What is the impact of regulatory proteins like thymosin-b4 on G-actin levels?

<p>They sequester G-actin, maintaining a high concentration in the cell. (B)</p> Signup and view all the answers

During treadmilling, what occurs at the barbed and pointed ends of actin filaments?

<p>Subunits are lost at the pointed end while added at the barbed end. (A)</p> Signup and view all the answers

Which statement best describes the critical concentration of actin at the barbed end compared to the pointed end?

<p>It is lower at the barbed end than at the pointed end. (D)</p> Signup and view all the answers

Which property of profilin makes it unique among actin-binding proteins?

<p>It acts as a nucleotide exchange factor. (C)</p> Signup and view all the answers

Why is it critical to maintain a high concentration of G-actin in cells like fibroblasts?

<p>To ensure there is a sufficient pool for assembly. (D)</p> Signup and view all the answers

What effect does ATP hydrolysis have on the state of actin?

<p>It leads to the formation of ADP-actin, promoting depolymerization. (B)</p> Signup and view all the answers

Flashcards

Actin assembly regulation

Actin assembly in cells is controlled by proteins that bind to actin filaments and subunits at both ends, impacting growth and disassembly.

Capping proteins

Proteins that bind to the barbed end of actin filaments, preventing further actin addition and thus regulating filament growth.

Thymosin-β4

A protein that sequesters G-actin (free actin monomers), preventing polymerization and thus regulating actin assembly.

Critical concentration

The concentration of G-actin required for net actin filament growth at a particular end.

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Treadmilling

Actin filament growth at one end and disassembly at the other, maintaining a steady-state.

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Actin-binding proteins

A diverse group of proteins that interact with actin in various ways, controlling assembly, crosslinking, and severing.

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G-actin

Globular actin, the monomeric form of actin.

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F-actin

Filamentous actin, the polymerized form of actin.

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Actin Polymerization

The process of actin monomers assembling into filaments.

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Barbed End (+ end)

The end of an actin filament where growth is faster.

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Profilin

A protein that binds to G-actin and regulates its assembly.

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ATP Hydrolysis

The breakdown of ATP (adenosine triphosphate) to ADP.

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Actin Structures

Filaments that form different structures in cells, including filopodia, lamellipodia, ruffles, and stress fibers.

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Rho GTPases

Small proteins that regulate actin organization, including Rac, Cdc42, and Rho.

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Rac

A Rho GTPase that promotes lamellipodium formation.

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Arp2/3 complex

A protein complex that nucleates actin filament branching, crucial for cell movement and shape changes.

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Nucleation-promoting factors

Proteins that activate the Arp2/3 complex, promoting actin filament assembly.

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Actin depolymerizing factor/cofilin

Proteins that regulate actin filament disassembly and sever actin filaments.

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Actin assembly at leading edge

The process of actin filament growth at the front of a moving or growing cell, crucial for motility.

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Signal-induced changes in actin cytoskeleton

Changes in actin structure caused by external signals, orchestrating cell response and movement.

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Cell surface receptors

Proteins on the cell surface detecting external signals, triggering actin cytoskeleton changes.

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Listeria hijacking actin

Bacteria use cell's actin machinery to propel themselves, causing infection.

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Dendritic nucleation model

A model that describes how actin filaments assemble into a branched network. It involves the Arp2/3 complex nucleating new filaments at the barbed ends of pre-existing filaments, forming Y-junctions.

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Actin dynamics in cell movement

Actin filament assembly and disassembly play a crucial role in cell movement. Rho-family GTPases like Cdc42, Rac, and Rho control actin dynamics, influencing protrusion formation, adhesion, and retraction.

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Actin Filament Structure

Actin filaments are long, thin, fibrous polymers made of globular actin monomers. They are crucial for cell structure and movement.

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Actin Filament Polarity

Actin filaments have distinct ends: a 'plus' end (+ end) where assembly is faster, and a 'minus' end (- end) where it is slower.

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Actin Filament Decoration

Myosin (S1) fragments bind to actin filaments in a specific way, creating an 'arrowhead' pattern. This allows us to see filament polarity.

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Actin Assembly in Cells

Actin assembly in cells involves multiple proteins that regulate the rate of filament formation and length.

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De Novo Nucleation

A fresh start! New actin filaments can be created from scratch by assembling individual actin monomers into trimers.

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Uncapping

It's like removing a cap! Some proteins might bind to the ends of actin filaments, blocking growth. Removing these proteins allows further assembly.

