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

Deciphering the assembly and signalling mechanisms of the unique BAFF 60-mer through the TACI cytokine receptor (#323)

Karen S Cheung Tung Shing 1 , Tracy L Nero 1 , Grace Gare 1 , William A Figgett 2 , Fabienne Mackay 3 , Michael W Parker 1 4
  1. Biochemistry and Molecular Biology, University of Melbourne, Melbourne, VIC, Australia
  2. Department of Microbiology and Immunology, Peter Doherty Institute, Melbourne, VIC, Australia
  3. QIMR Berghofer Medical Research Institute, Brisbane, QLD, Australia
  4. Australian Cancer Research Foundation Rational Drug Discovery Centre, St. Vincent’s Institute of Medical Research, Melbourne, VIC, Australia

The transmembrane activator and cyclophilin ligand interactor (TACI) receptor regulates the adaptive immune system and is involved in B-cell biology. TACI has been validated as a target for the treatment of chronic lymphocytic leukaemia. However, how TACI signals remains elusive. The TACI receptor has two extracellular domains, with the membrane-proximal domain binding to two cytokines, B-cell activating factor (BAFF) and A proliferation-inducing ligand (APRIL). It has been previously demonstrated that a trimer of APRIL binds to a trimer of the TACI receptor on the cell surface, activating signalling 1.  On the other hand, BAFF has been reported to exist both as a trimer and a 60-mer 2,3 . This is the only known example of a cytokine adopting a 60-meric conformation, reminiscent of a viral capsid. The physiological and pathological relevances of the two oligomeric states of BAFF remains unknown, as well as the assembly/disassembly mechanism by which BAFF switches between its two forms.

We have used a multi-pronged approach including mutagenesis, biophysical assays, X-ray crystallography and cryo-electron microscopy to explore BAFF biology . For the first time, we have identified environmental conditions and mutations that cause disassembly of the BAFF  60-mer into a trimer. These studies have revealed a novel intermediate disassembly step consisting of a pentamer of trimers that has been captured by cryo-electron microscopy. Computational studies have led to the design of a peptide that prevents the BAFF trimers from assembling into the 60-mer. The binding affinity of this peptide to BAFF was determined using surface plasmon resonance and preliminary co-crystals were obtained.  We have also successfully purified the ligand-binding domain of the TACI receptor to investigate its binding to the two forms of BAFF using biophysical assays, crystallography and electron microscopy. These studies provide a step-by-step mechanism for the activation of the TACI receptor by BAFF.

  1. Hymowitz, S. G. et al. Structures of APRIL-receptor complexes: like BCMA, TACI employs only a single cysteine-rich domain for high affinity ligand binding. J Biol Chem 280, 7218-7227, doi:10.1074/jbc.M411714200 (2005).
  2. Liu, Y. et al. Ligand-receptor binding revealed by the TNF family member TALL-1. Nature 423, 49-56, doi:10.1038/nature01543 (2003).
  3. Kim, H. M. et al. Crystal structure of the BAFF-BAFF-R complex and its implications for receptor activation. Nat Struct Biol 10, 342-348, doi:10.1038/nsb925 (2003).