Allosteric activation of T cell antigen receptor signalling by quaternary structure relaxation

Abstract

The mechanism of T cell antigen receptor (TCR-CD3) signalling remains elusive. Here, we identified mutations in the transmembrane region of TCR{beta} or CD3{zeta} that augmented pMHC-induced signalling, not explicable by enhanced ligand binding, lateral diffusion, clustering or co-receptor function. Using a novel biochemical assay and molecular dynamics simulation, we demonstrated that the gain-of-function mutations loosened interaction between TCR{beta} and CD3{zeta}. We found that, similar to the activating mutations, pMHC binding reduced TCR{beta} cohesion with CD3{zeta}. This event occurred prior to CD3{zeta} phosphorylation and at 0{degrees}C. Moreover, we demonstrated that soluble monovalent pMHC alone induced signalling and reduced TCR{beta} cohesion with CD3{zeta} in membrane-bound or solubilised TCR-CD3. Our data provide compelling evidence that pMHC binding suffices to activate allosteric changes propagating from TCR{beta} to the CD3 subunits, reconfiguring interchain transmembrane region interactions. These dynamic modifications could change the arrangement of TCR-CD3 boundary lipids to licence CD3{zeta} phosphorylation and initiate signal propagation.