Electron microscopy of myosin-II molecules and filaments reacted with monoclonal antibodies demonstrates directly where the antibodies bind and shows that certain antibodies can inhibit the polymerization of myosin-II into filaments. The binding sites of seven of 23 different monoclonal antibodies were localized by platinum shadowing of myosin monomer-antibody complexes. The antibodies bind to a variety of sites on the myosin-II molecule, including the heads, the proximal end of the tail near the junction of the heads and tail, and the tip of the tail. The binding sites of eight of the 23 antibodies were also localized on myosin filaments by negative staining. Antibodies that bind to either the myosin heads or to the proximal end of the tail decorate the ends of the bipolar filaments. Some of the antibodies that bind to the tip of the myosin-II tail decorate the bare zone of the myosin-II thin filament with 14-nm periodicity. By combining the data from these electron microscope studies and the peptide mapping and competitive binding studies we have established the binding sites of 16 of 23 monoclonal antibodies. Two of the 23 antibodies block the formation of myosin-II filaments and given sufficient time, disassemble preformed myosin-II filaments. Both antibodies bind near one another at the tip of the myosin-II tail and are those that decorate the bare zone of preformed bipolar filaments with 14-nm periodicity. None of the other antibodies affect myosin filament formation, including one that binds to another site near the tip of the myosin-II tail. This demonstrates that antibodies can inhibit polymerization of myosin-II, but only when they bind to key sites on the tail of the molecule.

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