Actin polymerization and depolymerization coupled to cooperative hydrolysis.

The hydrolysis of adenosine triphosphate (ATP) during actin (de)polymerization consists of two subprocesses, ATP cleavage and phosphate (P_{i}) release, which involve three nucleotide states of each actin protomer. A new theoretical model that explicitly incorporates these different subprocesses and states is introduced and compared with recent experimental data for actin depolymerization. These data can be explained by strongly cooperative ATP cleavage followed by strongly cooperative P_{i} release but are incompatible with random and/or vectorial subprocesses as proposed previously.