Role of Ca2+/Calmodulin-dependent Protein Kinase II in Drosophila Photoreceptors

William L Pak,Ning Wang,Hung-Tat Leung,Haiqin Lu,Bih-Hwa Shieh

doi:10.1074/jbc.m806956200

Abstract

Ca(2+) modulates the visual response in both vertebrates and invertebrates. In Drosophila photoreceptors, an increase of cytoplasmic Ca(2+) mimics light adaptation. Little is known regarding the mechanism, however. We explored the role of the sole Drosophila Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) to mediate light adaptation. CaMKII has been implicated in the phosphorylation of arrestin 2 (Arr2). However, the functional significance of Arr2 phosphorylation remains debatable. We identified retinal CaMKII by anti-CaMKII antibodies and by its Ca(2+)-dependent autophosphorylation. Moreover, we show that phosphorylation of CaMKII is greatly enhanced by okadaic acid, and indeed, purified PP2A catalyzes the dephosphorylation of CaMKII. Significantly, we demonstrate that anti-CaMKII antibodies co-immunoprecipitate, and CaMKII fusion proteins pull down the catalytic subunit of PP2A from fly extracts, indicating that PP2A interacts with CaMKII to form a protein complex. To investigate the function of CaMKII in photoreceptors, we show that suppression of CaMKII in transgenic flies affects light adaptation and increases prolonged depolarizing afterpotential amplitude, whereas a reduced PP2A activity brings about reduced prolonged depolarizing afterpotential amplitude. Taken together, we conclude that CaMKII is involved in the negative regulation of the visual response affecting light adaptation, possibly by catalyzing phosphorylation of Arr2. Moreover, the CaMKII activity appears tightly regulated by the co-localized PP2A.

Highlights

Visual transduction is the process that converts the signal of light into a change of membrane potential in photoreceptors
We explored the role of the sole Drosophila Ca2؉/calmodulin-dependent protein kinase II (CaMKII) to mediate light adaptation
To investigate the function of CaMKII in photoreceptors, we show that suppression of CaMKII in transgenic flies affects light adaptation and increases prolonged depolarizing afterpotential amplitude, whereas a reduced phosphatase 2A (PP2A) activity brings about reduced prolonged depolarizing afterpotential amplitude

Summary

CaMKII Modulates Light Adaptation in the Fly

We report the biochemical and electrophysiological analyses of CaMKII in Drosophila retina. We demonstrate that suppression of CaMKII in ala transgenic flies leads to a phenotype indicative of defective light adaptation. The ala flies display greater visual response, suggesting a defect in Arr. The ala flies display greater visual response, suggesting a defect in Arr2 These results support the notion that CaMKII plays a role in the negative regulation of the visual response. Our biochemical analyses demonstrate that dephosphorylation of CaMKII is mediated by protein phosphatase 2A (PP2A). We show that mts heterozygotes display reduced prolonged depolarizing potential (PDA) amplitude This PDA phenotype strongly suggests that Arr becomes more effective to terminate the visual signaling in mts flies. Our findings indicate that the ability of Arr to terminate metarhodopsin is increased upon phosphorylation by CaMKII, and the retinal CaMKII activity is regulated by PP2A

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION