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. 2015 Sep 28;73(9):ftv078. doi: 10.1093/femspd/ftv078

Figure 2.

Figure 2.

Models for the mechanism of actin nucleation and elongation by eukaryotic and bacterial nucleators. (A) After activation by NPFs, the Arp2/3 complex binds to the side of pre-existing actin filaments. WH2 domains and proline-rich regions (PRRs) within NPFs recruit actin monomers and profilin-actin, respectively. New filaments elongate at a 70° angle, originating a branched actin network with Arp2/3 bound at the pointed end. Formins act as homodimers that induce nucleation by stabilizing two actin subunits through their FH2 domains. As the filament elongates, formins remain bound to the barbed end to which the FH1 domains recruit profilin-actin. Nucleators belonging to the tandem monomer binding family contain tandem motifs that bind G-actin (generally WH2 motifs) that act as a scaffold that brings together actin subunits to form a nucleus (Firat-Karalar and Welch 2011). (B) In V. parahaemolyticus VopL, the VCD contributes to nucleation by stabilizing the pointed end of an actin nucleus and by allowing the dimerization of the WH2 domains that recruit G-actin. After nucleation, VopL dissociates from the growing filament but might remain bound to the pointed-end subunits, initiating a new cycle of polymerization (Namgoong et al. 2011). (C) Each V. cholerae VopF monomer contains three WH2 domains, but only the first and the third WH2 (the only WH2 represented in the figure for simplification) bind actin monomers. Nucleation involves dimerization of the WH2 domains and formation of a tetrameric actin nucleus where each terminal WH2 binds one barbed-end actin subunit. Processive elongation of the filament is accomplished by continuous displacement and re-association of the WH2 to newly added barbed-end subunits (Pernier et al. 2013). (D) Rickettsia conorii Sca2 monomers recruit profilin-actin via PRRs and G-actin via its WH2 domains. Interaction between its N- and C-terminal regions is thought to allow Sca2 to move processively during elongation, remaining at the barbed end and preventing filament capping (Madasu et al. 2013).