The Role of Drosophila ninaG Oxidoreductase in Visual Pigment Chromophore Biogenesis

Abstract

We previously reported (Sarfare, S., Ahmad, S. T., Joyce, M. V., Boggess, B., and O'Tousa, J. E. (2005) J. Biol. Chem. 280, 11895-11901) that the Drosophila ninaG gene encodes an oxidoreductase involved in the biosynthesis of the (3S)-3-hydroxyretinal serving as chromophore for Rh1 rhodopsin and that ninaG mutant flies expressing Rh4 as the major opsin accumulate large amounts of a different retinoid. Here, we show that this unknown retinoid is 11-cis-3-hydroxyretinol. Reversed phase high performance liquid chromatography coupled with a photodiode array UV-visible absorbance detector and mass spectrometer revealed a major product eluting at a retention time, t r , of 3.5 min with a max of ϳ324 nm and with a base peak in the mass spectrum at m/z 285. These observations are identical with those of the 3-hydroxyretinol standard. The base peak in the electrospray ionization mass spectrum arises from the loss of a water molecule from the protonated molecule at m/z 303 because of fragmentation in the ion source. These results suggest that 11-cis-3-hydroxyretinol is an intermediate required for chromophore biogenesis in Drosophila. We further show that ninaG mutants fed on retinal as the sole source of vitamin A are able to synthesize 3-hydroxyretinoids. Thus, the NinaG oxidoreductase is not responsible for the initial hydroxylation of the retinal ring but rather acts in a subsequent step in chromophore production. These data are used to review chromophore biosynthesis and propose that NinaG acts in the conversion of (3R)-3-hydroxyretinol to the 3S enantiomer.Photoisomerization of a retinoid chromophore covalently linked to the apoprotein opsin underlies the light-sensing ability of rhodopsins. Animals lack the ability to synthesize retinoids de novo and, therefore, derive retinoids from carotenoids present in the diet. In insects, three kinds of vitamin A derivatives, retinal, (3R)-3-hydroxyretinal, and (3S)-3-hydroxyretinal, function as chromophores (1). The Dipteran suborder Cyclorrapha that includes Drosophila is the only suborder described so far that utilizes (3S)-3-hydroxyretinal as the Rh1 rhodopsin chromophore (2). The use of (3S)-3-hydroxyretinal occurs in visual pigments exhibiting both UV and visible light sensitivity. This led to the suggestion that UV light sensitivity is because of the unique ability of (3S)-3-hydroxyretinal to accept energy transferred from a UV-sensitizing pigment (1).The Drosophila ninaG mutant is characterized by an abnormal electroretinogram response lacking the prolonged depolarization afterpotential. The lack of prolonged depolarization afterpotential, a characteristic of a group of "nina" 2 genes, is a manifestation of low levels of the major opsin, Rh1, expressed in the R1-6 photoreceptor cells (3). Low Rh1 rhodopsin in ninaG mutants is not due to defects in Rh1 opsin transcription, but rather, the ability to synthesize the Rh1 chromophore is defective. However, the ninaG defect does not affect the expression of minor opsins Rh4 and Rh6 (4). Rh4 and Rh6 are expressed in the R7 a...

Dissection of the pathway required for generation of vitamin A and for Drosophila phototransduction

“…The all-trans-retinal is then metabolized into vitamin A and transferred to the retinal pigment cells, where it is converted into the chromophore, through a process involving the PINTA retinoid binding protein (Wang and Montell, 2005). The NINAG oxidoreductase also participates in the production of the chromophore, in a step subsequent to the formation of vitamin A (Sarfare et al, 2005; Ahmad et al, 2006), although it remains to be determined whether it functions in the retinal pigment cells or in photoreceptor cells.…”

Phototransduction and retinal degeneration in Drosophila

“…The chromophore is transported to the photoreceptor cells where it binds to the opsin resulting in the generation of rhodopsin pigment epithelium (RPE) [72], it appears that Drosophila retinal pigment cells are the closest functional equivalent to the RPE. The ninaG gene encodes an oxidoreductase, which is proposed to act in the conversion of (3R)-3hydroxyretinol to the 3S enantiomer in the compound eye [82,83]. However, it is not known whether ninaG functions in the retinal pigment cells or in photoreceptor cells.…”

Molecular genetics of retinal degeneration: A Drosophila perspective

“…RDGC catalyzes the dephosphorylation of rhodopsin, 41,42 whereas CaMKII is involved in the phosphorylation of arrestin 2 (Arr2), 43,44 the major visual arrestin critical for the inactivation of activated rhodopsin. 45 Eye-PKC is a conventional PKC 46 activated by both 69 and santa maria (scavenger receptor acting in neural tissue and majority of rhodopsin is absent). 70 The stability of Drosophila Rh1 rhodopsin is dependent on its incorporation of the retinal chromophore: 71 flies defective in proteins that control the uptake or processing of vitamin A, contain a reduced level of Rh1.…”