Meta-Biol

• Pathways

Cyclic di-GMP (c-di-GMP) and cyclic-di-AMP (c-di-AMP) are ubiquitous secondary messengers secreted by bacteria, but not by eukarya. UV cross-linking experiment with radiolabeled c-di-GMP in lysates of human embryonic kidney 293T (HEK293T) cells expressing mouse Sting showed that STING recognizes and directly binds to c-di-GMP [Burdette DL et al 2011]. STING was reported to contain multiple trans-membrane regions at its N-terminus while its C-terminal domain (CTD) is cytosolic. Mutational analysis showed that the CTD is responsible for the binding to c-di-GMP and this binding enhances the recruitment of TBK1 by STING [Ouyang S et al 2012]. Furthermore, a C-terminal tail (CTT) within the CTD interacts with and activates TBK1 and IRF3 [Tanaka Y and Chen ZJ 2012]. Impotantly, Sting is required for both c-di-GMP and c-di-AMP induced type I IFN production in mouse cultured macrophages infected with intracellular pathogens in vitro [Jin L et al 2011; Sauer JD et al 2011]. Low levels of STING protein expressed in human embryonic kidney (HEK293T) cells were sufficient to reconstitute the responsiveness of the cells to both c-di-GMP and c-di-AMP [Burdette DL et al 2011]. However, structural studies of STING revealed, that STING prefers c-di-GMP over c-di-AMP [Ouyang S et al 2012].

Several studies have demonstrated that human STING functions as a dimer and STING dimerization was essential for the induction of IFN response [Sun W et al 2009; Burdette DL et al 2011; Jin L et al 2011; Ouyang S et al 2012]. Mouse Sting/Myps has been also reported to exist as a dimer constitutively [Jin L et al 2008]. Moreover, STING can function as a ROS sensor, which forms a disulfide-linked homodimer under conditions of oxidative stress in HEK293T cells [Jin L et al 2010]. Structure analysis of the C-terminal domain in complex with c-di-GMP revealed that two STING molecules associate with one molecule of c-di-GMP [Ouyang S et al 2012; Yin Q et al 2012; Scu C et al 2012]. The STING dimer is thought to have a V-shaped structure, and the c-di-GMP binding site is located at the bottom of the V of the dimer interface [Scu C et al 2012]. Isothermal titration calorimetry (ITC) experiments confirmed the stoichiometry of STING to c-di-GMP as 2:1 with a binding dissociation constant (Kd) of ~2.4 microM [Yin Q et al 2012; Scu C et al 2012]. The data are consistent with a previous measurement of mouse STING CTD binding affinity to c-di-GMP using equilibrium dialysis [Burdette DL et al 2011]. Although STING is considered as a direct sensor of bacterial c-di-GMP, it is noteworthy, that the binding affinity of c-di-GMP to mammalian STING is much weaker than to bacterial sensors. For example, E.coli protein YcgR binds to c-di-GMP with a Kd of ~0.84 microM [Ryjenkov DA et al 2006]. Also taking into account that, the normal concentration of c-di-GMP in bacteria varies from 0.1~10 microM, it remains to be determined whether STING binds to c-di-GMP under physiological conditions.