The antigen receptors on T and B cells, the T-cell receptor (TCR) and B-cell receptor (BCR), respectively, are the key initiators of adaptive immune responses and provide long-lasting protection against both internal and external threats to the body. Being single-spanning membrane protein complexes, structural characterisation by traditional biophysical techniques has proven challenging in the past due to their hydrophobic transmembrane (TM) domains. Recent advances in cryo-EM have enabled the first high-resolution structure of the intact αβ TCR complex to be resolved (extracellular and transmembrane domains) [1], providing novel structural insights into how these receptors may assemble and function in the membrane.
Reflected in this structure is the tight packing of the TCR’s 8 TM helices, with the stabilising αβ interface previously identified by our lab [2] at its core. This αβ interface, driven by highly conserved amino acids forming an inter-chain polar network, is a critical determinant of TCR assembly, and sequence alignments and molecular dynamics (MD) simulations indicate that this structure is also present in the γδ- and pre-TCR complexes and in all vertebrate species that have conventional T cells [2, 3]. We now show that a similar structure likely exists in the homodimeric BCR ligand-sensing subunit, the membrane-bound immunoglobulin (mIg) protein. Cysteine-crosslinking data and MD simulations support this hypothesis, and the effects of targeted mutations in the mIg TM domains are consistent with a key role in mIg assembly with the BCR signalling module CD79αβ. We are now using saturating mutagenesis, TM disulfide scanning, computational modelling and biophysical analyses to define the structural and functional role of this broadly conserved antigen receptor structural motif. We aim to better understand the evolution of antigen receptor structures and the structure-function relationship in these complex membrane protein assemblies.