Siva-1 Promotes K-48 Polyubiquitination of TRAF2 and Inhibits TCR-Mediated Activation of NF-kappaB

The proapoptotic protein Siva-1 plays an important role in some of the extrinsic and intrinsic apoptosis signaling pathways in cancer cells. Previously, we showed that Siva-1 inhibited the activity of the prosurvival transcription factor NF-kappa B. In the present study, upon TCR cross-linking of Jurkat T leukemia cells, we demonstrated that the inhibitory target of Siva-1 is upstream of the IKK complex in the NF-kappa B signaling pathway. Additionally, Siva-1 also suppressed the activity of another crucial transcription factor AP-1, and a common mediator of both these pathways is the adaptor protein TRAF2. Further, we observed that Siva-1 indeed interacted with TRAF2 and negatively regulated its activity by promoting K48-linked polyubiquitination. Siva-1 specifically interacted with the ring finger domain of TRAF2, which is essential for its E3 ligase activity and its ability to subsequently activate NF-kappa B. TCR cross-linking of Jurkat T cells that lacked Siva-1 revealed significantly lowered K48- but elevated K63-ubiquitinated TRAF2 levels upon TCR cross-linking, suggesting that the differential pattern of ubiquitination in these cells essentially contributed to a robust and sustained activation of NF-kappa B. The above results demonstrated an important role for endogenous Siva-1 in negatively regulating NF-kappa B activation by targeting TRAF2.