Human feline leukaemia virus subgroup C receptor-related proteins 1 and 2 (FLVCR1 and FLVCR2) are members of the major facilitator superfamily¹. Their dysfunction is linked to several clinical disorders, including PCARP, HSAN and Fowler syndrome^2,3,4,5,6,7. Earlier studies concluded that FLVCR1 may function as a haem exporter^8,9,10,11,12, whereas FLVCR2 was suggested to act as a haem importer¹³, yet conclusive biochemical and detailed molecular evidence remained elusive for the function of both transporters^14,15,16. Here, we show that FLVCR1 and FLVCR2 facilitate the transport of choline and ethanolamine across the plasma membrane, using a concentration-driven substrate translocation process. Through structural and computational analyses, we have identified distinct conformational states of FLVCRs and unravelled the coordination chemistry underlying their substrate interactions. Fully conserved tryptophan and tyrosine residues form the binding pocket of both transporters and confer selectivity for choline and ethanolamine through cation–π interactions. Our findings clarify the mechanisms of choline and ethanolamine transport by FLVCR1 and FLVCR2, enhance our comprehension of disease-associated mutations that interfere with these vital processes and shed light on the conformational dynamics of these major facilitator superfamily proteins during the transport cycle.

人猫白血病病毒亚组C受体相关蛋白1和2（FLVCR1和FLVCR2）是主要促进子超家族¹的成员。它们的功能障碍与多种临床疾病有关，包括 PCARP、HSAN 和 Fowler 综合征^2,3,4,5,6,7 。早期研究得出结论，FLVCR1 可能充当血红素输出者^8,9,10,11,12 ，而 FLVCR2 被认为充当血红素输入者¹³ ，但对于这两种转运蛋白的功能，仍缺乏确凿的生化和详细分子证据^{14 ， 15,16} 。在这里，我们证明 FLVCR1 和 FLVCR2 使用浓度驱动的底物易位过程促进胆碱和乙醇胺跨质膜的转运。通过结构和计算分析，我们已经确定了 FLVCR 的不同构象状态，并揭示了其底物相互作用背后的配位化学。完全保守的色氨酸和酪氨酸残基形成两种转运蛋白的结合袋，并通过阳离子-π相互作用赋予胆碱和乙醇胺选择性。我们的研究结果阐明了 FLVCR1 和 FLVCR2 转运胆碱和乙醇胺的机制，增强了我们对干扰这些重要过程的疾病相关突变的理解，并揭示了这些主要促进超家族蛋白在转运周期中的构象动力学。