The Matrix Protein of Nipah Virus Targets the E3-Ubiquitin Ligase TRIM6 to Inhibit the IKKε Kinase-Mediated Type-I IFN Antiviral Response

For efficient replication, viruses have developed mechanisms to evade innate immune responses, including the antiviral type-I interferon (IFN-I) system. Nipah virus (NiV), a highly pathogenic member of the Paramyxoviridae family (genus Henipavirus), is known to encode for four P gene-derived viral proteins (P/C/W/V) with IFN-I antagonist functions. Here we report that NiV matrix protein (NiV-M), which is important for virus assembly and budding, can also inhibit IFN-I responses. IFN-I production requires activation of multiple signaling components including the IκB kinase epsilon (IKKε). We previously showed that the E3-ubiquitin ligase TRIM6 catalyzes the synthesis of unanchored K48-linked polyubiquitin chains, which are not covalently attached to any protein, and activate IKKε for induction of IFN-I mediated antiviral responses. Using co-immunoprecipitation assays and confocal microscopy we show here that the NiV-M protein interacts with TRIM6 and promotes TRIM6 degradation. Consequently, NiV-M expression results in reduced levels of unanchored K48-linked polyubiquitin chains associated with IKKε leading to impaired IKKε oligomerization, IKKε autophosphorylation and reduced IFN-mediated responses. This IFN antagonist function of NiV-M requires a conserved lysine residue (K258) in the bipartite nuclear localization signal that is found in divergent henipaviruses. Consistent with this, the matrix proteins of Ghana, Hendra and Cedar viruses were also able to inhibit IFNβ induction. Live NiV infection, but not a recombinant NiV lacking the M protein, reduced the levels of endogenous TRIM6 protein expression. To our knowledge, matrix proteins of paramyxoviruses have never been reported to be involved in innate immune antagonism. We report here a novel mechanism of viral innate immune evasion by targeting TRIM6, IKKε and unanchored polyubiquitin chains. These findings expand the universe of viral IFN antagonism strategies and provide a new potential target for development of therapeutic interventions against NiV infections.

为实现高效复制，病毒进化出多种逃逸先天免疫应答的机制，其中包括抗病毒I型干扰素（IFN-I）系统。尼帕病毒（Nipah virus, NiV）是副黏病毒科（Paramyxoviridae）亨尼帕病毒属（Henipavirus）的高致病性成员，已知其可编码四种由P基因衍生的病毒蛋白（P/C/W/V），具备IFN-I拮抗功能。本研究发现，对病毒组装与出芽至关重要的尼帕病毒基质蛋白（NiV-M）同样能够抑制IFN-I应答反应。I型干扰素的产生需要多种信号分子的激活，其中包括IκB激酶ε（IKKε）。本团队此前的研究证实，E3泛素连接酶（E3-ubiquitin ligase）TRIM6可催化合成不与任何蛋白共价结合的游离K48位连接多聚泛素链，并激活IKKε以诱导IFN-I介导的抗病毒应答。本研究通过免疫共沉淀实验与共聚焦显微镜技术，证实NiV-M蛋白可与TRIM6相互作用并促进TRIM6降解。因此，NiV-M的表达会降低与IKKε结合的游离K48位连接多聚泛素链水平，进而损害IKKε的寡聚化过程、IKKε自身磷酸化，并削弱IFN介导的应答反应。NiV-M的这一IFN拮抗功能依赖于其双分型核定位信号中一个保守的赖氨酸残基（K258），该位点在不同亨尼帕病毒中均存在。与之相符的是，加纳病毒、亨德拉病毒与锡达病毒的基质蛋白同样能够抑制IFN-β的诱导产生。活尼帕病毒感染（而非缺失M蛋白的重组尼帕病毒）会降低内源性TRIM6蛋白的表达水平。据我们所知，此前从未有副黏病毒基质蛋白参与先天免疫拮抗的相关报道。本研究揭示了一种全新的病毒逃逸先天免疫的机制：通过靶向TRIM6、IKKε与游离多聚泛素链实现免疫逃逸。本研究结果拓展了病毒IFN拮抗策略的范畴，同时为尼帕病毒感染的治疗干预手段开发提供了全新的潜在靶点。