Folates (also known as vitamin B9) have a critical role in cellular metabolism as the starting point in the synthesis of nucleic acids, amino acids and the universal methylating agent S-adenylsmethionine^1,2. Folate deficiency is associated with a number of developmental, immune and neurological disorders^3,4,5. Mammals cannot synthesize folates de novo; several systems have therefore evolved to take up folates from the diet and distribute them within the body^3,6. The proton-coupled folate transporter (PCFT) (also known as SLC46A1) mediates folate uptake across the intestinal brush border membrane and the choroid plexus^4,7, and is an important route for the delivery of antifolate drugs in cancer chemotherapy^8,9,10. How PCFT recognizes folates or antifolate agents is currently unclear. Here we present cryo-electron microscopy structures of PCFT in a substrate-free state and in complex with a new-generation antifolate drug (pemetrexed). Our results provide a structural basis for understanding antifolate recognition and provide insights into the pH-regulated mechanism of folate transport mediated by PCFT.

叶酸（也称为维生素 B9）作为核酸、氨基酸和通用甲基化剂S-腺苷甲硫氨酸^1,2合成的起点，在细胞代谢中发挥着至关重要的作用。叶酸缺乏与许多发育、免疫和神经系统疾病有关^3,4,5 。哺乳动物不能从头合成叶酸；因此，一些系统已经进化为从饮食中吸收叶酸并将其分布在体内^3,6 。质子耦合叶酸转运蛋白 (PCFT)（也称为 SLC46A1）介导跨肠刷状缘膜和脉络丛的叶酸摄取^4,7 ，并且是癌症化疗中抗叶酸药物输送的重要途径^{8,9， 10} . PCFT 如何识别叶酸或抗叶酸剂目前尚不清楚。在这里，我们展示了无底物状态下的 PCFT 以及与新一代抗叶酸药物（培美曲塞）复合物的冷冻电子显微镜结构。我们的结果为理解抗叶酸识别提供了结构基础，并为 PCFT 介导的叶酸转运的 pH 调节机制提供了见解。