Questions and Answers
What effect does profilin have on G-actin binding?
What happens to actin filaments when the concentration of monomer is below Cc-?
Which actin-binding protein is responsible for destabilizing the minus end of actin filaments?
What is the role of thymosin-beta 4 in actin polymerization regulation?
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In which situation does treadmilling occur in actin filaments?
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What is the role of the Arp2/3 complex in actin polymerization?
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Which protein complex is crucial for the branching of the microfilament network?
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How do microfilaments maintain their dynamic properties?
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Which feature distinguishes microtubules from microfilaments?
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What is a primary function of filamin in microfilament networks?
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What structure in animal cells serves as the major microtubule organizing center?
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What is the effect of GTP binding on α-tubulin?
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Which characteristic of microfilaments allows them to 'treadmill'?
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What is the role of GTP in the dynamic instability of microtubules?
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What happens to the microtubule when GTP bound to beta-tubulin is hydrolyzed to GDP?
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Which of the following statements about microtubules is correct?
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What is the function of microtubule-associated proteins (MAPs)?
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How does min affect microtubule dynamics?
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What is the primary orientation factor for intracellular trafficking involving microtubules?
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What is a characteristic feature of microtubules?
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What happens to tubulin dimers after their addition to a microtubule?
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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.
