Polymerisation of cytoskeletal structures Flashcards
where is actin present.
In both muscle and non-muscle cells.
what binding sites does actin molecules’ have.
Each actin molecule has binding sites for Mg2+ ion with either ATP or ADP bound
what is the assembly of G and F actin accompanied by?
The assembly of G-actin into F-actin is accompanied by the hydrolysis of ATP to ADP and Pi
what are the three phases involved in the time course of actin polymerisation.
Nucleation (lag phase)
Elongation
steady state
explain what happens at the NUCLEATION phase
Time Frame: This is the initial lag phase.
Process:
Actin monomers (G-actin) come together to form small (often unstable) aggregates (Dimers -two monomers and trimmers- three monomers) called nucleation sites “nucleus”
A stable “nucleus” must form for polymerisation to proceed. This step is energetically unfavorable, making it the slowest part of the process.
Outcome:
Initiation of Filament Formation:
- Once a stable nucleus (often a trimer or larger) is formed, it acts as a template for further addition of G-actin monomers, marking the transition to the elongation phase.
Transition to Elongation:
- The successful nucleation sets the stage for rapid polymerization as additional G-actin monomers begin to add to the growing filament.
why is nucleation the slowest stage?
The nucleation phase has a higher energy barrier compared to the subsequent elongation phase, meaning it takes time for enough actin monomers to come together to form a stable nucleus.
what must there be for nucleation to occur?
To facilitate nucleation, the concentration of free G-actin must be relatively high. Once a nucleus is formed, it can effectively recruit additional G-actin monomers for elongation.
explain what happens at the Elongation Phase
Time Frame: This phase follows nucleation and can occur rapidly.
Process:
Once a stable nucleus is formed during the nucleation phase, G-actin monomers (ATP-bound) rapidly add to the growing ends of the actin filament, primarily at the barbed (+) end.
Once incorporated into the filament, each G-actin monomer hydrolyzes its bound ATP to ADP.
The rapid addition of G-actin leads to a significant increase in the length of the filament. This phase is characterized by a steep rise in the amount of filamentous actin (F-actin) over time.
Outcome:
The elongation phase results in the formation of long, dynamic actin filaments, which are essential for various cellular processes, including motility, shape maintenance, and intracellular transport.
Preparation for Steady State:
The elongation phase sets the stage for the steady state, where the dynamics of actin turnover become balanced
what does Cc stand for, and what does it mean?
Critical concentration:
concentration where the rate of subunit addition and loss are equal.
critical concentration (cc) during elongation phase?
As G-actin concentration decreases due to its incorporation into the filament, a point is reached where the rates of addition at the barbed end (+) and loss at the pointed end (-) become balanced.
Dynamic instability/equilibrium during elongation phase.
While elongation is occurring, the filament remains dynamic.
- When the concentration of G-actin is above the critical concentration, the filament will continue to grow as more monomers are added than are lost.
- When the concentration of G-actin is below the critical concentration, the filament will shrink as more monomers dissociate than are added.
- At concentrations around the critical concentration, Monomers can dissociate from the pointed (−) end, leading to a phenomenon known as “treadmilling” where the filament can maintain a constant length while while monomers are added at one end (barbed) and lost at the other end (pointed), leading to a dynamic turnover of actin.
Explain what happens at the steady state phase?
Time Frame: Eventually, a dynamic equilibrium is reached.
Process:
At steady state, the rate of addition of actin monomers at the barbed end is equal to the rate of loss at the pointed (−) end (Cc).
The total length of filaments remains constant, but there is turnover as G-actin is added on one end and simultaneously removed at the other end (treadmilling)- This allows for dynamic movement and flexibility in the cytoskeleton without a change in length
Outcome:
Stable Filament Length: The overall length of actin filaments remains constant despite continuous turnover of monomers.
Dynamic Equilibrium: A balance is achieved between actin monomer addition and loss, allowing for constant filament presence.
Treadmilling: Actin filaments can exhibit treadmilling, enabling movement and flexibility without changing length.
Facilitated Cellular Functions: The stable actin network supports critical functions such as cell motility, shape maintenance, and intracellular transport.
Adaptability: Cells can quickly respond to environmental changes, facilitating processes like migration and signaling.
Coordination with Other Cytoskeletal Components:
Cc during steady state phase?
The concentration of free actin monomers in the cytoplasm reaches a level known as the critical concentration. Below this concentration, filament disassembly dominates; above it, assembly occurs.
Role of regulatory proteins in steady state phase.
Actin-binding proteins (like profilin, cofilin, and tropomyosin) play a crucial role in modulating the dynamics of actin polymerization, affecting the rates of assembly and disassembly.
what is tubulin made up of and where is it found?
Tubulin consists of α-tubulin and β-tubulin, forming a HETERODIMER that is crucial for microtubule assembly.
found in the cytoskeleton in EUKARYOTIC cells.
