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.</p> Signup and view all the answers

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

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

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

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

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

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

Which of the following structures are generated through actin assembly?

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

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

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

Which of the following is NOT a transient actin structure?

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

How does actin contribute to cell polarity?

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

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

<p>Villin, fimbrin, and espin</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.</p> Signup and view all the answers

Which mechanism allows for the dynamic turnover of actin filaments?

<p>Treadmilling</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.</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.</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.</p> Signup and view all the answers

What occurs at both ends of actin filaments during assembly?

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

How does thymosin-b4 affect G-actin?

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

Which GTPase is primarily associated with the formation of lamellipodia?

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

What effect does C3 transferase have on Rho?

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

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

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

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

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

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

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

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

<p>Receptor tyrosine kinases</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.</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.</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.</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.</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.</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.</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.</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.</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.</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.</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.</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.</p> Signup and view all the answers

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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Description

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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