Unveiling EXOC4/SEC8: a key player in enhancing antiviral immunity by inhibiting the FBXL19-STING1-SQSTM1 signaling axis

As a core aptamer for anti-DNA viral immunity, STING1 (stimulator of interferon response cGAMP interactor 1) is tightly regulated to ensure the proper functioning of the natural antiviral immune response. However, many mechanisms underlying the regulation of STING1 remain largely unknown. In this study, we identify EXOC4/SEC8 (exocyst complex component 4) as a novel positive regulator of DNA virus-triggered type I interferon signaling responses through stabilizing STING1, thereby inhibiting DNA viral replication. Mechanistically, EXOC4 suppresses K27-linked ubiquitination of STING1 at K338, K347, and K370 catalyzed by the E3 ligase FBXL19 (F-box and leucine rich repeat protein 19), thereby preventing ubiquitinated-STING1 from recognition by SQSTM1 (sequestosome 1) for autophagic degradation. Importantly, mice conditionally knocked out for Exoc4/Sec8 are more susceptible to herpes simplex virus type 1 (HSV-1) infection and exhibit more severe lung pathology compared to control mice. This further confirms the important role of EXOC4/SEC8 in antiviral natural immunity. Taken together, our study reveals the importance of EXOC4/SEC8 in promoting STING1-centered antiviral natural immunity and highlights its potential as an anti-DNA viral therapeutic target.Abbreviations: ACTB/β-actin: actin beta; BMDMs: bone marrow-derived macrophages; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; cGAMP: cyclic GMP-AMP; CQ: chloroquine; ER: endoplasmic reticulum; EXOC4/SEC8: exocyst complex component 4; CGAS: cyclic GMP-AMP synthase; HAdV-4: human adenovirus type 4; HSV-1: herpes simplex virus type 1; IFIT1: interferon induced protein with tetratricopeptide repeats 1; IFIT2: interferon induced protein with tetratricopeptide repeats 2; IFNB1: interferon beta 1; IRF3: interferon regulatory factor 3; IFN-I: type I interferon; ISGs: IFN-stimulated genes; ISRE: IFN-stimulated response element; MG132/Z-LLL-CHO: carbobenzoxy-Leu-Leu-leucinal; MOI: multiplicity of infection; MST: microscale thermophoresis; PMs: peritoneal macrophages; Poly(dA:dT): polydeoxyadenylic-thymidylic acid; qPCR: quantitative real-time PCR; shRNAs: short hairpin RNAs; siRNA: small interfering RNA; SQSTM1: sequestosome 1; STING1: stimulator of interferon response cGAMP interactor 1; TBK1: TANK binding kinase 1; TCID₅₀: 50% tissue culture infectious dose; WT: wild-type.