Eight monoclonal antibodies that bind to specific sites on the tail of Dictyostelium discoideum myosin were tested for their effects on polymerization and ATPase activity. Two antibodies that bind close to the myosin heads inhibited actin activation of the ATPase either partially or completely, without having an effect on polymerization. Two other antibodies bind to sites within the distal portion of the tail that has been shown, by cleavage mapping, to be important for polymerization. One of these antibodies binds close to the sites of heavy chain phosphorylation which is known to regulate both myosin polymerization and actin-activated ATPase activity. Both antibodies showed strong inhibition of polymerization accompanied by complete inhibition of the actin-activated ATPase activity. A unique effect was obtained with an antibody that binds to the end of the myosin tail. This antibody prevented the formation of bipolar filaments. It caused myosin to assemble into unipolar filaments with heads at one end and the antibody molecules at the other. Only at concentrations higher than required for its effect on polymerization did this antibody show substantial inhibition of the actin-activated ATPase. These results indicate that, using a monoclonal antibody as a blocking agent, parallel assembly of myosin can be dissected out from antiparallel association, and that essentially normal actin-activated ATPase activity could be obtained after significant reductions in filament size.

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