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Bundling Proteins

These proteins act like glue, holding actin filaments together in bundles or networks. This creates the structure needed for things like cell movement.

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What are actin microfilaments?

Actin microfilaments are thin, fibrous structures composed of the protein actin. They are key for cell movement, shape, and internal organization.

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Actin Isoforms

There are different forms of actin, called isoforms, which are specialized for different functions. For example, α-actin is found in muscle, while β-actin is involved in cell movement.

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Where is actin found in the cell?

Actin is abundant throughout the cell but is particularly concentrated at the cell's periphery, creating a sub-plasmalemmal array. This array is important for cell movement and shape changes.

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What are lamellipodia?

Lamellipodia are thin, sheet-like extensions of the cell membrane, driven by actin polymerization. They play a crucial role in cell movement and migration.

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What are filopodia?

Filopodia are thin, finger-like projections of the cell membrane that are also driven by actin polymerization. They are involved in sensing the environment and cell adhesion.

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What are microvilli?

Microvilli are small, finger-like projections of the cell membrane that are supported by bundles of actin filaments. They increase the surface area of cells, important for absorption in tissues like the intestine.

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What are focal adhesions?

Focal adhesions are specialized structures where actin filaments attach to the cell's cytoskeleton and to the extracellular matrix. They are crucial for cell adhesion and migration.

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What are stress fibers?

Stress fibers are bundles of bundled actin filaments that provide structural support and generate tension within the cell. They contribute to cell shape and can be involved in transmitting forces.

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ATP Cap

The end of an actin filament where ATP-bound actin monomers are located. It is the growth end of the filament and is essential for maintaining a steady-state filament length.

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What does profilin do?

Profilin binds to G-actin and ATP, promoting the exchange of ADP for ATP and lowering the critical concentration for filament growth. It also facilitates the addition of actin monomers to the barbed-end of filaments.

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Why is G-actin maintained in cells?

Cells maintain a high concentration of G-actin because it allows for rapid polymerization and depolymerization in response to changing cellular needs.

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What is sequestration?

A process of binding and holding G-actin monomers in a reserve pool, preventing their incorporation into filaments. It is important for regulating actin assembly and maintaining a pool of available monomers.

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Profilin's double act

Profilin has two key roles: it promotes the exchange of ADP for ATP on G-actin monomers, preparing them for assembly and binds to ATP-actin and helps it add to the barbed end, making filament growth more efficient.

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How does profilin impact assembly?

Profilin lowers the critical concentration for filament growth, promoting assembly at the barbed end. However, too much or too little profilin can disrupt filament formation.

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Actin Assembly: De Novo Nucleation

A fresh start! New actin filaments can be created from scratch - individual actin monomers assemble into trimers, which then elongate.

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Actin Assembly: Uncapping

Like removing a cap! Some proteins block filament growth by binding to the ends. Removing these proteins allows further assembly.

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Actin Filament Bundling

Actin filaments can be cross-linked together to form bundles or networks by proteins like villin, fimbrin, and espin.

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ATP-ADP Hydrolysis in Actin Assembly

Actin assembly requires energy, which is provided by the process of ATP hydrolysis (breaking down ATP to ADP) in the actin subunits.

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Actin Binding Proteins: Regulators of Assembly

Actin binding proteins control the ratio of monomeric actin (G-actin) to filamentous actin (F-actin) by exploiting differences in assembly characteristics at each end.

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What are formins?

Formins are proteins that help to assemble actin filaments, particularly promoting filament growth at the barbed end by acting as nucleators.

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Filamin

A protein that acts as a cross-linker, connecting actin filaments into a network. This organization is crucial for the structural integrity of cells.

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ERM Proteins

A group of proteins (Ezrin, Radixin, Moesin) that link actin filaments to the plasma membrane, helping to anchor the cytoskeleton and regulate cell shape and movement.

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What does Rac do?

Rac is a Rho GTPase that promotes the formation of lamellipodia, sheet-like extensions that drive cell movement.

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What is the role of Rac in actin regulation?

Rac is a small GTPase that promotes the formation of lamellipodia, sheet-like extensions of a cell's plasma membrane, crucial for cell migration.

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What is the Arp2/3 complex?

The Arp2/3 complex is a protein complex that nucleates actin filament branching, essential for cell movement and shape changes.

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What are NPFs?

