enhanced actin depolymerization at the mDia1bound barbed end. This inhibition occurs in the submillimolar range of Pi, which is two orders of magnitude lower than the dissociation constant of Pi and G-actin (26). Thus, binding of Pi to F-actin inhibits profilin-induced depolymerization (Fig. 4E). ADP-G-actin (5 mM) elongated mDia1-bound F-actin faster in the presence of 20 mM P i than in its absence (Fig. 3C). This effect of Pi was more prominent in the presence of profilin than in its absence (Fig. 3C). The decrease in the actin offrate (Fig. 4D) corresponds well with the increase in the ADP-actin elongation rate by 3 to 20 mM P i (Fig. 4E). We thus suggest that P i cancels the inhibitory effect of profilin on ADP-actin elongatio
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