Organic anion transporters (OATs), organic cation transporters (OCTs) and organic carnitine and zwitterion transporters (OCTNs), which belong to the solute carrier 22 family (SLC22);TC# 2.A.1.19, transport a wide variety of metabolites, environmental toxins and drugs. Evolutionary analysis of these so called "drug" transporters may shed light on the endogenous role of SLC22 transporters which is only beginning to be explored. We took advantage of the sequencing of multiple genomes, and here, present a phylogenetic analysis of 31 known SLC22 members. Our studies indicate that they evolved from a common ancestor over 450 million years ago before the separation of bony fish and land vertebrates. Several putative SLC22 orthologs exist in worms, sea urchins, flies, and ciona while several other OAT, OCT, and OCTN family members first appear in mammals based on phylogenetic tree topologies. There is a particularly large expansion of SLC22 members in mammals, suggesting a physiological and/or toxicological role for these SLC22 transporters in the successful mammalian radiation into new environments. Moreover, phylogenetic analysis indicates that OATs (e.g. SLC22A6 also known as OAT1 or NKT) and OCTs (SLC22A1 or OCT1) represent two major subclades with the OAT subclade being by the largest; nevertheless there are clear OCTN (e.g. OCTN1 or SLC22A4, OCTN2 or SLC22A5), OAT-like (e.g. OAT10 or SLC22A13, SLC22A14 or OCTL2), OCTN-like subclades, as well as a distantly related group which we have named the OAT- related group containing SLC22A17, SLC22A18, SLC22A23, and SLC22A31. The uniqueness of the OAT-related subgroup raises the possibility of distinct functions for this set of transporters. Motif analysis suggests the first extracellular region is highly conserved within subclades but not between subclades and may be particularly important for subclade-specific function

The multispecific organic anion drug transporters OAT6 (SLC22A20) and OAT1 (SLC22A6) are expressed in nasal epithelial cells and both can bind odorants. Sequence analysis of OAT6 revealed an evolutionarily conserved 79-amino acid (AA) fragment present not only in OAT6 but also in other SLC22 transporters, such as the organic anion transporter (OAT), organic carnitine transporter (OCTN), and organic cation transporter (OCT) subfamilies. A similar fragment is also conserved in some odorant receptors (ORs) in both humans and rodents. This fragment is located in regions believed to be important for ligand/substrate preference and recognition in both classes of proteins, raising the possibility that it may be part of a potential common ligand/substrate recognition site in certain ORs and SLC22 transporters. In silico screening of an odorant database containing known OR ligands with a pharmacophore hypothesis (generated from a set of odorants known to bind OAT6 and/or OAT1), followed by in vitro uptake assays in transfected cells, identified OR ligands capable of inhibiting OAT6- and/or OAT1-mediated transport, albeit with different affinities. The conservation of the AA fragments between these two different classes of proteins, together with their coexpression in olfactory as well as other tissues, suggests the possibility that ORs and SLC22 transporters function in concert, and raises the question as to whether these transporters function in remote sensing and signaling and/or as transceptors.

The SLC22 family includes organic anion transporters (OATs), organic cation transporters (OCTs) and organic carnitine and zwitterion transporters (OCTNs). These are often referred to as drug transporters even though they interact with many endogenous metabolites and signaling molecules (Nigam, S.K., Nature Reviews Drug Discovery, 14:29-44, 2015). Phylogenetic analysis of SLC22 supports the view that these transporters may have evolved over 450 million years ago. Many OAT members were found to appear after a major expansion of the SLC22 family in mammals, suggesting a physiological and/or toxicological role during the mammalian radiation. Putative SLC22 orthologs exist in worms, sea urchins, flies, and ciona. At least six groups of SLC22 exist. OATs and OCTs form two Major clades of SLC22, within which (apart from Oat and Oct subclades), there are also clear Oat-like, Octn, and Oct-related subclades, as well as a distantly related group we term "Oat-related" (which may have different functions). Based on available data, it is arguable whether SLC22A18, which is related to bacterial drug-proton antiporters, should be assigned to SLC22. Disease-causing mutations, single nucleotide polymorphisms (SNPs) and other functionally analyzed mutations in OAT1, OAT3, URAT1, OCT1, OCT2, OCTN1, and OCTN2 map to the first extracellular domain, the large central intracellular domain, and transmembrane domains 9 and 10. These regions are highly conserved within subclades, but not between subclades, and may be necessary for SLC22 transporter function and functional diversification. Our results not only link function to evolutionarily conserved motifs but indicate the need for a revised sub-classification of SLC22.