Ca²⁺ Signaling in Drosophila Photoreceptor Cells

In Drosophila photoreceptor cells, Ca²⁺ exerts regulatory functions that control the shape, duration, and amplitude of the light response. Ca²⁺ also orchestrates light adaptation allowing Drosophila to see in light intensity regimes that span several orders of magnitude ranging from single photons to bright sunlight. The prime source for Ca²⁺ elevation in the cytosol is Ca²⁺ influx from the extracellular space through light-activated TRP channels. This Ca²⁺ influx is counterbalanced by constitutive Ca²⁺ extrusion via the Na⁺/Ca²⁺ exchanger, CalX. The light-triggered rise in intracellular Ca²⁺ exerts its regulatory functions through interaction with about a dozen well-characterized Ca²⁺ and Ca²⁺/CaM binding proteins. In this review we will discuss the dynamic changes in Ca²⁺ concentration upon illumination of photoreceptor cells. We will present the proteins that are known to interact with Ca²⁺ (/CaM) and elucidate the physiological functions of these interactions.