Abstract
The Drosophila ninaG mutant is characterized by low levels of Rh1 rhodopsin, because of the inability to transport this rhodopsin from the endoplasmic reticulum to the rhabdomere. ninaG mutants do not affect the biogenesis of the minor opsins Rh4 and Rh6. A genetic analysis placed the ninaG gene within the 86E4-86E6 chromosomal region. A sequence analysis of the 15 open reading frames within this region from the ninaG(P330) mutant allele identified a stop codon in the CG6728 gene. Germ-line transformation of the CG6728 genomic region rescued the ninaG mutant phenotypes, confirming that CG6728 corresponds to the ninaG gene. The NinaG protein belongs to the glucose-methanol-choline oxidoreductase family of flavin adenine dinucleotide-binding enzymes catalyzing hydroxylation and oxidation of a variety of small organic molecules. High performance liquid chromatography analysis of retinoids was used to gain insight into the in vivo role of the NinaG oxidoreductase. The results show that when Rh1 is expressed as the major rhodopsin, ninaG flies fail to accumulate 3-hydroxyretinal. Further, in transgenic flies expressing Rh4 as the major rhodopsin, 3-hydroxyretinal is the major retinoid in ninaG+, but a different retinoid profile is observed in ninaG(P330). These results indicate that the ninaG oxidoreductase acts in the biochemical pathway responsible for conversion of retinal to the rhodopsin chromophore, 3-hydroxyretinal.
Highlights
Vitamin A provides the retinal chromophores that mediate the sensing of light quanta by visual pigments
The Drosophila ninaG mutant is characterized by low levels of Rh1 rhodopsin, because of the inability to transport this rhodopsin from the endoplasmic reticulum to the rhabdomere. ninaG mutants do not affect the biogenesis of the minor opsins Rh4 and Rh6
We have carried out a molecular and functional analysis of the Drosophila ninaG gene. ninaG mutants have a severe reduction in the Rh1 rhodopsin because of a failure to transport newly synthesized rhodopsin from the endoplasmic reticulum to the rhabdomere
Summary
Vitamin A provides the retinal chromophores that mediate the sensing of light quanta by visual pigments. Rather vitamin A is obtained from the diet, either as preformed vitamin A such as retinol from animal sources, or as provitamin A carotenoids from plant sources These dietary forms must be both transported to the retina and chemically modified to play a key role in visual transduction [1, 2]. The ninaB encodes an oxygenase enzyme [7] responsible for cleaving -carotene to generate retinal Both of these gene products act outside of the retina to carry out their roles in vitamin A metabolism [8]. Five different retinoids are known to serve as visual pigment chromophores within the animal kingdom These are retinal, [3,4]-didehydroretinal, (3R)-3-hydroxyretinal, (3S)-3-hydroxyretinal, and (3R)-4-hydroxyretinal. This study provides the first molecular description of an enzyme active in modification of retinal to an alternative visual pigment chromophore
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