Role of Protein Phosphatase 2A in Regulating the Visual Signaling in Drosophila

Drosophilavisual signaling, a G-protein-coupled phospholipase Cβ (PLCβ)-mediated mechanism, is regulated by eye-protein kinase C (PKC) that promotes light adaptation and fast deactivation, most likely via phosphorylation of inactivation no afterpotential D (INAD) and TRP (transient receptor potential). To reveal the critical phosphatases that dephosphorylate INAD, we used several biochemical analyses and identified protein phosphatase 2A (PP2A) as a candidate. Importantly, the catalytic subunit of PP2A, microtubule star (MTS), is copurified with INAD, and an elevated phosphorylation of INAD by eye-PKC was observed in threemtsheterozygotes. To explore whether PP2A (MTS) regulates dephosphorylation of INAD by counteracting eye-PKC [INAC (inactivation no afterpotential C]in vivo, we performed ERG recordings. We discovered thatinaCP209was semidominant, becauseinaCP209heterozygotes displayed abnormal light adaptation and slow deactivation. Interestingly, the deactivation defect ofinaCP209heterozygotes was rescued by themtsXE2258heterozygous background. In contrast,mtsXE2258failed to modify the severe deactivation ofnorpAP16, indicating that MTS does not modulate NORPA (no receptor potential A) (PLCβ). Together, our results strongly indicate that dephosphorylation of INAD is catalyzed by PP2A, and a reduction of PP2A can compensate for a partial loss of function in eye-PKC, restoring the fast deactivation kineticsin vivo. We thus propose that the fast deactivation of the visual response is modulated in part by the phosphorylation of INAD.