Monoclonal antibodies demonstrate limited structural homology between myosin isozymes from Acanthamoeba.

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Abstract
A library of 31 monoclonal and 6 polyclonal antibodies was used to compare the structures of the 2 classes of cytoplasmic myosin isozymes isolated from Acanthamoeba: myosin-I, a 150,000-MW, globular molecule; and myosin-II, a 400,000-MW molecule with 2 heads and a 90-nm tail. This analysis confirms that myosin-I and -II are unique gene products and provides the first evidence that these isozymes have at least one structurally homologous region functionally important for myosin''s role in contractility. Characterization of the 23 myosin-II monoclonal antibody binding sites by antibody staining of 1-dimensional peptide maps and solid phase, competitive binding assays demonstrate that they bind to at least 15 unique sites on the myosin-II H chain. The antibodies can be grouped into 6 families, whose members bind close to one another. None of the monoclonal antibodies bind to myosin-II L chains, and polyclonal antibodies against myosin-II or H chain bind only to myosin-II L or H chains, respectively; no antibody binds both H and L chains. Six of 8 monoclonal antibodies and one of 2 polyclonal sera that react with the myosin-I H chain also bind to determinants on the myosin-II H chain. The cross-reactive monoclonal antibodies bind to the region of myosin-II recognized by the largest family of myosin-II monoclonal antibodies. In 2 subsequent studies, it is shown that this family of monoclonal antibodies to myosin-II binds to the myosin-II tail near the junction with the heads and inhibits both the actin-activated ATPase of myosin-II and contraction of gelled cytoplasmic extracts of Acanthamoeba cytoplasm. This structurally homologous region may play a key role in energy transduction by cytoplasmic myosins.