Monoclonal antibodies prepared against Dictyostelium actin: characterization and interactions with actin.

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Abstract
Three mouse monoclonal antibodies, Act I, Act II, and Act IV, against actin from the cellular slime mold D. discoideum, were made and characterized. All 3 antibodies are IgG. They form stable complexes with monomeric Dictyostelium actin, which prevents polymerization of the actin into filaments. On addition to preformed actin filaments, they cause a reduction in filament size and in the viscosity of the actin solution. They cross-react strongly with actins from the lower eukaryotes Physarum and Acanthamoeba, but not with .alpha.-actins from rabbit and human muscle or .beta.- and .gamma.-actins from human erythrocytes and a human B lymphoid cell line. Act II and Act IV recognize a similar antigenic determinant that is topographically distinct from that identified by Act I. In protein immunoblotting, only Act I bound strongly to Dictyostelium actin. Analysis of actin fragments with this technique showed that amino acids 13 to .apprx. 50 are required for Act I binding to actin. A comparison of the amino acid sequences of actins from lower eukaryotes and higher vertebrates implicates threonine 41 as a critical residue in the Act I antigenic site. The properties of Act II and Act IV suggest that they recognize antigenic sites involving the NH2-terminal 6 residues.