Crystal structure of the BAFF–BAFF-R complex and its implications for receptor activation

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Abstract
B-cell activating factor (BAFF) is a key regulator of B-lymphocyte development. Its biological role is mediated by the specific receptors BCMA, TACI and BAFF-R. We have determined the crystal structure of the extracellular domain of BAFF-R bound to BAFF at a resolution of 3.3 Å. The cysteine-rich domain (CRD) of the BAFF-R extracellular domain adopts a β-hairpin structure and binds to the virus-like BAFF cage in a 1:1 molar ratio. The conserved DxL motif of BAFF-R is located on the tip of the β-turn and is indispensable in the binding of BAFF. The crystal structure shows that a unique dimeric contact occurs between the BAFF-R monomers in the virus-like cage complex. The extracellular domain of TACI contains two CRDs, both of which contain the DxL motif. Modeling of TACI–BAFF complex suggests that both CDRs simultaneously interact with the BAFF dimer in the virus-like cage.