A simplified description of actin self-assembly. The process of actin assembly is complicated in vivo where proteins bind to the filament and influence its behavior. For simplicity, we will focus our discussion on the in vitro process that takes place in the presence of ATP. To simplify the discussion further, we shall initially focus on the process of adding a single monomer unit to a preformed ATP-actin filament. The discussion that follows is based in part on an excellent review 33b and detailed discussion of actin polymerization kinetics 35 reported by Pollard. In the presence of ATP, 34 actin is in equilibrium with an ATPactin complex, which is strongly favored ( Figure ). This undergoes very slow hydrolysis to generate a new ADP-Pi-actin complex. Subsequent dissociation of Pi and ADP regenerates actin. This process is slow and will henceforth be ignored for simplicity. Conversely, when an ATP-actin monomer associates with an actin filament, which is also a favorable equilibrium, the hydrolysis process is significantly accelerated ( Figure ). From this very simple description it is obvious that the hydrolysis of ATP is coupled to the reversible addition of an ATP-monomer unit to the filament ( Figure ) and we can draw a simple reaction network to demonstrate the cycle ( Figure ).