Evolutionary Analysis and Classification of OATs, OCTs, OCTNs, and Other SLC22 Transporters: Structure-Function Implications and Analysis of Sequence Motifs

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.溶质载体22家族（SLC22 family）涵盖有机阴离子转运体（organic anion transporters, OATs）、有机阳离子转运体（organic cation transporters, OCTs）以及有机肉碱与两性离子转运体（organic carnitine and zwitterion transporters, OCTNs）。尽管这类转运体可与多种内源性代谢物及信号分子相互作用，但它们常被称为药物转运体（Nigam, S.K.，《自然综述：药物发现》，14:29–44，2015）。对SLC22家族的系统发育分析表明，这类转运体的演化可追溯至4.5亿年前。研究发现，在哺乳动物中SLC22家族发生大规模扩张后，多数OAT成员才得以出现，这提示其在哺乳动物辐射演化过程中具有生理或毒理学相关功能。蠕虫、海胆、果蝇及柄海鞘中均存在推定的SLC22直系同源基因。目前已明确至少存在6组SLC22家族成员。OATs与OCTs构成SLC22的两个主要进化枝，在其内部（除Oat和Oct亚进化枝外）还存在明确的Oat样、OCTN以及Oct相关亚进化枝，以及一个被我们命名为“Oat相关”的远缘类群（其功能或存在差异）。基于现有数据，与细菌药物-质子反向转运体相关的SLC22A18是否应归入SLC22家族仍存在争议。OAT1、OAT3、URAT1、OCT1、OCT2、OCTN1及OCTN2中，致病突变、单核苷酸多态性（SNPs）及其他经功能分析验证的突变，均定位于首个细胞外结构域、大型中央细胞内结构域以及跨膜结构域9与10。这些区域在各亚进化枝内部高度保守，但在亚进化枝间并不保守，且可能是SLC22转运体行使功能及实现功能多样化的必要基础。本研究结果不仅将转运体功能与进化保守基序建立了关联，同时也表明需要对SLC22家族的亚分类进行修订。