Capping Protein Increases the Rate of Actin-Based Motility by Promoting Filament Nucleation by the Arp2/3 Complex

Department of Cellular and Molecular Pharmacology, School of Medicine, University of California, San Francisco, San Francisco, CA 94143, USA.
Cell (Impact Factor: 32.24). 06/2008; 133(5):841-51. DOI: 10.1016/j.cell.2008.04.011
Source: PubMed


Capping protein (CP) is an integral component of Arp2/3-nucleated actin networks that drive amoeboid motility. Increasing the concentration of capping protein, which caps barbed ends of actin filaments and prevents elongation, increases the rate of actin-based motility in vivo and in vitro. We studied the synergy between CP and Arp2/3 using an in vitro actin-based motility system reconstituted from purified proteins. We find that capping protein increases the rate of motility by promoting more frequent filament nucleation by the Arp2/3 complex and not by increasing the rate of filament elongation as previously suggested. One consequence of this coupling between capping and nucleation is that, while the rate of motility depends strongly on the concentration of CP and Arp2/3, the net rate of actin assembly is insensitive to changes in either factor. By reorganizing their architecture, dendritic actin networks harness the same assembly kinetics to drive different rates of motility.

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Available from: R. Dyche Mullins, Jan 11, 2014
    • "We then arrested network assembly by adding Phalloidin and Latrunculin B, together with a fluorescent derivative of CP (Fig- ure 3B). The two small-molecules rapidly freeze actin dynamics (Akin and Mullins, 2008), while the fluorescent CP labels free barbed ends of filaments in the network (Figures 3B and S4; Movie S3). Accumulation of labeled CP was biphasic (Figures S4D and S4E; Supplemental Experimental Procedures), with rapid binding to free barbed ends followed by a very slow exchange of labeled CP with unlabeled CP throughout the network (Reymann et al., 2011;Schafer et al., 1996). "
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    • "It is a highly conserved protein that is found from yeast to humans, and it is a heterodimer composed of two unrelated subunits, a and b (Wear and Cooper, 2004). Various lines of biochemical evidence (Akin and Mullins, 2008; Loisel et al., 1999) and cellular experiments (Fan et al., 2011; Mejillano et al., 2004; Pappas et al., 2008) have shown that capping protein is essential for various actin-mediated processes. For example, knockdown of capping protein in fast-moving fish keratinocytes causes filopodia-like structures to appear at cell edges (Mejillano et al., 2004), indicating the importance of this protein in cell migration. "
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    ABSTRACT: Actin polymerization is essential for various stages of mammalian oocyte maturation, including spindle migration, actin cap formation, polar body extrusion, and cytokinesis. The heterodimeric actin-capping protein (CP) is an essential element of the actin cytoskeleton. It binds to the fast-growing (barbed) ends of actin filaments and plays essential roles in various actin-mediated cellular processes. However, the roles of CP in mammalian oocyte maturation are poorly understood. We investigated the roles of CP in mouse oocytes and found that CP is essential for correct asymmetric spindle migration and polar body extrusion. CP mainly localized in the cytoplasm during maturation. By knockdown or ectopically overexpression of CP revealed that CP is critical for efficient spindle migration and maintenance of the cytoplasmic actin mesh density. Expression of the CP inhibiting protein CARMIL impaired spindle migration and polar body extrusion during oocyte maturation and decreased the cytoplasmic actin mesh density. Taken together, these findings show that CP is an essential component of the actin cytoskeleton machinery that plays crucial roles in oocyte maturation, presumably by controlling the cytoplasmic actin mesh density.
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    • "In support of this hypothesis, GFP-CP fusion proteins demonstrate that sites of actin assembling in living cells contain both CP and the actin-related protein2/3 (Arp2/3) complex, and CP is located in two types of structures: (1) motile regions of the cell periphery , which reflect movement of the edge of the lamella during extension and ruffling; and (2) dynamic spots within the lamella (Schafer et al., 1998). CP has been colocalized to the F-actin patches in fission yeast (Schizosaccharomyces pombe; Kovar et al., 2005), which promotes Arp2/3-dependent nucleation and branching and limits the extent of filament elongation (Akin and Mullins, 2008). These findings lend additional support for a model whereby CP cooperates with the Arp2/3 complex to regulate actin dynamics (Nakano and Mabuchi, 2006). "
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