2.A.30.1.8
Na⁺,K⁺,Cl^--cotransporter, NKCC1or Slc12a2/a4, of 1136 aas and 12 TMSs.  Ion transport via an ortholog is oxygen-sensitive and is regulated by two different oxygen sensors in crucian carp (Carassius carassius) (Berenbrink et al. 2006). Chew et al. 2019 determined the cryo-EM structure which defined the architecture of this protein family and revealed how cytosolic and transmembrane domains are strategically positioned for communication. Structural analyses, functional characterizations and computational studies revealed the ion-translocation pathway, ion-binding sites and key residues involved in transport activity, thus providing insights into ion selectivity, coupling and translocation, and establishing a framework for understanding the physiological functions of CCCs as well as interpreting disease-related mutations (Chew et al. 2019). The 3-D structures of this protein and the humanKCC! (TC# 2.A.30.1.4) have been compared and reviewed (Delpire and Guo 2020). While cation-Cl^- cotransporters share the overall architecture of carriers belonging to the amino acid-polyamine-organocation (APC) superfamily and some of their substrate binding sites, several new features have been revealed. (1) The large extracellular domain between TMSs 5 and 6 stabilizes the dimer and forms a cap that likely participates in extracellular gating. (2) Conservation of the K⁺ and Cl^- binding sites and a second Cl^- coordination site were revealed. (3) an extracellular gate is formed by conserved salt bridges between charged residues located toward the ends of TMSs 3 and 4, and there is an additional neighboring bridge in the hKCC1 structure. (4) Multiple points of contacts occur between the monomers forming the cotransporter homodimer units. These involve the TMSs, the COOH-terminal domains, and the large extracellular loop for hKCC1 (Delpire and Guo 2020).