The T Cell Antigen Receptor α Transmembrane Domain Coordinates Triggering through Regulation of Bilayer Immersion and CD3 Subunit Associations

Abstract

Summary Initial molecular details of cellular activation following αβT cell antigen receptor (TCR) ligation by peptide-major histocompatibility complexes (pMHC) remain unexplored. We determined the nuclear magnetic resonance (NMR) structure of the TCRα subunit transmembrane (TM) segment revealing a bipartite helix whose segmentation fosters dynamic movement. Positively charged TM residues Arg251 and Lys256 project from opposite faces of the helix, with Lys256 controlling immersion depth. Their modification caused step-wise reduction in TCR associations with CD3ζζ homodimers and CD3εγ plus CD3εδ heterodimers, respectively, leading to an activated transcriptome. Optical tweezers revealed that Arg251 and Lys256 mutations altered αβTCR-pMHC bond lifetimes, while mutations within interacting TCRα connecting peptide and CD3δ CxxC motif juxtamembrane elements selectively attenuated signal transduction. Our findings suggest that mechanical forces applied during pMHC ligation initiate T cell activation via a dissociative mechanism, shifting disposition of those basic sidechains to rearrange TCR complex membrane topology and weaken TCRαβ and CD3 associations.

Electrostatic interactions: From immune receptor assembly to signaling

“…Their study posits that rather than enabling the assembly of the αβTCR‐CD3, K256 was involved in regulating membrane depth of TCRα TM segment using both NMR and electron paramagnetic resonance spectroscopy approaches. Moreover, the αβTCR‐CD3 TCR was expressed at normal levels when R251 and K256 were mutated to non‐polar amino acid leucine (L) when compared to WT, although it no longer associated with the CD3εδ heterodimer . However, IP and cytokine secretion studies revealed severe defects in αβTCR‐CD3 assembly and signaling when R251 and K256 were mutated individually or simultaneously corroborating previous studies in the requirement of fully assembled TCRs for efficient expression and functions …”

Mechano-regulation of Peptide-MHC Class I Conformations Determines TCR Antigen Recognition

“…First, MHC conformational changes activate TCR-pMHC catch bonds, providing an extra layer of discrimination power for TCR antigen recognition (discussed above) and, possibly, promoting rapid propagation of conformational changes from TCR CDR loops to CD3 tails for rapid and efficient triggering of TCR signaling. For example, strengthened TCR-pMHC binding on TCR CDR loops may allow force to transmit to and change conformations of constant regions of the abTCR (Natarajan et al, 2017), which may further induce TCRa transmembrane domain structural movement to regulate the topological rearrangement of the abTCR-CD3 complex (Brazin et al, 2018) and alter the conformation of CD3 tails (Natarajan et al, 2016;Xu et al, 2008). These serial conformational changes may ultimately lead to successful CD3 phosphorylation by Lck.…”

The TCR is an allosterically regulated macromolecular machinery changing its conformation while working

“…A major handicap in TCR research is that a high‐resolution structure of the full TCR complex is still lacking even though structures of different domains of TCRαβ, CD3, and ζ are available . The first structures of the TCRαβ ectodomain bound and unbound to its pMHC ligand became available in the mid‐1990th .…”