The STING phase-separator suppresses innate immune signalling.

Area of Science:

• Cell Biology
• Immunology
• Structural Biology

Background:

• Biomolecular condensates (biocondensates) typically form from soluble proteins via liquid-liquid phase separation.
• The phase separation of endoplasmic reticulum (ER) transmembrane proteins into organized membranous structures has not been previously reported.

Purpose of the Study:

• To investigate the formation and function of novel membranous biocondensates involving the ER-resident stimulator of interferon genes (STING).
• To elucidate the molecular mechanisms underlying STING condensation and its role in innate immune regulation.

Main Methods:

• Observation of spherical ER membranous biocondensates with puzzle-like structures in STING-condensed cells.
• Analysis of STING condensation requirements, including transmembrane domains, intrinsically disordered region (IDR), and dimerization domain.
• Assessment of the impact of intracellular 2'3'-cGAMP concentrations on STING translocation and condensation.
• Investigation of the role of STING biocondensates in constraining STING and TBK1 (TANK binding protein 1).
• Evaluation of the effects of STING mutations (STING-E336G/E337G) on innate immune responses.
• Examination of the influence of microtubule inhibitors on STING condensate transition and type I-interferon production.

Main Results:

• Discovery of a novel spherical ER membranous biocondensate formed by STING condensation, termed the STING phase-separator.
• STING condensation requires its transmembrane domains, IDR, and dimerization domain, and is regulated by intracellular 2'3'-cGAMP levels.
• STING biocondensates act as a 'STING-TBK1-cGAMP sponge,' constraining these proteins to prevent overactivation of innate immunity.
• Impaired STING condensation due to mutations (STING-E336G/E337G) leads to enhanced innate immune responses during viral infection.
• Microtubule inhibitors disrupt the gel-like transition of STING condensates and enhance type I-interferon production in virus-infected cells.

Conclusions:

• The STING phase-separator represents a newly identified type of membranous biocondensate with a crucial role in regulating innate immunity.
• STING condensation provides a mechanism to control the inflammatory response, preventing excessive immune activation during viral infections.
• Targeting the STING phase-separator or its associated pathways may offer therapeutic strategies for modulating innate immune responses.