Mitochondrial voltage-dependent anion channel gene family in Drosophila melanogaster: Complex patterns of evolution, genomic organization, and developmental expression

Voltage-dependent anion channels (VDACs), also known as mitochondrial porins, are a family of small pore-forming proteins of the mitochondrial outer membrane found in all eukaryotes. VDACs play important roles in the regulated flux of metabolites between the cytosolic and mitochondrial compartments, energy metabolism, and apoptosis. Annotation of the genome sequence of Drosophila melanogaster revealed three genes (CG17137, CG31722-A, and CG31722-B) with homology to porin, the previously described Drosophila VDAC. Molecular analysis reveals a complex pattern of organization and expression. The genomic organization of these four genes and sequence comparisons with other insect VDAC homologs indicate that this gene family evolved through a mechanism of duplication and divergence from an ancestral VDAC gene during the radiation of the genus Drosophila. CG17137, CG31722-A, and CG31722-B are expressed in a male-specific pattern on both transcriptional and translational levels, while porin is equally expressed in both male and female flies. Additionally, CG31722-A and CG31722-B are expressed as a dicistronic transcript. Western blot analysis and immunofluorescence microscopy confirm that these proteins localize to the mitochondrion. Further expression analysis showed that CG17137 and CG31722-B are abundant in testes, while porin is ubiquitously expressed. While porin, CG17137, and CG31722-B are expressed to different degrees during embryogenesis, all of these proteins are dramatically reduced relative to cytochrome c content during larvogenesis. These studies illustrate a complex genomic organization and spatiotemporal pattern of expression for Drosophila VDACs as well as an evolutionary history consistent with either a partitioning of VDAC functions or an acquisition of novel functions among isoforms.