Comparison of the ADF-H domain–actin complex with other conserved actin-binding domains in complex with G-actin. (A) Twf-C binds to the “hot spot” between actin subdomains 1 and 3 similarly to gelsolin segment 1 (S1) and the WH2 domain of ciboulot. These three proteins inhibit the nucleotide exchange on the actin monomer and keep the cleft between actin subdomains 2 and 4 in a “closed” conformation (red arrow). Bovine profilin binds “behind” the hydrophobic cleft between actin subdomain 1 and 3. However, profilin appears to maintain the actin monomer in an “open” state (blue arrow) and promotes nucleotide exchange. (B) Comparison of G-actin interactions of gelsolin S1 and Twf-C. Gelsolin S1 (McLaughlin et al., 1993) is shown in yellow and Twf-C in blue. The most significant differences in the actin interactions are indicated by red arrows. These are: interaction of the loop before the C-terminal α-helix of Twf-C with actin subdomain 3 (left) and different interaction sites of N-terminal extensions of Twf-C and gelsolin S1 in actin (right).