Simultaneously four different laboratories identified a a new member of the tumor necrosis factor (TNF) family. This has been named as TALL-1 (1), THANK (TNF homologue that activates apoptosis, nuclear factor-kappaB, and c-Jun NH2-terminal kinase (2), BAFF (for B cell activating factor belonging to the TNF family) (3) and BlyS (B lymphocyte stimulator) (4). Membrane-bound BAFF is processed and secreted through the action of a protease whose specificity matches that of the furin family of proprotein convertases. The receptor for BlyS/BAFF appears to be expressed predominantly on B cells and activated T cells (5). Recently two orphan receptors TACI (6) and BCMA (,8) have been shown to be receptors for BlyS/BAFF (6-9). TACI is expressed on B cells and signals through CAML, activating the transcription factors NF-AT, NF-kB, and AP-1. TACI is a type III transmembrane protein, as defined by the lack of a cleaved signal sequence and its N-terminal extracellular exposure. The cDNA codes for 293 amino acids with predicted molecular weight of approximately 32 kDa.
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