Poster Presentation The 46th Lorne Conference on Protein Structure and Function 2021

The Transmembrane Organisation of the B Cell Antigen Receptor (#308)

Samyuktha Ramesh 1 2 , Soohyung Park 3 , Melissa J Call 1 2 , Wonpil Im 3 , Matthew E Call 1 2
  1. Walter and Eliza Hall Institute of medical research, Parkville, VIC, Australia
  2. Department of Medical Biology, University of Melbourne, Melbourne, Victoria, Australia
  3. Department of Biological Sciences and Bioengineering Program, Lehigh University, Bethlehem, Pennsylvania, USA

The antigen receptors on T and B cells, the T-cell receptor (TCR) and B-cell receptor (BCR), respectively, are key to initiating adaptive immune responses against both internal and external threats to the body. Being single-pass membrane protein complexes, structural characterisation by traditional biophysical techniques had proven challenging in the past due to their hydrophobic transmembrane domains (TMDs). Recent advances in cryo-EM enabled the first high-resolution structure of an intact receptor of this class to be resolved: the octameric αβ TCR complex [1]. The central αβ interface reflected in the TMD of this structure was previously identified by our lab using a combination of experimental and computational methods [2, 3]. We now employ a similar combination of techniques to map the organisation of the much less studied tetrameric BCR complex, composed of a homodimeric ligand-sensing membrane-bound immunoglobulin (mIg) protein and a signal-transducing CD79AB dimer.

We performed cysteine-crosslinking to identify points of close packing between the four helices of the BCR TMD in an in vitro translation and assembly system. This data was then used to restrain molecular dynamics (MD) simulations to generate models of how the TMDs would pack in a membrane. The mIg homodimer model thus generated shows that a polar network similar to an N-T-Y network stabilizing the TCRαβ TMD interface [2, 4] also exists in the BCR mIg, comprising S-Y-S-T residues. Mutations to remove the hydrogen bonding -OH groups from these residues reveal complex interactions between the mIg and CD79s. Modeling of the complete tetrameric TMD is currently under way.

Our work has provided novel structural details on the packing and interactions within the BCR TMD, and identified a structural motif shared with the TCR. We aim to better understand the evolution of antigen receptor structures and the structure-function relationship in these complex membrane protein assemblies.

  1. Dong, et al., Structural basis of assembly of the human T cell receptor-CD3 complex. Nature, 2019. 573(7775): p. 546-552.
  2. Krshnan, L., et al., A conserved alphabeta transmembrane interface forms the core of a compact T-cell receptor-CD3 structure within the membrane. Proc Natl Acad Sci U S A, 2016. 113(43): p. E6649-E6658.
  3. Ramesh, S., et al., Experimentally Guided Computational Methods Yield Highly Accurate Insights into Transmembrane Interactions within the T Cell Receptor Complex. The Journal of Physical Chemistry B, 2020. 124(46): p. 10303-10310.
  4. Park, S., et al., Structural Conservation and Effects of Alterations in T Cell Receptor Transmembrane Interfaces. Biophys J, 2018. 114(5): p. 1030-1035.