Phase separation in innate immunity: Teleost IL6Ra's evolutionary leap against viruses

Phase separation has been discovered as a new form of regulation in innate immunity. Here, we found that IL6Ra in teleost fish has a unique intrinsic disordered region (IDR) in its amino acid sequence, distinguishing it from the IL6Ra of higher vertebrates. This unique feature endows IL6Ra with the ability to undergo liquid-liquid phase separation, enabling the organism to swiftly initiate an immune response at the early stages of viral infection. Mechanistically, we discovered that zebrafish IL6Ra through its phase separation ability, can competitively recruit the G3bp1 protein. This competitive recruitment mechanism effectively blocks the early replication process of the virus while promoting the rapid activation of host antiviral signaling pathways. In addition, the synergy between IL6Ra and G3bp1 triggers a dynamic recruitment of neutrophils and robust host antiviral defense. Importantly, the IL6Ra protein without the IDR region that cannot undergo phase separation will significantly weaken its antiviral capability. Furthermore, we further confirmed that the characteristics and length of the IDR region of IL6Ra in teleost fish is positively correlated with its phase separation ability and antiviral capacity. In summary, our research results provide a phase separation immune defense strategy obtained from an evolutionary perspective. SIGNIFICANCE: This study reveals the evolutionary plasticity of IL6Ra gene's IDR sequences in bony fishes: Migratory species, facing dynamic viral pressures during freshwater-marine transitions, evolved elongated and complex IL6Ra IDR sequences to enhance LLPS capacity for rapid response to heterogeneous pathogens. Marine fishes developed highly complex IDR architectures to counter high viral diversity, while freshwater species retained simplified IDR sequences under low viral selection. This IDR sequence divergence demonstrates that vertebrates, constrained by innate immunity, can fine-tune non-domain sequences of key immune receptors to optimize environment-specific antiviral adaptation. The findings provide novel insights into "environment-immunogene" co-evolution and highlight the critical role of protein disordered regions in driving adaptive evolution, establishing a framework for understanding immune system diversification under ecological pressures.