Protein kinase C is required for light adaptation in Drosophila photoreceptors

PROTEIN kinase C (PKC) is a key enzyme for many cellular processes ^1,2 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 Drosophila^3-5 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 'hyper-adaptation' 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 cascade^7-12, responding to single photons with so-called quantum bumps¹³ which sum to form the macroscopic response to light^14-16. Light adaptation allows photoreceptors to adjust their sensitivity over the enormous range of ambient intensities^14,17. Although the molecular mechanism of light adaptation remains obscure, it is a negative-feedback process¹² mediated by a rise in cytosolic calcium^12,18 and a decrease in bump size^12,14-16. 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 calcium^4,5 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.