Nucleation-promoting factors (NPFs) are proteins that activate the Arp2/3 complex, stimulating actin filament assembly.

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How is the Arp2/3 complex activated?

The Arp2/3 complex is activated by signaling pathways involving cell surface receptors, Rho-family GTPases (cdc42, Rac, Rho), and WASP/Scar(WAVE) proteins.

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What is the role of ADF/cofilin in actin regulation?

Actin depolymerizing factor (ADF)/cofilin binds to both F-actin (filamentous actin) and G-actin (globular actin), promoting filament disassembly and severing.

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How do parasites exploit actin machinery?

Parasites like Listeria and Shigella hijack the cell's actin machinery, using it to build 'tails' that propel them through the cell and facilitate infection.

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What is the purpose of actin assembly at the leading edge of a cell?

Actin assembly at the leading edge of a cell, driven by the Arp2/3 complex, helps the cell extend outwards, allowing it to explore its environment and move.

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What are the two main mechanisms of actin filament assembly?

Actin filaments can assemble through 'de novo nucleation' (starting from scratch) or by 'uncapping' pre-existing filaments, allowing them to grow further.

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How does ATP hydrolysis contribute to actin dynamics?

ATP hydrolysis in actin subunits provides the energy required for both assembly and disassembly of filaments, controlling their length and dynamics.

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Study Notes

Actin Microfilaments

  • Actin is very abundant, comprising 20% of skeletal muscle mass and 5% of total non-muscle proteins.
  • Key functions include cell motility, contractility, shape changes, cytokinesis, cell polarity, phagocytosis, and macropinocytosis.
  • Actin assembly accompanies membrane ruffles and lamellipodia formation, crucial for cell movement in non-muscle cells.
  • The process of actin assembly initiation and regulation, as well as how actin drives cell movement, are areas of ongoing study.

Actin Proteins

  • Actin is a highly conserved protein with 375 amino acids, having a molecular weight of 43 kDa.
  • It's a globular monomer (G-actin) with two domains.
  • At least six isoforms of actin exist in mammals, each encoded by a separate gene.
  • Actin proteins are divided into three classes: Class 1 (non-muscle β, γ, and smooth muscle γ-actin), Class 2 (skeletal, cardiac, and vascular α-actin), and Class 3 (actin-RPV, centractin, and lower eukaryote actins).

Actin Isoform Localization

  • γ-actin is primarily located at the cell periphery (sub-plasmalemmal array).
  • α-actin is found in stress fibers.
  • β-actin is localized in membrane ruffles and is crucial for cell movement.
  • Increased β-actin often correlates with enhanced cell movement.

Actin Structures in Cells

  • Transient/labile structures: Lamellipodia, filopodia, ruffles (all actin-based).
  • Stable structures: Microvilli (bundles of actin filaments stabilized by cross-linking proteins).
  • Actin-associated junctions: Focal complexes, focal adhesions (these link the actin cytoskeleton to the extracellular matrix).

Actin Filaments - Control and Regulation

  • Actin filaments are polarized, with "+" and "-" ends exhibiting different kinetic properties.
  • Actin assembly requires energy from ATP-ADP nucleotide hydrolysis.
  • Actin binding proteins regulate the monomer-to-filament ratio, utilizing the differing properties of assembly at each end.
  • Cell signalling can alter actin assembly (e.g., small GTPases like rac/rho/cdc42).

Actin Assembly into Filaments

  • Pure actin spontaneously assembles into filaments, starting from ATP-bound G-actin.
  • ATP is hydrolyzed to ADP after polymerisation, leaving an ATP cap that lags behind.
  • ATP hydrolysis leads to an ADP-actin state, which is a depolymerization state.

Filament Elongation

  • Actin filament elongation is favored at the barbed (+)-end of the filament.
  • Critical concentrations of ATP-actin are lower at the plus end, driving primarily growth at this end.
  • Treadmilling is observed as a net addition of subunits to the barbed end and a net loss at the pointed end, creating a continuous flux through the filament.

Actin Treadmilling

  • ADP-actin depolymerizes at the minus end while ATP-actin polymerizes at the plus end.
  • The rate constants for these processes differ at each end, leading to a continuous turnover of actin subunits (treadmilling).

Formation of New Actin Filaments

  • New actin filaments form via three mechanisms: de novo nucleation, uncapping, and severing existing filaments.

