ARP2/ARP3 and Actin Polymerisation

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Questions and Answers

What effect does profilin have on G-actin binding?

It accelerates the binding of ATP to G-actin. (correct)

It destabilizes F-actin to promote depolymerization.

It inhibits filament elongation.

It sweeps G-actin away from filaments.

What happens to actin filaments when the concentration of monomer is below Cc-?

Treadmilling occurs at a rapid pace.

Filaments disassemble or fail to form. (correct)

Filaments grow steadily at both ends.

Filaments become stable and cease to change.

Which actin-binding protein is responsible for destabilizing the minus end of actin filaments?

Profilin

Capping proteins

Cofilin (correct)

Thymosin-beta 4

What is the role of thymosin-beta 4 in actin polymerization regulation?

It sequesters G-actin to slow down polymerization. Signup and view all the answers

In which situation does treadmilling occur in actin filaments?

When monomer concentration is between Cc- and Cc+. Signup and view all the answers

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

It acts as a primer for actin polymerization. Signup and view all the answers

Which protein complex is crucial for the branching of the microfilament network?

Arp2/3 complex Signup and view all the answers

How do microfilaments maintain their dynamic properties?

By regulating monomer concentration. Signup and view all the answers

Which feature distinguishes microtubules from microfilaments?

Microtubules have a heterodimer structure of tubulin. Signup and view all the answers

What is a primary function of filamin in microfilament networks?

To crosslink microfilaments into a gel-like network. Signup and view all the answers

What structure in animal cells serves as the major microtubule organizing center?

Centrosome Signup and view all the answers

What is the effect of GTP binding on α-tubulin?

It stabilizes the microtubule structure. Signup and view all the answers

Which characteristic of microfilaments allows them to 'treadmill'?

They have different rates of polymerization at each end. Signup and view all the answers

What is the role of GTP in the dynamic instability of microtubules?

It ensures that beta-tubulin remains bound to the microtubule during growth. Signup and view all the answers

What happens to the microtubule when GTP bound to beta-tubulin is hydrolyzed to GDP?

The microtubule experiences a loss of the GTP cap and can fall apart. Signup and view all the answers

Which of the following statements about microtubules is correct?

They are dynamic structures that can grow and shrink. Signup and view all the answers

What is the function of microtubule-associated proteins (MAPs)?

They enable crosslinking of microtubules and interactions with other cellular components. Signup and view all the answers

How does min affect microtubule dynamics?

It binds to αβ tubulin dimers, reducing tubule elongation. Signup and view all the answers

What is the primary orientation factor for intracellular trafficking involving microtubules?

The microtubule organizing center (MTOC). Signup and view all the answers

What is a characteristic feature of microtubules?

They exhibit dynamic instability and can switch between growing and shrinking. Signup and view all the answers

What happens to tubulin dimers after their addition to a microtubule?

They are hydrolyzed to GDP, which can lead to destabilization. Signup and view all the answers

Study Notes

ARP2/ARP3 Mediated Nucleation of Actin Polymerisation

Arp2 and Arp3 share structural similarity with G-actin but lack the ability to polymerise.
The Arp2/3 complex functions as a nucleation primer for actin polymerisation.
Arp2/3 complex binds to microfilaments, facilitating branching in the microfilament network.

Regulation of Microfilament Association

Filamin homodimers crosslink microfilaments, creating a gel-like network that enhances structural integrity.

Microfilaments

One of three cytoskeletal components, crucial for cellular structure, movement, and organization.
Microfilaments exhibit polarity, which influences their directional functions.
Formed from actin dimers that bind to ATP and ADP.
Dynamic structures capable of rapid growth and contraction.
Polymerisation occurs from both ends, affected by the concentrations of actin monomers.
Capable of treadmilling, where actin monomers are lost from the minus end and added at the plus end.
Monomer concentration is tightly regulated to control filament dynamics.

Microtubules

Present in both animal and plant cells, formed from α-tubulin and β-tubulin heterodimers (110 kDa).
Polymerisation results in tubular structures with a 25 nm diameter and a 14 nm internal pore.
α-Tubulin binds GTP, which is not hydrolyzed, while β-tubulin can have either GTP or GDP bound.

Microtubule Formation

Centrosome acts as the primary Microtubule Organising Center (MTOC) in animals.
Comprises a γ-tubulin ring complex essential for nucleating microtubule growth, organized around centrioles.

Actin Subunit Cycling

G-actin is the globular form of actin, binding one Ca²⁺ ion and a non-covalent ATP molecule.
G-actin polymerises into filamentous F-actin, with ATP hydrolysis occurring during polymerisation, although energy is not critical for the process.

Treadmilling

Filament stability depends on concentrations below Cc- and Cc+, where disassembly or failure to form occurs.
Above both Cc- and Cc+, filaments grow at both ends with increased monomer concentration.
Below Cc- but above Cc+, treadmilling occurs with filament turnover.

Regulation of Actin Polymerisation

Protein levels of profilin and thymosin can influence the rate of microfilament synthesis.
Profilin enhances ATP binding to G-actin, increasing polymerisation speed.
Cofilin destabilizes the minus end of actin filaments, facilitating rapid turnover.
Thymosin-beta 4 sequesters G-actin, slowing down filament assembly.

Dynamic Instability in Microtubules

Dynamic instability requires a GTP-β-tubulin cap to maintain microtubule growth.
Loss of GTP-bound β-tubulin leads to microtubule disassembly as GDP-bound β-tubulin falls off.
Hydrolysis of bound GTP can outpace subunit addition, transitioning from growth to shrinkage.

Stabilization of Microtubules

Microtubule Associated Proteins (MAPs) enable crosslinking and interaction with other cellular elements.
Overexpression of MAPs like Tau facilitates closer arrangement and stability of microtubules.

Microtubules (Main Points)

Serve as one of three types of cytoskeleton structures, essential for cellular organization.
Display polarity, influencing directionality for intracellular transport.
Dynamic behavior is influenced by binding of GTP and GDP.
Oriented relative to the MTOC, crucial for movement and trafficking.
MAPs provide stability and functional integrity to microtubule networks.

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Description

This quiz explores the role of the Arp2/3 complex in the nucleation of actin polymerisation. Learn how the Arp2 and Arp3 proteins, while similar to G actin, serve as a primer for the polymerisation process. Test your understanding of these critical cellular components.