PROTEIN-KINASE-C IS REQUIRED FOR LIGHT ADAPTATION IN DROSOPHILA PHOTORECEPTORS

PROTEIN kinase C (PKC) is a key enzyme for many cellular processes1,2 but its physiological roles are poorly understood. An excellent opportunity to investigate the function of PKC has been provided by the identification of an eye-specific PKC in Drosophila3-5 and a null PKC mutant, inaC(P209) (refs 5,6). Bright conditioning lights delivered to inaC photoreceptors lead to an abnormal loss of sensitivity in whole cell recordings from dissociated ommatidia; this has been interpreted as 'hyperadaptation' and PKC's role has been suggested to be distinct from light adaptation5. A presumably related finding is that during intense light, the response of inaC declines to baseline6. Invertebrate photoreceptors use the phosphoinositide signalling cascade7-12, responding to single photons with so-called quantum bumps13 which sum to form the macroscopic response to light14-16. Light adaptation allows photoreceptors to adjust their sensitivity over the enormous range of ambient intensities14,17. Although the molecular mechanism of light adaptation remains obscure, it is a negative-feedback process12 mediated by a rise in cytosolic calcium12,18 and a decrease in bump size12,14-16. We now show that under physiological conditions light adaptation is severely reduced in inaC, suggesting that eye-specific PKC, itself activated by a rise in cytosolic calcium4,5 and diacylglycerol, is required for adaptation. Furthermore, we show that in the absence of PKC individual bumps fail to terminate normally, an effect that can account for the pleiotropic manifestations of the inaC phenotype.