Evidence for an actin binding helix in gelsolin segment 2; have homologous sequences in segments 1 and 2 of gelsolin evolved to divergent actin binding functions?

Gelsolin is built up of six homologous segments that perform different functions on actin. Segments 1 and 2, which are suggested to be highly similar in their overall folds, bind monomeric and filamentous actin respectively. A long alpha-helix in segment 1 forms the major contact site of this segment with actin. We show that sequence 197-226 of segment 2, equivalent to the region around the actin binding helix in segment 1, contains F-actin binding activity. Consequently, positionally similar parts of segment 1 and 2 are implicated in the actin contact and solvent exposed residues in these parts must have evolved differentially to meet their different actin binding properties.