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Acanthamoeba castellanii myosins IA and IB demonstrate the catalytic properties of a myosin and can support analogues of contractile and motile activity in vitro, but their single, low molecular weight heavy chains, roughly globular shapes, and inabilities to self-assemble into filaments make them structurally atypical myosins. We now present the complete amino acid sequence of the 128-kDa myosin IB heavy chain, which we deduced from the nucleotide sequence of the gene and which reveals that the polypeptide is a fusion of myosin-like and non-myosin-like sequences. Specifically, the amino-terminal approximately 76 kDa of amino acid sequence is highly similar to the globular head sequences of conventional myosins. By contrast, the remaining approximately 51 kDa of sequence shows no similarity to any portion of conventional myosin sequences, contains regions that are rich in glycine, proline, and alanine residues, and lacks the distinctive sequence characteristics of an alpha-helical, coiled-coil structure. We conclude, therefore, that the protein is composed of a myosin globular head fused not to the typical coiled-coil rod-like myosin tail structure but rather to an unusual carboxyl-terminal domain. These results support the conclusion that filamentous myosin is not required for force generation and provide a further perspective on the structural requirements for myosin function. Finally, we find a striking conservation of intron/exon structure between this gene and a vertebrate muscle myosin gene. We discuss this observation in relation to the evolutionary origin of the myosin IB gene and the antiquity of myosin gene intron/exon structure.

The heavy chain of Acanthamoeba myosin IB is a fusion of myosin-like and non-myosin-like sequences.