A Mechanical Switch Couples T Cell Receptor Triggering to the Cytoplasmic Juxtamembrane Regions of CD3ζζ

Abstract

SUMMARY The eight-subunit T cell receptor (TCR)-CD3 complex is the primary determinant for T cell fate decisions. Yet how it relays ligand-specific information across the cell membrane for conversion to chemical signals remains unresolved. We hypothesized that TCR engagement triggers a change in the spatial relationship between the associated CD3ζζ subunits at the junction where they emerge from the membrane into the cytoplasm. Using three in situ proximity assays based on ID-PRIME, FRET, and EPOR activity we determined that the cytosolic juxtamembrane regions of the CD3ζζ subunits are spread apart upon assembly into the TCR-CD3 complex. TCR engagement then triggered their apposition. This mechanical switch resides upstream of the CD3ζζ intracellular motifs that initiate chemical signaling as well as the polybasic stretches that regulate signal potentiation. These findings provide a framework from which to examine triggering events for activating immune receptors and other complex molecular machines.

Transmembrane features governing Fc receptor CD16A assembly with CD16A signaling adaptor molecules

“…The rearrangement of the previously described CD247 interface (24) to accommodate the new contacts to CD16A is striking in view of the recent report suggesting that CD247 also undergoes a significant structural alteration upon incorporation into the TCR-CD3 complex (30), and this flexibility may be particularly important for the demonstrated ability of both CD247 and FceR1γ to form stable assemblies with many different receptor TM sequences. Interestingly, despite the apparent interchangeability of human FceR1γ and CD247, murine CD247 is essentially unable to function in the assembly of either human or mouse CD16A so that, in murine NK cells, only FceR1γ participates in the assembly of this receptor complex (4).…”

A conserved αβ transmembrane interface forms the core of a compact T-cell receptor–CD3 structure within the membrane

“…This type of signaling model implies a pathway between EC and cytoplasmic domains that may involve alterations in the subunit transmembrane (TM) domains with respect to each other and/ or the lipid bilayer (13)(14)(15)(16). Consistent with this view, Kuhns and colleagues (17) recently reported a ligand-induced change in intersubunit proximity at the TM-juxtamembrane juncture in the ζζ dimer and proposed that this "mechanical switch" is coupled to signal initiation at the ζζ cytoplasmic tails. However, the nature of the upstream structural changes that trigger this switch, and whether it is required for signal initiation, remain to be determined.…”

Mechanosensing in the immune response

“…Soluble monomeric pMHC are poor activators of TCR triggering, while surface-bound pMHC efficiently activate TCRs, suggesting that TCR triggering requires additional steps beyond pMHC binding [25, 32]. Structural studies indicate that electrostatic interactions between the TCR-α and CD3 transmembrane regions create a pivot which can mechanically couple antigen binding and force application to conformational changes within the CD3ζζ cytosolic regions [33] (Fig. 1a, inset).…”

“…While forces tangential to the cell membrane can engage the TCR-CD3 pivot driving the conformational changes leading to triggering [33], forces normal to the cell membrane along the TCR axis are required to engage the catch-bond and induce signaling [28]. Overall, these observations raise the following questions: what is the magnitude and direction of forces in situ ?…”