Department of Pathology, College of Medicine, University of Florida, Gainesville, Florida, United States of America.
PLoS One. 2011;6(8):e23122. doi: 10.1371/journal.pone.0023122. Epub 2011 Aug 12.
Most vertebrates have three paralogous genes with identical intron-exon structures and a high degree of sequence identity that encode mitochondrial adenine nucleotide translocase (Ant) proteins, Ant1 (Slc25a4), Ant2 (Slc25a5) and Ant3 (Slc25a6). Recently, we and others identified a fourth mammalian Ant paralog, Ant4 (Slc25a31), with a distinct intron-exon structure and a lower degree of sequence identity. Ant4 was expressed selectively in testis and sperm in adult mammals and was indeed essential for mouse spermatogenesis, but it was absent in birds, fish and frogs. Since Ant2 is X-linked in mammalian genomes, we hypothesized that the autosomal Ant4 gene may compensate for the loss of Ant2 gene expression during male meiosis in mammals. Here we report that the Ant4 ortholog is conserved in green anole lizard (Anolis carolinensis) and demonstrate that it is expressed in the anole testis. Further, a degenerate DNA fragment of putative Ant4 gene was identified in syntenic regions of avian genomes, indicating that Ant4 was present in the common amniote ancestor. Phylogenetic analyses suggest an even more ancient origin of the Ant4 gene. Although anole lizards are presumed male (XY) heterogametic, like mammals, copy numbers of the Ant2 as well as its neighboring gene were similar between male and female anole genomes, indicating that the anole Ant2 gene is either autosomal or located in the pseudoautosomal region of the sex chromosomes, in contrast to the case to mammals. These results imply the conservation of Ant4 is not likely simply driven by the sex chromosomal localization of the Ant2 gene and its subsequent inactivation during male meiosis. Taken together with the fact that Ant4 protein has a uniquely conserved structure when compared to other somatic Ant1, 2 and 3, there may be a specific advantage for mammals and lizards to express Ant4 in their male germ cells.
大多数脊椎动物都有三个具有相同内含子-外显子结构和高度序列同一性的基因，它们编码线粒体腺嘌呤核苷酸转运蛋白（Ant）蛋白，Ant1（Slc25a4）、Ant2（Slc25a5）和 Ant3（Slc25a6）。最近，我们和其他人发现了第四个哺乳动物 Ant 基因的同源物 Ant4（Slc25a31），它具有独特的内含子-外显子结构和较低的序列同一性。Ant4 在成年哺乳动物的睾丸和精子中选择性表达，确实对小鼠精子发生至关重要，但在鸟类、鱼类和青蛙中不存在。由于 Ant2 在哺乳动物基因组中是 X 连锁的，我们假设常染色体 Ant4 基因可能在哺乳动物减数分裂期间补偿 Ant2 基因表达的丧失。在这里，我们报告说 Ant4 同源物在绿鬣蜥（Anolis carolinensis）中是保守的，并证明它在鬣蜥睾丸中表达。此外，在鸟类基因组的同源区域中鉴定出了推定的 Ant4 基因的退化 DNA 片段，表明 Ant4 存在于共同的羊膜动物祖先中。系统发育分析表明 Ant4 基因的起源甚至更为古老。尽管鬣蜥被认为是雄性（XY）异型配子，与哺乳动物一样，雄性和雌性鬣蜥基因组中 Ant2 及其邻近基因的拷贝数相似，这表明鬣蜥 Ant2 基因要么是常染色体的，要么位于性染色体的假常染色体区域，与哺乳动物的情况相反。这些结果表明，Ant4 的保守性不太可能仅仅是由 Ant2 基因的性染色体定位及其在雄性减数分裂期间的随后失活所驱动的。再加上与其他体细胞 Ant1、2 和 3 相比，Ant4 蛋白具有独特保守结构的事实，哺乳动物和蜥蜴在其雄性生殖细胞中表达 Ant4 可能具有特定的优势。
