Objective To elucidate the correlation between specific nucleocapsid(N)protein mutant of the SARS-CoV-2 B.1.1.7 variant and clinical stratification in COVID-19 patients,revealing their impact on N protein liquid-liquid phase separation(LLPS)and host innate immune response.Methods Based on whole-genome sequencing data of the SARS-CoV-2 B.1.1.7 lineage from the GISAID database,non-synonymous mutation sites significantly associated with mild/severe clinical phenotypes were screened.For high-frequency N protein mutant,IFN-β promoter transcriptional activity was quantitatively measured using a dual-luciferase reporter system.qPCR was used to detect the mRNA expression levels of interferon(IFN)-β,interleukin(IL)-6 and tumor necrosis factor(TNF)-α.LLPS characteristics were observed by confocal microscopy.The ubiquitination status of host MAVS was detected by Western blot assay.Results A total of 17 640 non-synonymous mutation sites were identified,among which 65 were associated with mild cases and 20 were related to severe cases,with a mutation frequency>1%.The N protein mutation sites associated with severe cases were D3L,M234I and R203K-G204R-T205I.N protein and the mutants NM234I,NR203K-G204R-T205I inhibited the promoter activity of IFN-β(P＜0.05).Compared to the wild type N protein,NR203K-G204R-T205I mutation significantly reduced the mRNA levels of IFN-β,IL-6 and TNF-α(P＜0.05),and altered the phase separation state by dispersing the formation of LLPS condensates.However,N mutant did not affect the ubiquitination modification of host MAVS.Conclusion N protein mutants of the SARS-CoV-2 B.1.1.7 variant can influence the clinical prognosis of COVID-19 patients by altering LLPS status and suppressing the innate immune responses.These finding provides a theoretical basis for the design of antiviral drugs targeting the N protein.

Objective To elucidate the correlation between specific nucleocapsid(N)protein mutant of the SARS-CoV-2 B.1.1.7 variant and clinical stratification in COVID-19 patients,revealing their impact on N protein liquid-liquid phase separation(LLPS)and host innate immune response.Methods Based on whole-genome sequencing data of the SARS-CoV-2 B.1.1.7 lineage from the GISAID database,non-synonymous mutation sites significantly associated with mild/severe clinical phenotypes were screened.For high-frequency N protein mutant,IFN-β promoter transcriptional activity was quantitatively measured using a dual-luciferase reporter system.qPCR was used to detect the mRNA expression levels of interferon(IFN)-β,interleukin(IL)-6 and tumor necrosis factor(TNF)-α.LLPS characteristics were observed by confocal microscopy.The ubiquitination status of host MAVS was detected by Western blot assay.Results A total of 17 640 non-synonymous mutation sites were identified,among which 65 were associated with mild cases and 20 were related to severe cases,with a mutation frequency>1%.The N protein mutation sites associated with severe cases were D3L,M234I and R203K-G204R-T205I.N protein and the mutants NM234I,NR203K-G204R-T205I inhibited the promoter activity of IFN-β(P＜0.05).Compared to the wild type N protein,NR203K-G204R-T205I mutation significantly reduced the mRNA levels of IFN-β,IL-6 and TNF-α(P＜0.05),and altered the phase separation state by dispersing the formation of LLPS condensates.However,N mutant did not affect the ubiquitination modification of host MAVS.Conclusion N protein mutants of the SARS-CoV-2 B.1.1.7 variant can influence the clinical prognosis of COVID-19 patients by altering LLPS status and suppressing the innate immune responses.These finding provides a theoretical basis for the design of antiviral drugs targeting the N protein.