The sodium solute symporters (SSS) and neurotransmitter sodium symporters (NSS) are two families of secondary transporters that are not related in amino acid sequence. Nonetheless, recent crystal structures showed that the Na(+)/galactose (SSS) and Na(+)/leucine (NSS) transporters have similar core structures. The structural relatedness highlights the need for classification methods for membrane protein structures based on other criteria than amino acid similarity. Here, we demonstrate that a method based on hydropathy profile alignments convincingly identifies structural similarity between the NSS and SSS families. Most importantly, the method shows that one of the largest transporter families for which a crystal structure is elusive (the amino acid/polyamine/organocation or APC superfamily), also shares the similar core structure observed for the Na(+)/galactose and Na(+)/leucine transporters. The APC superfamily contains the major amino acid transporter families that are found throughout life. Insight into their structure will significantly facilitate the studies of this important group of transporters.

中文翻译：

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主要的氨基酸转运蛋白超家族具有与Na +-半乳糖和Na +-亮氨酸转运蛋白相似的核心结构。

钠溶质转运蛋白（SSS）和神经递质钠转运蛋白（NSS）是二级转运蛋白家族，与氨基酸序列无关。尽管如此，最近的晶体结构显示Na（+）/半乳糖（SSS）和Na（+）/亮氨酸（NSS）转运蛋白具有相似的核心结构。结构相关性强调需要基于氨基酸相似性以外的其他标准对膜蛋白结构进行分类的方法。在这里，我们证明了基于亲水性谱比对的方法令人信服地确定了NSS和SSS系列之间的结构相似性。最重要的是，该方法表明，晶体结构难以捉摸的最大转运蛋白家族之一（氨基酸/多胺/有机或APC超家族），还具有与Na（+）/半乳糖和Na（+）/亮氨酸转运蛋白相似的核心结构。APC超家族包含一生中发现的主要氨基酸转运蛋白家族。对它们的结构的深入了解将极大地促进这一重要转运蛋白的研究。