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#G-actin

2 public questions tagged with this topic.

Cytoskeleton

Which actin-binding protein promotes actin polymerization by catalyzing the exchange of ADP for ATP on G-actin?

Actin nucleotide state dictates polymerization competence. ATP G actin incorporates into barbed end with high affinity Kd 0.1 micromolar, ADP G actin weak Kd 1 micromolar and depolymerizes. After filament turnover ADP actin accumulates needing recharge. Profilin, 15 kilodalton ubiquitous protein present at 50 to 100 micromolar, binds actin subdomain 1 and 3 cleft with 1 to 1 stoichiometry. Structural studies show binding opens nucleotide binding pocket between subdomains 2 and 4, reducing affinity for ADP 1000 fold and accelerating its dissociation rate from 0.02 to 1 per second, allowing abundant cellular ATP 1 to 5 millimolar to bind. Resulting profilin ATP actin complex sterically inhibited from adding to pointed end due to clash with incoming filament, but adds efficiently to barbed end, upon incorporation profilin falls off. Profilin also binds poly L proline stretches via aromatic cradle, targeting complex to enabled VASP and formin FH1 for processive elongation. Thymosin beta4 sequesters ATP actin blocking all assembly, tropomodulin caps minus end, formin nucleates without exchange activity, making profilin unique catalyst of ADP to ATP exchange promoting polymerization.

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Profilin and Nucleotide Exchange on G-Actin.

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Cytoskeleton

What happens when the G-actin concentration is lower than Cc at both ends?

Filament mass reflects equilibrium between monomer pool and polymers determined by two Cc values. Plus end higher affinity due to ATP cap and hydrophobic pocket, minus end lower affinity bearing ADP actin. When total free G actin concentration falls below Cc for both ends, for instance after dilution into low ionic buffer, sequestration by thymosin beta4 or latrunculin A, or after activation of cofilin increasing off rate, association cannot compensate dissociation. Consequently subunits dissociate from both termini, filament shortens from both directions and ultimately disappears. This explains experimental observation where lowering monomer below 0.1 micromolar triggers rapid disassembly throughout cytoplasm, confirmed by loss of phalloidin staining. If monomer concentration lies between 0.1 and 0.8 micromolar, plus end still grows while minus shrinks, creating treadmilling without net change. Concentrations above 0.8 drive elongation at both ends. Filament stability without growth demands accessory proteins like phalloidin or capping proteins preventing subunit exchange, not merely low monomer levels alone.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Actin Critical Concentration and Dynamics.

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