An Atypical Tumor Necrosis Factor (TNF) Receptor-associated Factor-binding Motif of B Cell-activating Factor Belonging to the TNF Family (BAFF) Receptor Mediates Induction of the Noncanonical NF-κB Signaling Pathway

BAFF receptor (BAFFR) is a member of the TNF receptor (TNFR) superfamily that regulates the survival and maturation of B cells. BAFFR exerts its signaling function by inducing activation of NF-κB, although the underlying mechanism has not been well defined. By using a chimeric BAFFR, we show that BAFFR preferentially induces the noncanonical NF-κB signaling pathway. This specific function of BAFFR is mediated by a sequence motif, PVPAT, which is homologous to the TRAF-binding site (PVQET) present in CD40, a TNFR known to induce both the canonical and noncanonical NF-κB pathways. Mutation of this putative TRAF-binding motif within BAFFR abolishes its interaction with TRAF3 as well as its ability to induce noncanonical NF-κB. Interestingly, modification of the PVPAT sequence to the typical TRAF-binding sequence, PVQET, is sufficient to render the BAFFR capable of inducing strong canonical NF-κB signaling. Further, this functional acquisition of the modified BAFFR is associated with its stronger and more rapid association with TRAF3. These findings suggest that the PVPAT sequence of BAFFR not only functions as a key signaling motif of BAFFR but also determines its signaling specificity in the induction of the noncanonical NF-κB pathway.