Calmodulin binding to Drosophila NinaC required for termination of phototransduction

The ninaC locus encodes two unconventional myosins, p132 and p174, consisting of fused protein kinase and myosin head domains expressed in Drosophila photoreceptor cells. NinaC are the major calmodulin-binding proteins in the retina and the NinaC-calmodulin interaction is required for the normal subcellular localization of calmodulin as well as for normal photo-transduction. In the current report, we present evidence for two calmodulin-binding sites in NinaC, C1 and C2, which have different in vitro binding properties. C1 was found to be common to both p132 and p174 while C2 was unique to p174. To address the requirements for calmodulin binding at each site in vivo, we generated transgenic flies expressing ninaC genes deleted for either C1 or C2. We found that the spatial localization of calmodulin depended on binding to both C1 and C2. Furthermore, mutation of either site resulted in a defective photoresponse. A prolonged depolarization afterpotential (PDA) was elicited at lower light intensities than necessary to produce a PDA in wild-type flies. These results suggest that calmodulin binding to both C1 and C2 is required in vivo for termination of phototransduction.