Co-degradation of interferon signaling factor DDX3 by PB1-F2 as a basis for high virulence of 1918 pandemic influenza

The multifunctional influenza virus protein PB1-F2 plays several roles in deregulation of host innate immune responses, and is a known immunopathology enhancer of the 1918 influenza pandemic. Here, we show that the 1918 PB1-F2 protein not only interferes with the mitochondria-dependent pathway of type-I interferon (IFN) signaling, but also acquired a novel IFN antagonist function by targeting the DEAD-box helicase DDX3, a key downstream mediator in antiviral interferon signaling, towards proteasome-dependent degradation. Interactome analysis revealed that 1918 PB1-F2, but not PR8 PB1-F2, binds to DDX3 and causes its co-degradation. Consistent with intrinsic protein instability as basis for this gain-of-function, internal structural disorder is associated with the unique cytotoxic sequences of the 1918 PB1-F2 protein. Infusing mice with recombinant DDX3 protein completely rescued them from lethal infection with the 1918 PB1-F2-producing virus. Alongside NS1 protein, 1918 PB1-F2 therefore constitutes a potent IFN antagonist causative for the severe pathogenicity of the 1918 influenza strain. Our identification of molecular determinants of pathogenesis should be useful for the future design of new antiviral strategies against influenza pandemics.

多功能流感病毒蛋白PB1-F2可在宿主先天免疫应答失调过程中发挥多种作用，且是1918年流感大流行的已知免疫病理增强因子。本研究发现，1918株PB1-F2蛋白不仅可干扰I型干扰素（IFN）信号通路的线粒体依赖性途径，还通过靶向抗病毒干扰素信号通路中的关键下游介质——DEAD盒解旋酶DDX3（DEAD-box helicase DDX3）——使其发生蛋白酶体依赖性降解（proteasome-dependent degradation），从而获得了全新的IFN拮抗功能。相互作用组分析（interactome analysis）显示，仅1918株PB1-F2（而非PR8株PB1-F2）可结合DDX3并引发其协同降解。与这种功能获得性突变的内在蛋白不稳定性相符，1918株PB1-F2蛋白的独特细胞毒性序列与其内部结构无序性密切相关。向小鼠体内注射重组DDX3蛋白，可完全挽救其因产1918株PB1-F2病毒引发的致死性感染。因此，与NS1蛋白一同，1918株PB1-F2构成了强效IFN拮抗因子，是1918流感毒株高致病性的致病根源。本研究鉴定的致病性分子决定簇，可为未来开发抗流感大流行的新型抗病毒策略提供重要参考。