Water in actin polymerization

We have addressed the question whether water is part of the G- to F-actin p olymerization reaction. Under osmotic stress, the critical concentration fo r G-Ca-ATP actin was reduced for six different osmolytes. These results are interpreted as showing that reducing water activity favored the polymerize d state. The magnitude of the effect correlated, then saturated, with incre asing MW of the osmolyte and suggested that up to 10-12 fewer water molecul es were associated with actin when it polymerized. By contrast, osmotic eff ects were insignificant for Mg-ATP actin. The nucleotide binding site of th e Mg conformation is more closed than the Ca and more closely resembles the closed actin conformation in the polymerized state. These results suggest that the water may come from the cleft of the nucleotide binding site.