Structure of the human cation–chloride cotransporter NKCC1 determined by single-particle electron cryo-microscopy

The secondary active cation–chloride cotransporters (CCCs) utilize the existing Na(+) and/or K(+) gradients to move Cl(−) into or out of cells. NKCC1 is an intensively studied member of the CCC family and plays fundamental roles in regulating trans-epithelial ion movement, cell volume, chloride homeostasis and neuronal excitability. Here, we report a cryo-EM structure of human NKCC1 captured in a partially loaded, inward-open state. NKCC1 assembles into a dimer, with the first ten transmembrane (TM) helices harboring the transport core and TM11-TM12 helices lining the dimer interface. TM1 and TM6 helices break α-helical geometry halfway across the lipid bilayer where ion binding sites are organized around these discontinuous regions. NKCC1 may harbor multiple extracellular entryways and intracellular exits, raising the possibility that K(+), Na(+), and Cl(−) ions may traverse along their own routes for translocation. NKCC1 structure provides a blueprint for further probing structure–function relationships of NKCC1 and other CCCs.