Myosin VII is an unconventional myosin widely expressed in organisms ranging from amoebae to mammals that has been shown to play vital roles in cell adhesion and phagocytosis. Here we present the first study of the mechanism of action of a myosin VIIa from Drosophila melanogaster. We have expressed a truncated single-headed Drosophila myosin VIIa construct in the baculovirus-Sf9 system that bound calmodulin light chains. By using steady-state and transient kinetic methods, we showed that myosin VIIa exhibits a fast release of phosphate and a slower, rate-limiting ADP release from actomyosin. As a result, myosin VIIa will be predominantly strongly bound to actin during steady-state ATP hydrolysis (its duty ratio will be at least 80%). This kinetic pattern is, in many respects, similar to that of the single-molecule vesicle transporters myosin V and VI. We expressed two HMM-like fragments of Drosophila myosin VIIa, including one in which a leucine zipper motif was added after the endogenous predicted coiled-coil forming domain. Our studies indicate that only the zippered HMM effectively moved processively on actin filaments, suggesting that the non-zippered form may not dimerize effectively on its own. The zippered HMM was shown to have 30-nm steps and to move in a hand over hand manner on actin. This step size is consistent with this myosin having a neck, which is 5IQ motifs long.
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