Myosin superfamily evolutionary history.

The superfamily of myosin proteins found in eukaryotic cells is known to contain at least 18 different classes. Members are classified based on the phylogenetic analysis of the head domains located at the amino terminus of the polypeptide. While phylogenetic relationships provide insights into the functional relatedness of myosins within and between families, the evolutionary history of the myosin superfamily is not revealed by such studies. In order to establish the evolutionary history of the superfamily, we analyzed the representation of myosin gene families in a range of organisms covering the taxonomic spectrum. The amino acid sequences of 232 myosin heavy chains, as well as 65 organisms representing the protist, plant, and animal kingdoms, were included in this study. A phylogenetic tree of organisms was constructed based on several complementary taxonomic classification schemes. The results of the analysis support an evolutionary hypothesis in which myosins II and I evolved the earliest of all the myosin groups. Myosins V and XI evolved from a common myosin II-like ancestor, but the two families diverged to either the plant (XI) or animal (V) lineage. Class VII myosin appeared fourth among the families, and classes VI and IX appeared later during the early period of metazoan radiation. Myosins III, XV, and XVIII appeared after this group, and X appeared during the formative phases of vertebrate evolution. The remaining members of the myosin superfamily (IV, VI, XII, XIII, XIV, XVI, and XVII) are limited in distribution to one or more groups of organisms. The evolutionary data permits one to predict the likelihood that myosin genes absent from a given species are either missing (not found yet because of insufficient data) or lost due to a mutation that removed the gene from an organism's lineage. In conclusion, an analysis of the evolutionary history of the myosin superfamily suggests that early-appearing myosin families function as generalists, carrying out a number of functions in a variety of cell types, while more recently evolved myosin families function as specialists and are limited to a few organisms or a few cell types within organisms.