CryoEM structure of the human SLC4A4 sodium-coupled acid-base transporter NBCe1

Abstract

Na ⁺-coupled acid–base transporters play essential roles in human biology. Their dysfunction has been linked to cancer, heart, and brain disease. High-resolution structures of mammalian Na ⁺-coupled acid–base transporters are not available. The sodium-bicarbonate cotransporter NBCe1 functions in multiple organs and its mutations cause blindness, abnormal growth and blood chemistry, migraines, and impaired cognitive function. Here, we have determined the structure of the membrane domain dimer of human NBCe1 at 3.9 Å resolution by cryo electron microscopy. Our atomic model and functional mutagenesis revealed the ion accessibility pathway and the ion coordination site, the latter containing residues involved in human disease-causing mutations. We identified a small number of residues within the ion coordination site whose modification transformed NBCe1 into an anion exchanger. Our data suggest that symporters and exchangers utilize comparable transport machinery and that subtle differences in their substrate-binding regions have very significant effects on their transport mode.

Abstract

Na ⁺-coupled acid-base membrane transport proteins regulate blood pressure, ion homeostasis and acid-base chemistry. Here the authors present the 3.9 Å resolution cryoEM structure of the sodium-bicarbonate cotransporter NBCe1 and characterize its ion coordination site and ion accessibility pathway with mutagenesis experiments.