The Inhibited, Interacting-Heads Motif Characterizes Myosin II from the Earliest Animals with Muscles

Abstract

Electron microscopic studies have shown that the inhibited (relaxed) state of myosin II is characterized by an intramolecular interaction between myosin heads, in both thick filaments and isolated monomers. Interaction inhibits head activity by blocking actin-binding in one head and ATPase activity in the other. Interacting-heads are present in vertebrate smooth muscle and nonmuscle monomers, and in thick filaments of invertebrate smooth muscle (flatworms), invertebrate striated muscle (mollusks, arthropods) and vertebrate striated muscle (zebrafish, mouse and human); furthermore, head interactions underlie the off-state in myosins regulated by both Ca2+ binding and RLC phosphorylation. Head-head interaction has thus been conserved through evolution from mollusks and arthropods to humans. Our goal has been to determine how early this self-inhibiting motif arose, by studying the off-state of myosin monomers and/or filaments from primitive species. Filaments from sea anemones (Cnidaria), the most primitive animals with muscles, have the same 14.5-nm repeat as other muscles, but EM images have not yet been reconstructed. However, sea anemone myosin monomers, under relaxing conditions, have the same folded tail and bent-back, interacting-heads motif as myosins from the higher species, further demonstrating the very early origin of this structure. Interestingly, unlike the animals mentioned above, Acanthamoeba myosin monomers under relaxing conditions appear to lack a folded tail or interacting heads. The sequence of this myosin is strikingly different from the myosin IIs of animals, and its regulatory mechanism is quite different, suggesting that self-inhibition by interacting heads evolved after amoebae and animals diverged. Conservation of interacting heads over time suggests that it is a highly successful mechanism for regulating myosin activity. We are now studying the more primitive sea sponges (Porifera), one of the earliest animal groups, which lack muscles.