NUCLEOTIDE EXCHANGE AND RHEOMETRIC STUDIES WITH F-ACTIN PREPARED FROMATP-MONOMERIC OR ADP-MONOMERIC ACTIN

It has recently been reported that polymer actin made from monomer con taining ATP (ATP-actin) differed in EM appearance and rheological char acteristics from polymer made from ADP-containing monomers (ADP-actin) . Further, it was postulated that the ATP-actin polymer was more rigid due to storage of the energy released by ATP hydrolysis during polyme rization (Janmey et al. 1990. Nature 347:95-99). Electron micrographs of our preparations of ADP-actin and ATP-actin polymers show no major differences in appearance of the filaments. Moreover, the dynamic visc osity parameters G' and G'' measured for ATP-actin and ADP-actin polym ers are very different from those reported by Janmey et al., in absolu te value, in relative differences, and in frequency dependence. We sug gest that the relatively small differences observed between ATP-actin and ADP-actin polymer rheological parameters could be due to small dif ferences either in flexibility or, more probably, in filament lengths. We have measured nucleotide exchange on ATP-actin and ADP-actin polym ers by incorporation of alpha-P-32-ATP and found it to be very slow, i n agreement with earlier literature reports, and in contradiction to t he faster exchange rates reported by Janmey et al. This exchange rate is much too slow to cause ''reversal'' of ADP-actin polymer to ATP-act in polymer as reported by Janmey et al. Thus our results do not suppor t the notion that the energy of actin-bound ATP hydrolysis is trapped in and significantly modifies the actin polymer structure.