The STING Phase-separator Suppresses Innate Immunity

Biomolecular condensates(biocondensates)are micron-scaled non-classic cellular compartments that concentrate biological molecules without a physical barrier to separate interior components from surrounding materials.They are generally driven by liquid-liquid phase-separation(LLPS)of soluble proteins Biocondensates often become viscoelastic by maturing into gel-like condensates to stop exchanging molecules with the outside,thus inhibiting the separated proteins.Biocondensates have multiple functions by affecting biological reaction kinetics and specificity via increasing local protein concentration or decreasing molecular motion through the increased viscoelasticity.Emerging evidence shows that cellular membranes are involved in the formation and dynamics of biocondensates by serving as assembly platforms or by direct contacts.However,no biocondensate formed by transmembrane proteins with specified membrane structures has been reported.cGAMP,catalyzed by the cytosolic DNA sensor cGAS(cyclicGMP-AMP [cGAMP]synthase)following DNA binding,binds to and activates STING(Stimulator of interferon genes,also named ERIS or MITA),an endoplasmic reticulum(ER)-membrane protein,which translocates to sub-micrometer-sized perinuclear vesicles(translocators),where it functions as a scaffold protein,recruiting the kinase TANK-binding kinase 1(TBK1)and interferon regulatory factor 3(IRF3),to induce type I interferon production.Here we discovered that STING was condensed into a puzzle-like ER membranous biocondensate in DNA virus-infected cells,which was triggered by cGAMP and formed in a phase-separation–like manner.STING-condensates insulated STING and TBK1 from IRF3,thus prevented innate immunity from over-activation.STING condensates contained puzzlelike structures,annulate lamellae and tangle fraction.Unexpectedly,microtubule inhibitors were found to disrupt STING-condensate gel-like transition,thus significantly augmented IFN-I production in DNA virus-infected cells.STING mutants from patients with autoimmune diseases showed apparently reduced STING-condensate formation,probably led to uncontrolled STING activation.This membranous biocondensate was therefore named the STING Phase-separator.Our findings thus identified a novel form of membranous biocondensate,provide a new level of cGAS-STING regulation from the point of view of biocondensates and shed lights on the long-sought function of Cubic Membranes.