Practice question

Assembly of G actin into F actin follows reversible nucleation condensation kinetics characterized by critical concentration. Defined as monomer concentration at which rate of subunit addition equals rate of loss at filament ends, Cc reflects equilibrium constant Kd equal to k off divided by k on. Experimental determination uses pyrene fluorescence plateau where increasing total actin beyond Cc goes into polymer, free monomer stays constant. Because barbed plus end and pointed minus end have distinct kinetic constants due to ATP versus ADP actin states, each end possesses its own Cc approximately 0.1 micromolar for barbed and 0.8 micromolar for pointed under physiological 50 millimolar KCl 2 millimolar MgCl2. Between these values treadmilling operates, barbed elongates while pointed shrinks, net polymer constant. Complete depolymerization requires dropping below both Cc values or adding sequestering drugs like latrunculin. Branch angle 70 degrees relates to Arp2/3 geometry not Cc, and ATP hydrolysis rate is consequence not definition. Understanding Cc enables prediction of growth versus shrinkage and design of polymerization assays.