Functional characterization of the three Drosophila retinal degeneration C (RDGC) protein phosphatase isoforms.

Drosophila retinal degeneration C (RDGC) is the founding member of the PPEF family of protein phosphatases. RDGC mediates dephosphorylation of the visual pigment rhodopsin and the TRP ion channel. From the rdgC locus, three protein isoforms, termed RDGC-S, -M, and -L, with different N-termini are generated. Due to fatty acylation, RDGC-M and -L are attached to the plasma membrane while RDGC-S is soluble. To assign physiological roles to these RDGC isoforms, we constructed flies that express various combinations of RDGC protein isoforms. Expression of the RDGC-L isoform alone did not fully prevent rhodopsin hyperphosphorylation and resulted in impaired photoreceptor physiology and in decelerated TRP dephosphorylation at Ser936. However, expression of RDGC-L alone as well as RDGC-S/M was sufficient to prevent degeneration of photoreceptor cells which is a hallmark of the rdgC null mutant. Membrane-attached RDGC-M displayed higher activity of TRP dephosphorylation than the soluble isoform RDGC-S. Taken together, in vivo activities of RDGC splice variants are controlled by their N-termini.

Solubility and subcellular localization of the three Drosophila RDGC phosphatase variants are determined by acylation.

Protein phosphorylation is an abundant molecular switch that regulates a multitude of cellular processes. In contrast to other subfamilies of phosphoprotein phosphatases, the PPEF subfamily is only poorly investigated. Drosophila retinal degeneration C (RDGC) constitutes the founding member of the PPEF subfamily. RDGC dephosphorylates the visual pigment rhodopsin and the ion channel TRP.However, rdgC null mutant flies exhibit rhodopsin and TRP hyperphosphorylation, altered photoreceptor physiology, and retinal degeneration. Here, we report the identification of a third RDGC protein variant and show that the three RDGC isoforms harbor different N-termini that determine solubility and subcellular targeting due to fatty acylation. Taken together, solubility and subcellular targeting of RDGC splice variants are determined by their N-termini.

The latency of the light response is modulated by the phosphorylation state of Drosophila TRP at a specific site.

Drosophila photoreceptors respond to oscillating light of high frequency (∼100 Hz), while increasing the oscillating light intensity raises the maximally detected frequency. Recently, we reported that dephosphorylation of the light-activated TRP ion channel at S936 is a fast, graded, light-, and Ca2+-dependent process. We further found that this process affects the detection limit of high frequency oscillating light. Accordingly, transgenic Drosophila, which do not undergo phosphorylation at the S936-TRP site (trpS936A), revealed a short time-interval before following the high stimulus frequency (oscillation-lock response) in both dark- and light-adapted flies. In contrast, the trpS936D transgenic flies, which mimic constant phosphorylation, showed a long-time interval to oscillation-lock response in both dark- and light-adapted flies. Here we extend these findings by showing that dark-adapted trpS936A flies reveal light-induced current (LIC) with short latency relative to trpWT or trpS936D flies, indicating that the channels are a limiting factor of response kinetics. The results indicate that properties of the light-activated channels together with the dynamic light-dependent process of TRP phosphorylation at the S936 site determine response kinetics.