Human STING Is Regulated by an Autoinhibitory Mechanism for Type I Interferon Production

Abstract

Stimulator of interferon genes (STING) plays a pivotal role in type I interferon-mediated innate immune response to the cytoplasmic detection of aberrant DNA. STING is a membrane protein localized in endoplasmic reticulum (ER), which upon stimulation translocates to Golgi apparatus and activates downstream signaling cascades. However, the mechanism regulating STING activity and significance of its intracellular traffic are not completely understood. Here we identify a novel region of human STING comprising thirteen residues within its C-terminal tail (CTT) for downstream nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) activation. We also discover that STING CTT fragment can activate downstream signaling regardless of its ER localization. In addition, we reveal that ligand-binding domain (LBD) in the middle of STING binds and confers autoinhibition to its CTT for both NF-κB- and interferon regulatory factor 3-activation. Furthermore, STING LBD can inhibit the interferon-stimulating activity of STING CTT in trans and demonstrate a dominant negative effect on endogenous STING for interferon induction. We thus uncover an important autoinhibitory mechanism modulating STING activity.

Activation of STING Based on Its Structural Features

“…Besides the LBDβ2–β3 loop region, the distances between the tips of the two protomers vary among these species, which raises the concern of crystal packing artifact of apo STING LBD. The CTT conformation in all solved structures has never been determined, although several studies claim that the CTT could bind to the other part of the LBD, playing a role in STING autoinhibition ( 13 , 23 , 36 ). In contrast to the in-depth structural studies of STING LBD, the knowledge about TMD is scarce owing to the challenging property of the full-length protein.…”

“…The key point to understand the activation mechanism may exist in the knowledge of how STING was inhibited. Several studies proposed that the CTT is the inhibitory element that can bind to the LBD domain and prevents STING activation, while the ligand binding could release the CTT from LBD sequestration and breaks its autoinhibition ( 13 , 23 , 36 ). However, the structural basis for STING autoinhibition involving CTT remains unclear and needs further investigation.…”