Structural and mechanistic insights into human choline and ethanolamine transport

Abstract

Human feline leukaemia virus subgroup C receptor-related proteins 1 and 2 (FLVCR1 and 2) are major facilitator superfamily transporters from the solute carrier family 49. Dysregulation of these ubiquitous transporters has been linked to various haematological and neurological disorders. While both FLVCRs were initially proposed to hold a physiological function in heme transport, subsequent studies questioned this notion. Here, we used structural, computational and biochemical methods and conclude that these two FLVCRs function as human choline transporters. We present cryo-electron microscopy structures of FLVCRs in different inward- and outward-facing conformations, captured in the apo state or in complex with choline in their translocation pathways. Our findings provide insights into the molecular framework of choline coordination and transport, largely mediated by conserved cation-π interactions, and further illuminate the conformational dynamics of the transport cycle. Moreover, we identified a heme binding site on the protein surface of the FLVCR2 N-domain, and observed that heme actively drives the conformational transitions of the protein. This auxiliary binding site might indicate a potential regulatory role of heme in the FLVCR2 transport mechanisms. Our work resolves the contested substrate specificity of the FLVCRs, and sheds light on the process of maintaining cellular choline homeostasis at the molecular level.

Biallelic variation in the choline and ethanolamine transporterFLVCR1underlies a pleiotropic disease spectrum from adult neurodegeneration to severe developmental disorders

“…In summary, our report reveals a broad and pleiotropic phenotypic spectrum resulting from biallelic FLVCR1 variants; these range from adult neurodegeneration to severe developmental disorders with variable anemia and skeletal malformations. Combined with recent studies demonstrating FLVCR1 encodes a choline and ethanolamine transporter 4,13,16,36 , these data suggest choline and ethanolamine transport into the central and peripheral nervous systems is essential to prevent neurodegeneration and required for neurodevelopment. The observation that the most severe FLVCR1-related phenotypes cause stillbirth and resemble Diamond-Blackfan anemia establishes FLVCR1 should be considered in the differential diagnosis of recurrent miscarriage, multiple congenital anomalies, and severe Diamond-Blackfan anemia-like phenotypes.…”

“…Choline deficiency also causes anemia, liver disease, growth retardation, and immune deficiency [45][46][47] . Neurodevelopment is also disrupted by defective choline uptake 48 Finally, the protein encoded by closely related gene FLVCR2 is expressed at the blood-brain barrier and transports choline and ethanolamine 16,52 . Pathogenic variation in FLVCR2 causes Fowler syndrome, a severe AR disorder characterized by proliferative vasculopathy, hydranencephaly, fetal akinesia deformation sequence, and prenatal lethality [MIM: 225790] 53 .…”

“…Finally, the protein encoded by closely related gene FLVCR2 is expressed at the blood-brain barrier and transports choline and ethanolamine 16,52 . Pathogenic variation in FLVCR2 causes Fowler syndrome, a severe AR disorder characterized by proliferative vasculopathy, hydranencephaly, fetal akinesia deformation sequence, and prenatal lethality [MIM: 225790] 53 .…”

“…Both FLVCR1 isoforms are required for murine viability; mice retaining FLVCR1b but lacking FLVCR1a have normal erythropoiesis but develop hemorrhages, edema, and skeletal malformations 9 . The physiologic significance of FLVCR1 heme transport has been called in question especially considering recent evidence that FLVCR1 is a choline and ethanolamine transporter 4,[12][13][14][15][16] .…”

Unearthing FLVCR1a: tracing the path to a vital cellular transporter

“…At the time of writing, different works on FLVCR1a structure have appeared [ 21 , 22 ], although not already peer-reviewed, that seem to confirm at least in part the predicted structure described.…”