Actin-Binding Proteins: Sequestering Proteins

  • Regulatory proteins like profilin, thymosin-β4, and capping protein control actin assembly, maintaining a pool of free G-actin.
  • Profilin binds G-actin/ATP, thus lowering the critical concentration, thereby limiting elongation at the plus end, by suppressing spontaneous nucleation, crucial for controlling actin assembly.

Actin-Binding Proteins: Polymer Modifying Proteins

  • Various actin-binding proteins like formins, cofilin, gelsolin, α-actinin, filamin, and tropomyosin regulate actin assembly, branching, severing, and bundling.
  • These proteins have crucial roles in different cell processes, including cell movement, the establishment of stress fibers, and the connections to the cell membrane and extracellular matrix.

Actin Organization in a Migrating Cell; Rho-GTPases

  • Rho-family GTPases like Rac, Cdc42, and Rho regulate actin organization in migrating cells, controlling stress fiber formation, leading to cell movement.

Regulation of Actin State in Cells

  • Actin within cells is in constant flux, with its configuration changes controlled locally through signaling pathways.
  • Signaling cascades involving small GTPases (like Rac, Rho, and Cdc42) are key regulators.

How the Role of Rac/Rho/cdc42 in Actin Regulation Was Discovered

  • Experimentation with serum-starved Swiss 3T3 cells revealed the crucial role of these GTPases in regulating actin cytoskeletal dynamics and cell movement.
  • C3 transferase experiments further elaborated that the Rho-family proteins are essential in inducing actin-based cell movement by activating downstream signaling cascades.

Actin Assembly at Leading Edge; Arp2/3 Complex

  • Arp2/3 complex plays a particularly crucial role in promoting actin branching at cell leading edges, driving the development of lamellipodia.

Fish Keratocytes as Model Systems

  • Fish keratocytes are useful models for studying cell movement.
  • By combining video microscopy with immunoelectron microscopy, scientists can scrutinise the interplay between actin-related proteins and regulatory proteins during cell movement.

Spatial and Temporal Regulation in Response to External Stimuli

  • Signalling pathways involving cell surface receptors, Rho family GTPases (such as Cdc42, Rac, and Rho), and proteins like WASP/Scar (WAVE) mediate the temporal and spatial regulation of Arp2/3 activation.
  • Receptor tyrosine kinases (like PDGF and EGF) and PIP2 (phosphatidylinositol 4,5-biphosphate) are among the signalling molecules in these mechanisms.

Steps in Cell Movement

  • A series of steps including protrusion (extension via actin polymerization), adhesion, translocation of the cell body, followed by de-adhesion (and endocytic recycling of the old adhesion sites) are involved in cell movement.

Dendritic Actin Network

  • Arp2/3 complex promotes the generation of dendritic actin networks that enable cell protrusions and movement.
  • The structure forms complex branched, three-dimensional networks necessary for maintaining the dynamism of cells.

Actin Cross-linking Proteins

  • Actin cross-linking proteins include, but are not limited to, fimbrin, α-actinin, spectrin, and filamin.
  • These proteins regulate and influence cell morphology, strength, and stability by binding to and linking actin filaments.

Actin Associations at a Cell Edge

  • Actin at the leading edge of a cell demonstrates a combination of stress fiber connections toward the cell interior as well as a more loosely arranged actin network at the front of the cell, facilitating movement.
  • A complex interplay of many proteins dynamically regulates and integrates actin filament behavior within the lamellipodium and the cell body.

Actin Dynamics and Cell Protrusions

  • Actin dynamics and cell protrusions are controlled by a network of proteins interacting with cell surface receptors, Rho family GTPases, WASP/Scar proteins, the Arp2/3 complex, capping proteins, profilin, and cofilin.

Nucleation-promoting factors and rapid filament assembly at plasma membrane

  • Nucleation-promoting factors (NPFs) play a crucial role in initiating actin filament assembly at the cell membrane. Different regulatory factors, such as Arp2/3 and others, work in concert to drive rapid filament polymerization.

Hijacked by Parasites

  • Some parasites, such as Listeria, Shigella, and Helicobacter, exploit host cell actin machinery to promote their own infection. Specifically, they hijack the actin polymerization machinery to facilitate invasion and spread.

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L3 Actin Microfilaments PDF

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Test your knowledge on actin dynamics, including the roles of Thymosin-b4, capping proteins, and actin-binding proteins. This quiz covers various aspects of actin polymerization phases and their regulatory mechanisms.

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