Activation of Drosophila Melanogaster Myosin-5 Motor Function by Calcium and Cargo-Binding Protein

Abstract

In Drosophila melanogaster compound eye, myosin-5 (DmM5) plays two distinct roles in response to light stimulation: to transport pigment granules from cytosol to the rhabdomere base to decrease light exposure and to transport rhodopsin-bearing vesicles to the rhabdomere base to compensate for the rhodopsin loss during light exposure. The association of DmM5 with pigment granule and rhodopsin-bearing vesicle are mediated by two cargo-binding proteins, i.e., by dRab11 and Ltd, respectively. However, little is known how these two cargo-binding proteins affect the motor function of DmM5. Here we succeeded in expressing the recombinant DmM5 and studied its regulation by calcium and cargo-binding proteins. The actin-activated ATPase activity of DmM5 is significantly lower than that of the truncated DmM5 without the C-terminal globular tail domain (GTD), indicating that the GTD is the inhibitory domain. The actin-activated ATPase activity of full-length DmM5 is significantly stimulated by micromolar level calcium. In contrast, the actin-activated ATPase activity of the truncated DmM5 without GTD is partially inhibited by calcium and this inhibition could not be rescued by exogenous calmodulin (CaM). Among the two cargo-binding proteins of DmM5 in Drosophila compound eye, GTP-bound dRab11 significantly activates the actin-activated ATPase activity of DmM5, whereas Ltd has little effect on it. A single residue mutation in the GTD, Q1689A, disrupts the interaction between the GTD and dRab11 and abolishes the dRab11-dependent activation of the actin-activated ATPase activity of DmM5, indicating that dRab11 abolishes the inhibition of DmM5 by the GTD. Based on those results, we propose that DmM5-dependent transport of pigment granule is directly activated by light-induced calcium influx and DmM5-dependent transport of rhodopsin-bearing vesicle is activated by active GTP-bound dRab11, whose formation is stimulated by light-induced calcium influx.