Genetic evidence that Acanthamoeba myosin I is a true myosin

Acanthamoeba castellanii contains two enzymes, myosins IA and IB, that exhibit the catalytic properties of a myosin but possess very unusual physical properties, the most striking of which are their single, low molecular weight heavy chain, their globular shape, and their inability to form bipolar filaments. We have now isolated a putative myosin IB heavy chain gene from Acanthamoeba, using as a heterologous probe a portion of a sarcomeric myosin heavy chain gene from Caenorhabditis elegans. The amoeba genomic clone hybridizes to a 4250-nucleotide RNA species and hybrid-selects an mRNA encoding a 125-kDa polypeptide. This polypeptide comigrates exactly with the heavy chain of purified amoeba myosin IB and is specifically immunoprecipitated with antiserum to myosin IB. We sequenced two restriction enzyme fragments of this gene, and the deduced amino acid sequences show strong homology with the regions of muscle myosins that contain the reactive thiols and the ATP binding site. Our identification of a myosin IB heavy chain gene demonstrates that myosin IB, despite the unusually low molecular weight of its heavy chain, is a true gene product. The sequence results show that, despite its atypical physical properties, myosin IB is clearly related to conventional myosins.