The experimental basis of some recent hypotheses on the mechanism of the polymerization of actin: A reappraisal

We have studied the effect of sonication on the fluorescence of N-(1-pyrenyl)iodoacetamide-labeled F-actin as well as of native actin-pyrenyl-actin mixed oligomers in which the subunits were covalently attached to each other by phenylenebismaleimide. In both cases the fluorescence of the solution was largely decreased by sonication. We have found that this effect is due (a) to a 20-30% decrease of the specific fluorescence of the polymers; (b) to the release of monomers (or other nonfluorescing species) from the polymers. These results question the validity of the novel mechanism for the polymerization of actin recently proposed (D. Pantaloni et al. (1984) J. Biol. Chem. 259, 6274-6283). In these studies, in fact, the implicit assumption was made that the quenching of the fluorescence of the solution under sonication was due exclusively to the conversion of F-actin into G-actin.