STING Activation and its Role in IFN Production

Abstract

The immune system’s ability to respond to foreign DNA is a highly conserved mechanism. One crucial part of this mechanism is Stimulator of Interferon Genes (STING), a central innate immune protein located in the endoplasmic reticulum. It is a pattern recognition receptor that is activated in response to intracellular infection and, ultimately, mediates type 1 interferon (IFN) production. Encoded by the gene TMEM173, STING is expressed in T‐cells, monocytes, natural killer cells, and dermal fibroblasts.. It is composed of four transmembrane helices and a cytoplasmic ligand binding and signaling domain, which form a domain‐swapped dimer assembly. The STING pathway is initiated when double‐stranded DNA (dsDNA) is sensed, causing cyclic‐GMP‐AMP‐synthase (cGAS) to cyclize GTP and ATP and produce 2′3′‐cyclic‐GMP‐AMP (cGAMP), the main ligand that activates STING. Upon STING activation, the ligand‐binding domain closes, inducing a 180 degree rotation of the ligand‐binding domain relative to the transmembrane domain. The C‐terminal tail of STING has been found to autoinhibit the protein by blocking the polymerization interface of inactive STING, preventing auto activation. The conformational change made in response to cGAMP releases the CTT, exposing the polymer interface and allowing disulfide link formation. Alternatively, STING can be activated by bacterial cyclic‐di‐GMP in a cooperative manner, with cyclic‐di‐GMP serving as a partial antagonist of the cGAMP signaling pathway. Structural and biochemical studies have provided elegant insight into the mechanism of differential of STING by cGAMP and cDi‐GMP. STING activates tank binding kinase 1 (TBK1) which phosphorylates transcription factor IRF‐3 and causes it to translocate to the nucleus. Gain‐of‐function mutations of TMEM173 causing elevated transcription of IFNs have been proven to cause autoimmune syndromes such as systemic lupus erythematosus and, most recently, three mutations of TMEM173 have been found to directly cause STING‐associated vasculopathy with onset in infants (SAVI). New research proposes a therapeutic strategy that uses molecules that induces a different STING conformation than cGAMP‐bound STING as a STING inhibitor. This method would prevent constitutive STING activation in autoimmune syndromes while being less immunosuppressive than previously proposed therapies involving STING inhibition. The Walton High School MSOE Center for Biomolecular Modeling SMART Team has designed a 3‐D model of STING using a recently published cryo‐electron microscopy structure that reveals the transmembrane domain structure to further investigate and examine the structure‐function relationship of STING.Support or Funding InformationMSOE Center for Biomolecular Modeling