Regulation Of Anti-viral Innate Immune Responses By SCF Complex SCF^FBXO21 And The Underlying Mechanisms

Innate immune system is the first defense that protects the host from infection by pathogenic organisms. Recognition of viral components by pattern-recognition receptors(PRRs) and initiation of programmed cell death are cucial for host to restrain viral infection. Ubiquitin modification is one of the most important protein post-translational modifications(PTM) and exists in almost all of the cell activities. E3 ubiquitin ligase which is the key component of the ubiquitin system, and its new function and regulation mode in innate anti-viral immunity is a research hotspot in innate immune responses. Therefore, by GeneChip assays, we examined the gene expression profile of E3 ligases in mouse peritoneal macrophages and RAW264.7 cell line treated with several pathogens associated molecular patterns (PAMPs) and virus. Based on the GeneChip results, a group of E3 ubiquitin ligase complex whose function in innate anti-viral immune responses remain unknown attract our attention. As F-BOX protein is the key substrate recongnition component of this complex, we analyse F-BOX proteins’ expression pattern in GeneChip results. We selected 8 FBXO proteins for function study and found the F-BOX only protein FBXO21 deserved for further research.Here we report that F-box only protein Fbxo21, a component of the SCF (SKP1-CUL1-F-BOX protein) ubiquitin ligases complexes, facilitates Lys29-linkage and activation of ASK1 and promotes innate antiviral responses and cell death. ASK1 (Map3k5) is essential for TLR signaling and apoptosis induction. However, roles of ASK1 in innate antiviral response have not been fully elucidated.Next, we find FBXO21 deficiency impairs Lys29-linkage of ASK1, leading to attenuated cytokine production and cell death upon viral infection. Lys29-linkage of ASK1 by SCFFBXO21 is required for ASK1 activation and constitutive JNK1/2 activation is required for FBXO21-mediated regulation of innate antiviral response. Our study suggests that FBXO21 is required for antiviral innate response by modifying Lys29-linkage of ASK1, providing new mechanistic insights into ubiquitination-mediated regulation of ASK1 activation.