Ubiquitin-specific Peptidase 21 Inhibits Tumor Necrosis Factor α-induced Nuclear Factor κB Activation via Binding to and Deubiquitinating Receptor-interacting Protein 1

Ubiquitination and deubiquitination of receptor-interacting protein 1 (RIP1) play an important role in the positive and negative regulation of the tumor necrosis factor alpha (TNF alpha)-induced nuclear factor kappa B (NF-kappa B) activation. Using a combination of functional genomic and proteomic approaches, we have identified ubiquitin-specific peptidase 21 (USP21) as a deubiquitinase for RIP1. USP21 is constitutively associated with RIP1 and deubiquitinates RIP1 in vitro and in vivo. Notably, knockdown of USP21 in HeLa cells enhances TNF alpha-induced RIP1 ubiquitination, I kappa B kinase beta (IKK beta), and NF-kappa B phosphorylation, inhibitor of NF-kappa B alpha (I kappa B alpha) phosphorylation and ubiquitination, as well as NF-kappa B-dependent gene expression. Therefore, our results demonstrate that USP21 plays an important role in the down-regulation of TNF alpha-induced NF-kappa B activation through deubiquitinating RIP1.