SYNERGY BETWEEN ACTIN DEPOLYMERIZING FACTOR COFILIN AND PROFILIN IN INCREASING ACTIN FILAMENT TURNOVER

The mechanism of control of the steady state of actin assembly by acti n depolymerizing factor (ADF)/cofilin and profilin has been investigat ed. Using TP, as an indicator of the concentration of ATP-G-actin, we show that ADF increases the concentration of ATP-G-actin at steady sta te. The measured higher concentration of ATP-G-actin is quantitatively consistent with the increase in treadmilling, caused by the large inc rease in the rate of depolymerization from the pointed ends induced by ADF (Carlier, M.-F., Laurent, V., Santolini, J., Didry, D., Melki, R. , Xia, G.-X., Hong, Y., Chua, N.-H., and Pantaloni, D. (1997) J. Cell Biol. 136, 1307-1322). Experiments demonstrate that profilin synergize s with ADF to further enhance the turnover of actin filaments up to a value 125-fold higher than in pure F-actin solutions. Profilin and ADF act at the two ends of filaments in a complementary fashion to increa se the processivity of treadmilling. Using the capping protein CapZ, w e show that ADF increases the number of filaments at steady state by 1 .3-fold, which cannot account for the 25-fold increase in turnover rat e. Computer modeling of the combined actions of ADF and profilin on th e dynamics of actin filaments using experimentally determined rate con stants generates a distribution of the different actin species at stea dy state, which is in quantitative agreement with the data.