The Enzymatic Activity And Cellular Localization Of Drosophila Myosin 7a Is Regulated By A Novel Binding Protein

All myosin 7 isoforms have a conserved motor domain, 5 IQ motifs, a putative coiled-coil motif, and two MyTH4-FERM domains separated by an SH3 domain. Myosin 7a was found in regions of high actin density such as the stereocilia of the hair cells and bristles in flies. We have previously shown that the MgATPase of myosin 7a from Drosophila requires large amounts of actin to obtain Vmax (KATPase = 30 uM). Electron microscopy (EM) reveals that baculovirus-expressed myosin 7a is single-headed and the tail bends back upon the head. EM and truncation experiments suggest that an interaction between the third subdomain (termed MyTH7) of the second MyTH4-FERM domain and the myosin head are responsible for this regulation. A yeast-two-hybrid screening uncovered a myosin 7a binding partner, termed M7BP by using the last FERM domain as bait. M7BP has a C-terminal myosin binding domain and a putative Rab binding domain at the N-terminus. M7BP binds to the MyTH4-FERM domain of myosin 7a and activates myosin's MgATPase activity at low actin concentration (KATPase =2 μM), suggesting that M7BP may unfold myosin 7a. Co-expression of GFP-myosin 7a and M7BP-mCherry in Drosophila S2 cells resulted in a marked shape change with large areas of ruffling membrane, extensive polymerization of actin within the cell body, and the extension of numerous filopodia. The two proteins are extensively co-localized and are found predominantly in actin-rich regions, including filopodia. There is a tendency for the two proteins to be localized near the tips of filopodia and large punctuates of fluorescence can be observed moving in both directions in these structures. Present findings support that the novel drosophila myosin 7a binding-partner M7BP activates its ATPase enzymatic activities and motor functions.