Wild-type body wall muscle cells of Caenorhabditis elegans produce at a constant ratio two myosin heavy chain isoforms, A and B, that form homodimeric myosins. Electron microscopy of negatively stained complexes of isoform-specific antibodies with isolated thick filaments shows that the surface of the 9.7 =B5m long filament is differentiated with respect to myosin content: a medial 1.8 =B5m zone contains myosin A and two polar 4.4 = =B5m zones contain myosin B. Biochemical and electron microscopic studies show that at 0.45 M KC1, pH 6.35, myosin B and paramyosin are solubilized. The medial all-myosin A region with novel core structures extending in a polar manner remain. These dissociation experiments suggest a sequential model for wild-type thick filament assembly in which myosins A and B would participate in the initiation and termination of assembly, respectively. Analysis of mutant thick filaments clarifies the relationship of the myosin isoforms. CB190 (
unc-54 I) thick filaments contain myosin A only and have normal length. CB1214 (
unc-15 I) mutants produce no paramyosin, and their thick filaments are composed of a medial myosin region