Analysis of the actin binding site of ABP-120 from Dictyostelium discoideum

Abstract

Proteolysis experiments of ABP-120 from Dictyostelium discoideum demonstrate that removal of residues 89-115 from a 17-kDa tryptic peptide which retains actin binding activity, abolishes actin binding. Antibodies made against a synthetic peptide of this 27 amino acid sequence (27mer) specifically immunoprecipitate native ABP-120 from Dictyostelium high speed supernatants, demonstrating that at least a portion of the 27mer sequence is on the surface of the molecule as expected for an active site. ABP-120 is also inhibited in its binding to F-actin by Fab{dollar}\sp\prime{dollar} fragments of the anti-27mer IgG. Half-maximal inhibition occurs at an approximate molar ratio of 7 Fab{dollar}\sp\prime{dollar} fragments per ABP-120 monomer. Viscoelastic measurements indicate that ABP-120 forms fewer cross-links with F-actin in the presence of the 27mer synthetic peptide than in its absence. In F-actin cosedimentation assays, the binding of ABP-120 to actin is inhibited by the 27mer synthetic peptide, as well. In addition, the 27mer synthetic peptide cosediments with F-actin, whereas a control hydrophobic peptide and a synthetic peptide of residues 69-88 of ABP-120 do not cosediment with F-actin. These observations suggest a direct involvement of the 27mer sequence in the actin binding activity of ABP-120. The 27mer sequence is highly conserved in a number of diverse proteins including fimbrin/plastin, {dollar}\alpha{dollar}-actinin, {dollar}\beta{dollar}-spectrin, dystrophin and actin binding protein (ABP-280/non-muscle filamin). Given the conservation of the 27mer sequence in a number of actin cross-linking proteins, we suggest that the 27mer sequence is an archetypical binding site for a class of proteins which bind to the sides of actin filaments.