Secondary active cation-chloride cotransporters (CCCs) catalyze electroneutral symport of Cl- with Na+ and/or K+ across membranes1,2. CCCs are fundamental in cell volume homeostasis, transepithelia ion movement, maintenance of intracellular Cl- concentration, and inhibitory synaptic transmission3-6. K+-Cl- cotransport 1 (KCC1) was first characterized in red blood cells and later in many other cell types as a crucial player in regulatory volume decrease in defense against cell swelling upon

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... onic challenges7,8. Here we present two cryo-EM structures of human KCC1: one captured in an inward-open state and another arrested in an outward-open state by a small molecule inhibitor. KCC1 can surprisingly adopt two distinct dimeric architectures via homotypic association of different protein domains and conversion between these two forms of dimers may entail dynamic formation and rupture of two interdigitating regulatory cytoplasmic domains. The inhibitor wedges into and forces open an extracellular ion permeation path and arrests KCC1 in an outward-open conformation. Concomitantly, the outward-open conformation involves inward movement of the transmembrane helix 8 and occlusion of the intracellular exit by a conserved short helix within the intracellular loop 1. Our structures provide a blueprint for understanding the mechanisms of CCC transporters and their inhibition by small molecule compounds.