Structure of the human cation–chloride cotransport KCC1 in an outward-open state

Cation–chloride cotransporters (CCCs) catalyze electroneutral symport of Cl(−) with Na(+) and/or K(+) across membranes. CCCs are fundamental in cell volume homeostasis, transepithelia ion movement, maintenance of intracellular Cl(−) concentration, and neuronal excitability. Here, we present a cryoelectron microscopy structure of human K(+)–Cl(−) cotransporter (KCC)1 bound with the VU0463271 inhibitor in an outward-open state. In contrast to many other amino acid–polyamine–organocation transporter cousins, our first outward-open CCC structure reveals that opening the KCC1 extracellular ion permeation path does not involve hinge-bending motions of the transmembrane (TM) 1 and TM6 half-helices. Instead, rocking of TM3 and TM8, together with displacements of TM4, TM9, and a conserved intracellular loop 1 helix, underlie alternate opening and closing of extracellular and cytoplasmic vestibules. We show that KCC1 intriguingly exists in one of two distinct dimeric states via different intersubunit interfaces. Our studies provide a blueprint for understanding the mechanisms of CCCs and their inhibition by small molecule compounds.