Purealin is a small bioactive compound obtained from the marine sponge. The compound modulates various types of ATPase activity of myosin from skeletal muscle, cardiac muscle, and smooth muscle. To elucidate the structural basis of these effects of purealin on myosin ATPases, we examined the effect of purealin on the conformation of skeletal muscle myosin in aqueous solution and in glycerol. Analysis of the circular dichroism spectrum of subfragment 1, a single-headed fragment of myosin, revealed that in 10% glycerol purealin decreased the β-sheet content of S1, but in aqueous solution it had little effect on the secondary structure of S1. A myosin monomer conforms to two pear-like globular heads attached to a long tail. Electron microscopy observations with rotary shadowing revealed that purealin unfolded each globular head to an extended single strand. The tips of the unfolded strand bound each other and formed a ring in one molecule. These results suggest that binding of purealin affects the critical parameters of myosin folding.
Keywords: Circular dichroism; Electron microscopy; Myosins; Porifera; Skeletal muscle.