Myosin II Head Interaction in Primitive Species

Myosin II is a two-headed motor protein with an elongated α-helical tail. The motor domain of each head interacts with actin to convert the chemical energy of ATP into movement. Myosin II activity in muscle and nonmuscle cells is switched off by intramolecular interaction between its two heads, which inhibits their activity. This has been shown by EM and image processing of myosin filaments and isolated myosin molecules. In switched-off single molecules, the myosin tail folds into three segments, with the interacting heads folded back on the tail. The interacting-heads motif is highly conserved, being found in vertebrate and invertebrate smooth and striated muscle and in nonmuscle cells. We are investigating its evolutionary origins by EM imaging of isolated myosin molecules in the off-state. In previous work we found that the motif was present as far back as Cnidaria (sea anemones), the earliest animals with muscles. Here we have studied additional animal and non-animal species. At high (0.5 M) salt, all the myosin IIs showed the typical appearance of an extended tail and non-interacting heads. At low salt (0.15 M), under relaxing conditions (MgATP), insect indirect flight and embryonic skeletal muscle myosins showed a folded tail and similar head-head interactions to other muscles. Three species of primitive, non-animal myosins gave differing results. Acanthamoeba (reported previously) and Schizosaccharomyces pombe showed extended tails and no head-head interactions. The tails of these two myosins were approximately 30-40% shorter than the animal myosins, possibly accounting for their inability to fold. In contrast, Dictyostelium, with a tail 10% longer than animal myosin, showed head-head interactions similar to animal myosin; however, the conformation of the folded tail was different. These results suggest that head-head interaction arose before the evolution of animals.