Transport unplugged: KCCs are regulated through an N-terminal plug of the ion pathway

Abstract

The ability to regulate transmembrane ion transport in response to various cues is vital to any living cell. In neurons, one key example of critical ion control relates to the extrusion of chloride mediated by the potassium-chloride-cotransporters (KCC1-4). In a recent hallmark study, Chi et␣al (2021) report cryo-EM structures of human KCC1 and KCC3b, delineating in detail how regulation by phosphorylation inhibits the transport activity. The authors also identify a stabilizing binding site for nucleotides and speculate on its functional role.

Figure 1. Ion transport by SLC12 proteins and regulatory mechanisms

(A) KCC1‐4 transporters are responsible for extrusion of chloride ions utilizing the cellular potassium electrochemical gradient (top panel). The activity of KCCs is inhibited by phosphorylation. Uptake of chloride ions utilizing the sodium electrochemical gradient is mediated by NKCC1‐2 and NCC (bottom panel). The activity of NKCC1‐2/NCC is stimulated by phosphorylation. (B) In response to phosphorylation, KCCs adopt an auto‐inhibited state where part of the N‐terminal domain (yellow ribbon) binds to the transmembrane domain (gray surface) preventing ion access to the binding sites (top panel). Upon removal of phosphoryl modifications, the N‐terminal domain dislodges allowing potassium ions (purple) and chloride ions (green) to access the ion‐binding sites in the transmembrane domains (bottom panel). (C) Phosphorylated residues in the C‐terminal domains (orange dots) might perturb the local spatial arrangement of the nucleotide binding sites preventing ATP/ADP from associating with the protein (top panel). When phosphoryl modifications are not present, as in the constitutive active KCC1 mutant, the nucleotide binding sites can stably engage with ATP/ADP.