The sequence of the dictyostelium myo J heavy chain gene predicts a novel, dimeric, unconventional myosin with a heavy chain molecular mass of 258 kDa.

The complete sequence of the Dictyostelium myo J heavy chain gene has been determined from overlapping genomic clones. The gene spans approximately 7400 base pairs, is split by two small introns, and encodes a 2241-residue, 258-kDa heavy chain polypeptide that that is composed of an N-terminal 944-residue myosin head domain, a central 863-residue domain that is predicted to form an alpha helical coiled-coil containing six hinges, and a C-terminal 434-residue globular domain. The head domain is notable in that it contains a approximately 30 residue insert near the nucleotide binding pocket, and five potential calmodulin/myosin light chain binding sites at the head/tail junction. The existence within the Myo J tail domain of both an extensive coiled-coil structure and a large globular domain suggests that this myosin is dimeric and incapable of self-assembly into filaments. While these properties, as well as the overall predicted structure of the Myo J protein, are reminiscent of class V myosins, the sequence of the 434-residue globular tail piece of Myo J shows no similarity to that of either yeast or vertebrate myosins V. Consistent with this, phylogenetic analyses based on myosin head sequence comparisons do not classify Myo J as a type V myosin. These and other sequence comparisons indicate that Myo J and two as-yet-unclassified unconventional myosins from Arabidopsis represent members of the newest class within the myosin superfamily (class XI). Northern blots analyses suggest that Myo J may function predominantly in vegetative Dictyostelium cells. Finally, Southern blot analyses suggest that Dictyostelium possesses another myosin that is very closely related to Myo J.