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Nature Cell Biology 8, 1337–1347 (1 December 2006) | doi:10.1038/ncb1502

Regulation of cell shape by Cdc42 is mediated by the synergic actin-bundling activity of the Eps8|[ndash]|IRSp53 complex

Andrea Disanza , Sara Mantoani , Maud Hertzog , Silke Gerboth , Emanuela Frittoli , Anika Steffen , Kerstin Berhoerster , Hans-Juergen Kreienkamp , Francesca Milanesi , Pier Paolo Di Fiore , Andrea Ciliberto , Theresia E. B. Stradal & Giorgio Scita

Actin-crosslinking proteins organize actin into highly dynamic and architecturally diverse subcellular scaffolds that orchestrate a variety of mechanical processes, including lamellipodial and filopodial protrusions in motile cells. How signalling pathways control and coordinate the activity of these crosslinkers is poorly defined. IRSp53, a multi-domain protein that can associate with the Rho-GTPases Rac and Cdc42, participates in these processes mainly through its amino-terminal IMD (IRSp53 and MIM domain). The isolated IMD has actin-bundling activity in vitro and is sufficient to induce filopodia in vivo. However, the manner of regulation of this activity in the full-length protein remains largely unknown. Eps8 is involved in actin dynamics through its actin barbed-ends capping activity and its ability to modulate Rac activity. Moreover, Eps8 binds to IRSp53. Here, we describe a novel actin crosslinking activity of Eps8. Additionally, Eps8 activates and synergizes with IRSp53 in mediating actin bundling in vitro, enhancing IRSp53-dependent membrane extensions in vivo. Cdc42 binds to and controls the cellular distribution of the IRSp53–Eps8 complex, supporting the existence of a Cdc42–IRSp53–Eps8 signalling pathway. Consistently, Cdc42-induced filopodia are inhibited following individual removal of either IRSp53 or Eps8. Collectively, these results support a model whereby the synergic bundling activity of the IRSp53–Eps8 complex, regulated by Cdc42, contributes to the generation of actin bundles, thus promoting filopodial protrusions.