A “Knob Switch” Model for the Phosphoregulatory Mechanism of KCC3 at the Carboxy-Terminal Domain

Phosphorylation is a reversible post-translational modification that can modulate protein function. For example, phosphorylation modifications of solute carrier family 12 (SLC12) proteins function as molecular switches that precisely regulate cation-chloride ion transport. Elucidating the phosphoregulatory mechanism of SLC12 at the carboxy-terminal domain (CTD) through structural determination approaches remains challenging due to the domain's disordered and flexible nature. In this study, molecular dynamics (MD) simulations and enhanced sampling techniques were employed to investigate the CTD phosphoregulatory mechanism of SLC12A6 (also known as KCC3). Atomistic MD and metadynamics simulations revealed that the dephosphorylation of residues T940 and T997 stabilizes CTD to a favorable state that "switches on" the solvent accessibility of the inward-facing pocket. Meanwhile, phosphorylation induces distinct orientations of the CTD, transitioning the dimer into another favorable state that "switches off" the solvent accessibility. The alteration of solvent accessibility in the inward-facing pocket influences the water and ion dynamics. Based on these findings, we propose a "knob switch" model to illustrate how CTD phosphorylation regulates ion transport in KCC3.