A millennial myosin census

The past decade has seen a remarkable explosion in our knowledge of the siz e and diversity of the myosin superfamily. Since these actin-based motors a re candidates to provide the molecular basis for many cellular movements, i t is essential that motility researchers be aware of the complete set of my osins in a given organism. The availability of cDNA and/or draft genomic se quences from humans, Drosophila melanogaster, Caenorhabditis elegans, Arabi dopsis thaliana, Saccharomyces cerevisiae, Schizosaccharomyces pombe, and D ictyostelium discoideum has allowed us to tentatively define and compare th e sets of myosin genes in these organisms. This analysis has also led to th e identification of several putative myosin genes that may be of general in terest. In humans, for example, we find a total of 40 known or predicted my osin genes including two new myosins-I, three new class II (conventional) m yosins, a second member of the class III/ninaC myosins, a gene similar to t he class XV deafness myosin, and a novel myosin sharing at most 33% identit y with other members of the superfamily. These myosins are in addition to t he recently discovered class XVI myosin with N-terminal ankyrin repeats and two human genes with similarity to the class XVIII PDZ-myosin from mouse. We briefly describe these newly recognized myosins and extend our previous phylogenetic analysis of the myosin superfamily to include a comparison of the complete or nearly complete inventories of myosin genes from several ex perimentally important organisms.