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.

A molecular analysis of the Na(+)-independent cation chloride cotransporters.

The homologous genes encoding the electroneutral solute carrier family 12A (SLC12A) were identified more than 20 years ago, however, over the last few years, it has become clear that each of the genes within this family potentially encode for more than one cation-chloride cotransporter (CCC). Even more surprising, despite more than 30 years of functional studies and a wealth of knowledge on the activators, inhibitors, ion affinities, and kinetics of these cotransporters, we still cannot sufficiently explain why some cells express only one CCC isoform, while others express two, three, or more CCC isoforms. In 2009, Drs. Alvarez-Leefmans and Di Fulvio published an extensive in silico molecular analysis of the potential splice variants of the Na(+)-dependent cation-chloride cotransporters. In this review, we will look at the exceptionally large variety of potential splice variants within the Na(+)-independent cation-chloride cotransporter (SLC12A4-SLC12A7) genes, their initial tissue identification, and their physiological relevance.