DS151 is the first 151 amino acids of the Dictyostelium discoidium protein severin, which shares high sequence similarity with segment 1 of the actin-severing protein gelsolin. DS151 is able to mediate the depolymerization of F-actin in a calcium-dependent fashion, much like segment 1 of gelsolin. A structural model of DS151 was obtained by comparative modeling studies with segment 1 of gelsolin. This model was tested by studies of chemical cross linking between DS151 and bound actin, suggesting that Cys residues on DS151 are cross linked with Lys residues of actin. The model suggests that Cys125 of DS151 cross links with either Lys326 or Lys328 of actin. Mutagenesis of DS151 demonstrates that Cys125 of DS151 dominates the cross linking, whereas Cys25 of DS151 makes a minor contribution through a longer-range cross link with Cys374 of actin, which likely involves flexibility of both proteins in that region.