Endogenous association of TRAF2, TRAF3, cIAP1, and Smac with lymphotoxin β receptor reveals a novel mechanism of apoptosis

Lymphotoxin-beta receptor (LTbetaR) is a member of tumor necrosis factor receptor family and plays essential roles in the embryonic development and organization of secondary lymphoid tissues. It binds two types of tumor necrosis factor family cytokines, heterotrimer LTalpha1beta2 and homotrimer LIGHT, and activates multiple signaling pathways including transcriptional factor NFkappaB, c-Jun N-terminal kinase, and cell death. However, the molecular mechanism of the activation of these signaling pathways by LTbetaR is not clear. Because there is no enzymatic activity associated with the receptor itself, the signal transduction of LTbetaR is mediated by cytoplasmic proteins recruited to receptors. To identify these proteins, we took a proteomic approach. The endogenous LIGHT-LTbetaR complex was affinity-purified from U937 cells, and proteins associated with the complex were identified by mass spectrometry. Four of five proteins identified, TRAF2, TRAF3, cIAP1, and Smac, are reported here. Their association with LTbetaR was further confirmed by coimmunoprecipitation in U937 cells and HEK293 cells. The presence of cIAP1 and Smac in LIGHT-LTbetaR complex revealed a novel mechanism of LIGHT-LTbetaR-induced apoptosis.