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      ADF/Cofilin Accelerates Actin Dynamics by Severing Filaments and Promoting Their Depolymerization at Both Ends

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          Summary

          Actin-depolymerizing factor (ADF)/cofilins contribute to cytoskeletal dynamics by promoting rapid actin filament disassembly. In the classical view, ADF/cofilin sever filaments, and capping proteins block filament barbed ends whereas pointed ends depolymerize, at a rate that is still debated. Here, by monitoring the activity of the three mammalian ADF/cofilin isoforms on individual skeletal muscle and cytoplasmic actin filaments, we directly quantify the reactions underpinning filament severing and depolymerization from both ends. We find that, in the absence of monomeric actin, soluble ADF/cofilin can associate with bare filament barbed ends to accelerate their depolymerization. Compared to bare filaments, ADF/cofilin-saturated filaments depolymerize faster from their pointed ends and slower from their barbed ends, resulting in similar depolymerization rates at both ends. This effect is isoform specific because depolymerization is faster for ADF- than for cofilin-saturated filaments. We also show that, unexpectedly, ADF/cofilin-saturated filaments qualitatively differ from bare filaments: their barbed ends are very difficult to cap or elongate, and consequently undergo depolymerization even in the presence of capping protein and actin monomers. Such depolymerizing ADF/cofilin-decorated barbed ends are produced during 17% of severing events. They are also the dominant fate of filament barbed ends in the presence of capping protein, because capping allows growing ADF/cofilin domains to reach the barbed ends, thereby promoting their uncapping and subsequent depolymerization. Our experiments thus reveal how ADF/cofilin, together with capping protein, control the dynamics of actin filament barbed and pointed ends. Strikingly, our results propose that significant barbed-end depolymerization may take place in cells.

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          Highlights

          • ADF/cofilin domains grow symmetrically and sever mostly at their pointed-end border

          • Pointed-end depolymerization is accelerated by ADF/cofilin saturation

          • ADF/cofilin domains reach capped barbed ends and rapidly uncap them

          • ADF/cofilin-saturated barbed ends depolymerize and are hard to cap or re-elongate

          Abstract

          Wioland et al. monitor the action of ADF/cofilin isoforms on individual actin filaments and characterize the reactions leading to filament disassembly. ADF/cofilin-saturated filaments differ greatly from bare filaments, as their barbed ends can hardly stop depolymerizing. This situation can arise from severing or from a synergy with capping protein.

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          Most cited references49

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          Actin, a central player in cell shape and movement.

          The protein actin forms filaments that provide cells with mechanical support and driving forces for movement. Actin contributes to biological processes such as sensing environmental forces, internalizing membrane vesicles, moving over surfaces, and dividing the cell in two. These cellular activities are complex; they depend on interactions of actin monomers and filaments with numerous other proteins. Here, we present a summary of the key questions in the field and suggest how those questions might be answered. Understanding actin-based biological phenomena will depend on identifying the participating molecules and defining their molecular mechanisms. Comparisons of quantitative measurements of reactions in live cells with computer simulations of mathematical models will also help generate meaningful insights.
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            Regulation of the actin cytoskeleton in cancer cell migration and invasion.

            Malignant cancer cells utilize their intrinsic migratory ability to invade adjacent tissues and the vasculature, and ultimately to metastasize. Cell migration is the sum of multi-step processes initiated by the formation of membrane protrusions in response to migratory and chemotactic stimuli. The driving force for membrane protrusion is localized polymerization of submembrane actin filaments. Recently, several studies revealed that molecules that link migratory signals to the actin cytoskeleton are upregulated in invasive and metastatic cancer cells. In this review, we summarize recent progress on molecular mechanisms of formation of invasive protrusions used by tumor cells, such as lamellipodia and invadopodia, with regard to the functions of key regulatory proteins of the actin cytoskeleton; WASP family proteins, Arp2/3 complex, LIM-kinase, cofilin, and cortactin.
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              Mechanism of actin filament turnover by severing and nucleation at different concentrations of ADF/cofilin.

              ADF/cofilins are key regulators of actin dynamics during cellular motility, yet their precise role and mechanism of action are shrouded in ambiguity. Direct observation of actin filaments by evanescent wave microscopy showed that cofilins from fission yeast and human do not increase the rate that pointed ends of actin filaments shorten beyond the rate for ADP-actin subunits, but both cofilins inhibit elongation and subunit dissociation at barbed ends. Direct observation also showed that cofilins from fission yeast, Acanthamoeba, and human sever actin filaments optimally at low-cofilin binding densities well below their K(d)s, but not at high binding densities. High concentrations of cofilin nucleate actin assembly. Thus, the action of cofilins in cells will depend on the local concentration of active cofilins: low concentrations favor severing, whereas high concentrations favor nucleation. These results establish a clear paradigm for actin turnover by cofilin in cells.
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                Author and article information

                Contributors
                Journal
                Curr Biol
                Curr. Biol
                Current Biology
                Cell Press
                0960-9822
                1879-0445
                10 July 2017
                10 July 2017
                : 27
                : 13
                : 1956-1967.e7
                Affiliations
                [1 ]Institut Jacques Monod, CNRS, Université Paris Diderot, 75013 Paris, France
                [2 ]Institute of Biotechnology, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland
                Author notes
                []Corresponding author antoine.jegou@ 123456ijm.fr
                [∗∗ ]Corresponding author romet@ 123456ijm.fr
                [3]

                Lead Contact

                Article
                S0960-9822(17)30614-0
                10.1016/j.cub.2017.05.048
                5505867
                28625781
                e466ef32-32c3-4a4a-9b3f-16a5eb29a76a
                © 2017 The Author(s)

                This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

                History
                : 18 January 2017
                : 4 April 2017
                : 16 May 2017
                Categories
                Article

                Life sciences
                actin-depolymerizing factor,cofilin,capping protein,actin dynamics,single filaments,microfluidics,non-muscle actin,barbed-end depolymerization

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