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. 2020 Apr 23;12(4):1045. doi: 10.3390/cancers12041045

Figure 1.

Figure 1

Ligand-receptor interactions in the BAFF/APRIL and BAFF-R/BCMA/TACI system. BAFF and APRIL are first synthetized as type II transmembrane proteins, mainly in myeloid and stromal cells, and processed by furin protease into trimeric soluble cytokines BAFF 3-mer and APRIL 3-mer. Twenty soluble BAFF 3-mers can be further oligomerized into a virus-like particle, called BAFF 60-mer. BAFF-R, BCMA, and TACI are type I transmembrane proteins belonging to tumor necrosis factor receptor (TNFR) superfamily and mainly expressed by B cells at different stages of differentiation. BAFF-R and BCMA have only one cysteine-rich domains (CRD) extracellularly, whereas TACI has two CRDs for ligand-binding. BAFF-R only binds BAFF, which can be BAFF 3-mer, BAFF 60-mer or membrane-bound BAFF. BCMA can bind both BAFF and APRIL and has higher binding affinity with APRIL than that with BAFF. TACI also binds to BAFF and APRIL, but it seems to respond better to oligomeric ligands, i.e., BAFF 60-mer or HSPG-bound APRIL, than trimeric soluble ligands.