Effects of the IQ1 motif of Drosophila myosin-5 on the calcium interaction of calmodulin

Abstract

In Drosophila compound eyes, myosin-5 (DmMyo5) plays a key role in organelle transportation, including transporting pigment granules from the distal end to the proximal end of the photoreceptor cells to regulate the amount of light reaching the photosensitive membrane organelle rhabdomere. It is generally accepted that, upon exposure to light, the dark-adapted compound eyes produce a rapid rise of free Ca2+ concentration, which in turn activates DmMyo5 to transport pigment granules. Considering the dynamic and compartmentation of Ca2+ signaling in photoreceptor cells during light exposure, it is necessary to understand the kinetics of Ca2+ interaction with DmMyo5. Here, we investigated the interaction of Ca2+ with Drosophila calmodulin (CaM) in complex with the IQ1 of DmMyo5 using steady-state and kinetic approaches. Our results show that IQ1 binding substantially increases the Ca2+ affinity of CaM and decreases the dissociation rate of Ca2+ from CaM. In addition, we found that Mlc-C, the light chain associated with the IQ2 of DmMyo5, has little effect on the Ca2+ kinetics of CaM in IQ1. We propose that, by decreasing the Ca2+ dissociation rate from CaM, IQ1 delays the deactivation of DmMyo5 after Ca2+ transition, thereby prolonging the DmMyo5-driven transportation of pigment granules.