Figure : Simplified models that can be used to discuss actin polymerization at non-equilibrium steady state. (a) The behavior of actin in solution with ATP; ATP-actin hydrolyses slowly. (b) ATP-actin hydrolyses rapidly when assembled in an actin filament. (c) The coupled processes involved in the assembly of ATP-actin to a preformed filament. (d) A simple reaction network that confirms that the coupling of assembly and hydrolysis is reasonable even when the details of the hydrolysis process are ignored. (f) An expanded model of ATP-actin selfassembly that can be used to account for treadmilling. (f) Schematic that demonstrates that the disassembly of aged actin filaments is qualitatively different from the disassembly of a simple supramolecular polymer; ADP-Pi-actin monomer units in the bulk are kinetically stable and strongly bound to the filament but can rapidly lose Pi at the termini resulting in the dissociation of ADP-actin.