Practice question

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.