Effects of a mutation in the Drosophila porin gene encoding mitochondrial voltage-dependent anion channel protein on phototransduction

Mitochondrial porins, also know as VDACs (voltage-dependent anion channels), play an important role in regulating energy metabolism, apoptosis, and the transport of metabolites across the mitochondrial outer membrane. So far three distinct isoforms of VDAC (VDAC1-3) have been reported in vertebrates, but their functions remain unknown. The annotation database of the Drosophila melanogaster genome sequence has identified four genes (porin, CG17137, CG17139, and CG17140) encoding different isoforms of VDACs. We identified post-translational modifications of PORIN that are specific to D. melanogaster eyes. We also identified the P-element insertion in the porin gene, porin ^G2294, that is homozygous viable whereas all the porin mutants previously reported are homozygous lethal at the pupal stage. The mutant does not show any defects in fly morphology, survival, and photoreceptor structure. The mutant, however, produces <10% of the normal level of wild-type (WT) porin transcripts and 16.5% of WT level of the PORIN protein. The P-element insertion affects only the expression of Class I transcript but not Class II transcript of the porin gene. Unlike in WT, the mutant displays an ERG (electroretinogram) that is not maintained during a prolonged light stimulus. The revertant obtained from remobilization of the P-element in the mutant produces the WT level of porin transcripts and PORIN protein, and shows a normal ERG response. Our data suggest that the PORIN protein is important in maintaining a photoreceptor response during prolonged Stimulation.