Expression and localization in spermatozoa of the mitochondrial porin isoform 2 in Drosophila melanogaster

Mitochondrial porins or VDACs (voltage-dependent anion-selective channels) are transmembrane pore-forming proteins. In eukaryotic genomes multiple genes coding for VDAC homologues have been discovered, but their function remains unknown. In Drosophila melanogaster three additional genes homologous to the gene porin have been found. In a previous report we have expressed in vitro Porin 2 (gene GC17137) and we have found that the reconstituted protein shows pore-forming activity but it is cation-selective and poorly dependent from voltage. In this work we have characterized the expression pattern of Porin 2. Amplification upon germinal and somatic or stage specific mRNA showed that the highest transcription level of Porin 2 is in testis. Western blot analysis performed with antibodies raised against the recombinant Porin 2 confirmed a high level of expression in the fly spermatozoa. Immuno-histochemical studies indicate that Porin 2 is selectively present in spermatozoa tail, where the mitochondria are located, but not in spermatocytes. A lethal mutant of D. melanogaster carrying a P-element in the first intron of the porin (Porin 1) gene hinders the expression of both Porin 1 and 2. Our results suggest that Porin 2 is truly expressed and that it is required for functional germinal tissues.