Conservation of the actin monomer binding site in ADF/cofilins and twinfilins. (A) Three major sites of interaction are present in the Twf-C–G-actin structure: (left) The N-terminal extension of Twf-C, (middle) the long α-helix, and (right) the loop before the C-terminal helix. Close-up figures illustrate some of the major contacts observed in the structure. (B) A structural sequence alignment between Twf-C, Twf-N, and ADF/cofilin. The residues shown to be important for G-actin interactions by mutagenesis are indicated by asterisks. Residues identified in a synchrotron footprinting study as G-actin–interacting residues in ADF/cofilin are indicated by hash marks. Interacting peptides from the same study are shown as black lines. Interface residues identified from our crystal structure are displayed as red lines below the sequences.