Ca2+ regulates T-cell receptor activation by modulating the charge property of lipids

Calcium–lipid electrostatic interactions are shown to amplify the tyrosine phosphorylation of CD3ε and CD3ζ in T-cell antigen receptor complex. Calcium control of T cells These authors show that Ca 2+ can regulate the ionic interaction between the T-cell receptor–CD3 complex (TCR) and anionic phospholipids — important components in adaptive immunity — by modulating the electrostatic property of phospholipids. Ca 2+ is shown to bind to the phosphate group in anionic phospholipid headgroups and undermines CD3 CD –membrane ionic interaction, thus facilitating TCR phosphorylation. This positive feedback regulation by Ca 2+ can amplify TCR signalling to potentiate the effector function of T cells against invading pathogens. Ionic protein–lipid interactions are critical for the structure and function of membrane receptors, ion channels, integrins and many other proteins 1 , 2 , 3 , 4 , 5 , 6 , 7 . However, the regulatory mechanism of these interactions is largely unknown. Here we show that Ca 2+ can bind directly to anionic phospholipids and thus modulate membrane protein function. The activation of T-cell antigen receptor–CD3 complex (TCR), a key membrane receptor for adaptive immunity, is regulated by ionic interactions between positively charged CD3ε/ζ cytoplasmic domains (CD3 CD ) and negatively charged phospholipids in the plasma membrane 1 , 8 , 9 , 10 . Crucial tyrosines are buried in the membrane and are largely protected from phosphorylation in resting T cells. It is not clear how CD3 CD dissociates from the membrane in antigen-stimulated T cells. The antigen engagement of even a single TCR triggers a Ca 2+ influx 11 and TCR-proximal Ca 2+ concentration is higher than the average cytosolic Ca 2+ concentration 12 . Our biochemical, live-cell fluorescence resonance energy transfer and NMR experiments showed that an increase in Ca 2+ concentration induced the dissociation of CD3 CD from the membrane and the solvent exposure of tyrosine residues. As a consequence, CD3 tyrosine phosphorylation was significantly enhanced by Ca 2+ influx. Moreover, when compared with wild-type cells, Ca 2+ channel-deficient T cells had substantially lower levels of CD3 phosphorylation after stimulation. The effect of Ca 2+ on facilitating CD3 phosphorylation is primarily due to the charge of this ion, as demonstrated by the fact that replacing Ca 2+ with the non-physiological ion Sr 2+ resulted in the same feedback effect. Finally, 31 P NMR spectroscopy showed that Ca 2+ bound