Despite many biochemical and structural similarities between muscle and nonmuscle myosins, their genes appear to have completely diverged, since muscle myosin molecular clones will not hybridize to RNA from nonmuscle sources. Here we report the isolation and characterization of a partial myosin heavy chain (MHC) cDNA clone from the slime mold Dictyostelium discoideum. We have isolated this clone from a Xgtll expression cDNA library by antibody screening. In contrast to the highly conserved sarcomeric muscle MHC multigene families in other organisms, there appears to be only one gene encoding MHC in the Dictyostelium genome. The cloned portion of this gene does not hybridize to the genomic DNAs of other eukaryotic organisms. Analysis of the predicted amino acid sequence of the partial Dictyostelium MHC done shows that while there is no sequence homology to known striated muscle MHCs, the structure-and coiled-coil-forming capacities have been conserved.Myosins are ubiquitous in eukaryotes, where they convert chemical energy into mechanical force through the hydrolysis of ATP. This generation of force manifests itself in muscle cells by contraction and in nonmuscle cells (e.g., macrophages and fibroblasts) by many fundamental cellular processes such as phagocytosis and cell division. The myosin molecule consists of two heavy chains (about 200 kDa each) and two pairs of light chains (15-20 kDa each). The carboxylterminal portion of the heavy chain forms an a-helical tail, and the globular amino-terminal portion contains the ATPase activity and the binding sites for the myosin light chains and actin. An intriguing aspect of myosin structure and function is that there are many biochemical similarities between muscle and nonmuscle myosins, and yet they appear to have completely diverged at the sequence level. Myosin heavy chain (MHC) cDNA probes from skeletal or cardiac muscle will hybridize to each other but will not cross-hybridize with RNA from smooth muscle or nonmuscle tissue (1).The availability of cloned DNA sequences corresponding to sarcomeric MHCs has been invaluable in elucidating features of their primary structure and regulated expression. Sarcomeric MHCs are encoded by highly conserved multigene families of at least 10 members (2-5). Rat MHC genes will cross-hybridize with DNA from such diverse organisms as humans, goldfish, and sea urchins (6). Protein and nucleic acid studies have shown that within striated muscle, there are multiple tissue-and developmental-specific MHC forms that are very closely related, but distinct (e.g., see ref. 7). It is presumed that these various forms are functionally significant. Molecular genetic analysis of different MHC forms has begun to identify those areas of the molecule that are type specific and those that are more highly conserved. Similar approaches to the study of nonmuscle MHC have not been possible due to the inability to isolate such molecular clones. Here we present the isolation of a partial MHC cDNA clone from the slime mold Dictyostelium discoideum...