Participation of actin in cellular processes relies on the dynamics of filament assembly. Filament elongation is fed by monomeric actin in complex with either profilin or a Wiscott-Aldrich syndrome protein (WASP) homology domain 2 (WH2)/beta-thymosin (betaT) domain. WH2/betaT motif repetition (typified by ciboulot) or combination with nonrelated domains (as found in N-WASP) results in proteins that yield their actin to filament elongation. Here, we report the crystal structures of actin bound hybrid proteins, constructed between gelsolin and WH2/betaT domains from ciboulot or N-WASP. We observe the C-terminal half of ciboulot domain 2 bound to actin. In solution, we show that cibolout domains 2 and 3 bind to both G- and F-actin, and that whole ciboulot forms a complex with two actin monomers. In contrast, the analogous portion of N-WASP WH2 domain 2 is detached from actin, indicating that the C-terminal halves of the betaT and WH2 motifs are not functionally analogous.
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