Actin Depolymerizing Factor (ADF/Cofilin) Enhances the Rate of Filament Turnover: Implication in Actin-based Motility

Abstract

Actin-binding proteins of the actin depolymerizing factor (ADF)/cofilin family are thought to control actin-based motile processes. ADF1 from 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 methods and actin-based motility assays, under physiological ionic conditions and at pH 7.8. ADF binds the ADPbound forms of G- or F-actin with 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 comparable to that observed in vivo in motile lamellipodia. ADF increases the 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 cannot be accounted for by filament severing. The main functionally relevant effect is a 25-fold increase in the rate of actin dissociation from the pointed ends, while the rate of dissociation from the barbed ends 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 function of ADF is not to sequester G-actin. ADF uses ATP hydrolysis in actin assembly to enhance filament dynamics.

BABY BOOM target genes provide diverse entry points into cell proliferation and cell growth pathways

“…ADF/cofilin binds both to G-and F-actin and enhances actin dynamics by severing actin filaments and increasing their depolymerization from the pointed end (Carlier et al 1997;Gungabissoon et al 1998). Since the activity of ADF/cofilin on the actin cytoskeleton has been studied in detail, and since ADF9 provides a potential link from BBM signaling to actin dynamization, we chose to further characterize the up-regulation of ADF9 in 35S::BBM:GR seedlings.…”

Actin bundling in plants

“…The ADF/cofilin family has emerged as a central regulator of actin turnover in eukaryotes including plants [Bamburg, 1999;Maciver and Hussey, 2002;Van Troys et al, 2008]. Through its AF severing and pointed enddepolymerizing activities, ADF/cofilin enhances actin cytoskeleton dynamics [Carlier et al, 1997;Blanchoin and Pollard, 1999;Pavlov et al, 2007]. Dong et al [2001] reported that both an increase and a decrease of the Arabidopsis ADF1 (AtADF1) expression level induce a significant effect on the overall cellular actin-bundling state.…”

Relationships between Actin Regulatory Mechanisms and Measurable State Variables

“…The only known influences on these rates are proteins of the ADF/cofilin family, which are present at significant concentrations in cells, $20 lM. 1,12,14,44 Cofilins bind preferentially to ADP-bound subunits in actin filaments and accelerate depolymerization of these filaments by increasing the off-rate of ADP-actin subunits, 9 severing filaments to create more ends, 31 and increasing the Pi release rate on ADPAEPi-bound subunits. 5 However, it is not clear what the relative contributions of these three effects are and how they relate to the concentration of cofilin.…”

“…These proteins bind the ADP-bound subunits of filaments found most prominently near pointed ends. 9 Cofilins destablize filament structure leading to both filament severing 31 and enhanced depolymerization. 9 Also regulating the actin cycle in cells are proteins that cap the barbed and pointed ends.…”

“…9 Cofilins destablize filament structure leading to both filament severing 31 and enhanced depolymerization. 9 Also regulating the actin cycle in cells are proteins that cap the barbed and pointed ends. Barbed end specific 'capping protein' blocks assembly and disassembly at barbed ends.…”