The Conformational State of Actin Filaments Regulates Branching by Actin-related Protein 2/3 (Arp2/3) Complex

From the Department of Physiology and Biophysics, Boston University, Boston, Massachusetts 02118.
Journal of Biological Chemistry (Impact Factor: 4.57). 07/2012; 287(37):31447-53. DOI: 10.1074/jbc.M112.350421
Source: PubMed

ABSTRACT Actin is a highly ubiquitous protein in eukaryotic cells that plays a crucial role in cell mechanics and motility. Cell motility is driven by assembling actin as polymerizing actin drives cell protrusions in a process closely involving a host of other actin-binding proteins, notably the actin-related protein 2/3 (Arp2/3) complex, which nucleates actin and forms branched filamentous structures. The Arp2/3 complex preferentially binds specific actin networks at the cell leading edge and forms branched filamentous structures, which drive cell protrusions, but the exact regulatory mechanism behind this process is not well understood. Here we show using in vitro imaging and binding assays that a fragment of the actin-binding protein caldesmon added to polymerizing actin increases the Arp2/3-mediated branching activity, whereas it has no effect on branch formation when binding to aged actin filaments. Because this caldesmon effect is shown to be independent of nucleotide hydrolysis and phosphate release from actin, our results suggest a mechanism by which caldesmon maintains newly polymerized actin in a distinct state that has a higher affinity for the Arp2/3 complex. Our data show that this new state does not affect the level of cooperativity of binding by Arp2/3 complex or its distribution on actin. This presents a novel regulatory mechanism by which caldesmon, and potentially other actin-binding proteins, regulates the interactions of actin with its binding partners.

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Available from: C-L Albert Wang, Jan 10, 2014
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    • "There is a conformational-dependent maturation of the filament during polymerization [Carlier, 1990; Bryan and Rubenstein, 2005] which has been shown to be important for its interaction with different actin binding proteins. Such an example is caldesmon [Jensen et al., 2012], a protein involved in regulating filament stability in smooth muscle and non-muscle cells. Furthermore, the binding of different proteins to the filament can cause propagated conformational changes along the filament such as occurs with formin and cofilin [Muhlrad et al., 2004; Prochniewicz et al., 2005; Bobkov et al.,2004, 2006; Bugyi et al., 2006; Papp et al., 2006; De La Cruz and Sept, 2010; Ujfalusi et al., 2010; Galkin et al., 2003, 2011], proteins involved in the first case with filament nucleation and elongation and in the second case with filament severing. "
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