Potassium suppresses allosteric activation of ZAP-70-dependent T cell receptor signaling

Ionic imbalance in the tumor microenvironment alters the function of tumor-infiltrating T lymphocytes. High extracellular K⁺ suppresses T cell function by negatively regulating T cell receptor (TCR) signaling. The mechanism of how monovalent cations regulate T lymphocyte function is unknown. Here, we present a mechanism that explains how cellular potassium dynamics regulate TCR function. At rest, high intracellular K⁺ uncouples allosteric recruitment of ZAP-70, a key signaling module, to the TCR complex. Elevated K⁺ concentration imparts a higher thermodynamic penalty on the binding of the ZAP-70 regulatory module to the phosphotyrosine residues in the ITAM motifs of the CD3 chain. Our data suggest that K⁺ functions as a key allosteric modulator, stabilizing the autoinhibited conformation of ZAP-70. Thus, it prevents spontaneous TCR activation in the resting state. Formation of the antigen-TCR complex induces K⁺ efflux, leading to spontaneous recruitment of ZAP-70 to the TCR. Increasing extracellular K⁺ concentration perturbs K⁺ efflux and slows ZAP-70 recruitment to the TCR complex, even upon antigen binding. Impaired ZAP-70 activation partially dampens TCR signaling, thereby altering downstream signaling. In contrast, the regulatory module in the paralogous kinase Syk, which is expressed in B cells, is insensitive to potassium concentration. At elevated K⁺ concentration, the interaction between the Syk regulatory module and phosphorylated ITAM motifs remains unaltered. We conclude that K⁺ dynamics are integral to T cell ligand discrimination and fundamental to turning off the signaling during T cell quiescence.