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

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 Ca concentration, which in turn activates DmMyo5 to transport pigment granules. Considering the dynamic and compartmentation of Ca signaling in photoreceptor cells during light exposure, it is necessary to understand the kinetics of Ca interaction with DmMyo5. Here, we investigated the interaction of Ca 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 Ca affinity of CaM and decreases the dissociation rate of Ca from CaM. In addition, we found that Mlc-C, the light chain associated with the IQ2 of DmMyo5, has little effect on the Ca kinetics of CaM in IQ1. We propose that, by decreasing the Ca dissociation rate from CaM, IQ1 delays the deactivation of DmMyo5 after Ca transition, thereby prolonging the DmMyo5-driven transportation of pigment granules.