A comprehensive survey on the neurotransmitter sodium symporter family in prokaryotes and eukaryotes

Abstract

olute carriers (SLCs) are membrane proteins for transport of many types of substances over the cell membrane. They consist of 52 families and one of them is the SLC6 family. This family composes of neurotransmitter sodium symporters (NSSs) and therefore is also named as the NSS family. In this study, we have genome-widely identified 128 NSSs encoded by 9 species from archaea, bacterium, fungus, invertebrates, vertebrates and plants. Phylogenetic analysis showed that these NSSs could be classified into 5 different subfamilies and one of them is the most ancient subfamily containing members from all the 9 species. In the plant kingdom, only alga species encode NSSs with only one member in each genome. In archaea, bacterium and fungus species, they encode 1-6 members each and their expansion mainly occurred after species divergence. For invertebrates and vertebrates, they experienced several rounds of expansion and the large scale of expansion was observed at the most recent common ancestor (MRCA) of Caenorhabditis elegans , and Drosophila melanogaster , or MRCA of Danio rerio , Mus musculus , and Homo sapiens . Both tandem and segmental duplications have been regarded as the major driver for the family expansion. These expanded NSSs were generally under strong purifying selection during the long evolutionary history with limited functional divergence. Further investigation showed that these expanded NSSs exhibited significant difference in either expression abundance or tissue specificity. Thus, expression divergence was regarded as the main selection force for these genes to survive after their birth.