A FERM domain autoregulates Drosophila myosin 7a activity

Abstract

Full-length Drosophila myosin 7a (myosin 7a-FL) has a complex tail containing a short predicted coiled coil followed by a MyTH4-FERM domain, an SH3 domain, and a C-terminal MyTH4-FERM domain. Myosin 7a-FL expressed in Sf9 cells is monomeric despite the predicted coiled coil. We showed previously that Subfragment-1 (S1) from this myosin has MgATPase of V_max ≈ 1s⁻¹ and K_ATPase ≈ 1 μM actin. We find that myosin 7a-FL has V_max similar to S1 but K_ATPase ≈ 30 μM. Thus, at low actin concentrations (5 μM), the MgATPase of S1 is fully activated, whereas that of myosin 7a-FL is low, suggesting that the tail regulates activity. Electron microscopy of myosin 7a-FL with ATP shows the tail is tightly bent back against the motor domain. Myosin 7a-FL extends at either high ionic strength or without ATP, revealing the motor domain, lever, and tail. A series of C-terminal truncations show that deletion of 99 aa (the MyTH7 subdomain of the C-terminal FERM domain) is sufficient to abolish bending, and the K_ATPase is then similar to S1. This region is highly conserved in myosin 7a. We found that a double mutation in it, R2140A-K2143A, abolishes bending and reduces K_ATPase to S1 levels. In addition, the expressed C-terminal FERM domain binds actin with K_d ≈ 30 μM regardless of ATP, similar to the K_ATPase value for myosin 7a-FL. We propose that at low cellular actin concentrations, myosin 7a-FL is bent and inactive, but at high actin concentrations, it is unfolded and active because the C-terminal FERM domain binds to actin.