Identification and characterization of receptor-interacting protein 2 as a TNFR-associated factor 3 binding partner

Tumor necrosis factor receptor-associated factor 3 (TRAF3) is a highly versatile immune regulator that positively controls type I interferon production, but negatively regulates the activation of mitogen-activated protein kinase and alternative nuclear factor-κB signaling. The precise function of TRAF3 in different signaling pathways remains unclear. Thus, in a yeast two-hybrid assay, TRAF3 was used as the bait to screen a human spleen cDNA library for TRAF3 interactors that may potentially mediate TRAF3-regulated functions. Receptor-interacting protein 2 (RIP2) was identified as a TRAF3 binding partner. The interaction between TRAF3 and RIP2 was further confirmed by mammalian two-hybrid, co-immunoprecipitation and GST pull-down assays, and this interaction was also verified by immunoprecipitation of endogenous proteins in Ramos cells, a human B lymphoma cell line. RIP2 is an activator of NF-κB. We therefore examined the effect of TRAF3 in RIP2-induced NF-κB activation. The result showed that TRAF3 could inhibit RIP2-induced NF-κB activation. Given the high expression of RIP2 in the B lymphoma cell line and endogenous interaction between TRAF3 and RIP2 in Ramos cells, the role of RIP2 was further studied. The result demonstrated that RIP2 knockdown was capable of increasing the expression of TRAF3 and suppressing the activation of alternative NF-кB pathway in Ramos cells. These findings suggest that functional interactions between RIP2 and TRAF3 may provide some clues to the mechanisms of TRAF3-involvement in both positive and negative regulatory functions.