ACTIN DEPOLYMERIZING FACTOR (ADF COFILIN) ENHANCES THE RATE OF FILAMENT TURNOVER - IMPLICATION IN ACTIN-BASED MOTILITY/

Actin-binding proteins of the actin depolymerizing factor (ADF)/cofili n family are thought to control actin-based motile processes. ADF1 fro m Arabidopsis thaliana appears to be a good model that is functionally similar to other members of the family. The function of ADF in actin dynamics has been examined using a combination of physical-chemical me thods and actin-based motility assays, under physiological ionic condi tions and at pH 7.8. ADF binds the ADP-bound forms of G- or F-actin wi th an affinity two orders of magnitude higher than the ATP- or ADP-Pi- bound forms. A major property of ADF is its ability to enhance the in vitro turnover rate (treadmilling) of actin filaments to a value compa rable to that observed in vivo in motile lamellipodia. ADF increases t he rate of propulsion of Listeria monocytogenes in highly diluted, ADF -limited platelet extracts and shortens the actin tails. These effects are mediated by the participation of ADF in actin filament assembly, which results in a change in the kinetic parameters at the two ends of the actin filament. The kinetic effects of ADF are end specific and c annot be accounted for by filament severing, The main functionally rel evant effect is a 25-fold increase in the rate of actin dissociation f rom the pointed ends, while the rate of dissociation from the barbed e nds is unchanged. This large increase in the rate-limiting step of the monomer-polymer cycle at steady state is responsible for the increase in the rate of actin-based motile processes. In conclusion, the funct ion of ADF is not to sequester G-actin. ADF uses ATP hydrolysis in act in assembly to enhance filament dynamics.