A CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE MODULATES DROSOPHILA PHOTORECEPTOR K+ CURRENTS - A ROLE IN SHAPING THE PHOTORECEPTOR POTENTIAL/

Light activation of Drosophila photoreceptors leads to the generation of a depolarizing receptor potential via opening of transient receptor potential and transient receptor potential-like cationic channels. Co unteracting the light-activated depolarizing current are two voltage-g ated K+ conductances, I-A and I-K, that are expressed in these sensory neurons. Here we show that Drosophila photoreceptors I-A and I-K are regulated by calcium-calmodulin (Ca2+/calmodulin) via a Ca2+/calmoduli n-dependent protein kinase (CaM kinase), with I-K being far more sensi tive than I-A. Inhibition of Ca2+/calmodulin by N-(6 aminohexyl)-5-chl oro-1-naphthalenesulfonamide or trifluoperazine markedly reduced the K + current amplitudes. Likewise, inhibition of CaM kinases by KN-93 pot ently depressed I-K and accelerated its C-type inactivation kinetics. The effect of KN-93 was specific because its structurally related but functionally inactive analog KN-92 was totally ineffective. In Drosoph ila photoreceptor mutant Sh(KS133), which allows isolation of I-K, we demonstrate by current-clamp recording that inhibition of I-K by quini dine or tetraethylammonium increased the amplitude of the photorecepto r potential, depressed light adaptation, and slowed down the terminati on of the light response. Similar results were obtained when CaM kinas es were blocked by KN-93. These findings place photoreceptor K+ channe ls as an additional target for Ca2+/calmodulin and suggest that I-K is well suited to act in concert with other components of the signaling machinery to sharpen light response termination and fine tune photorec eptor sensitivity during light adaptation.