Cryo-EM - the technology credited with the "resolution revolution" underway in structural biology - uses a massive microscope to shoot a narrow stream of electrons through thin, flash-frozen samples. The STING (Stimulator of Interferon Genes) protein studied in is about half the size of the next-smallest membrane protein determined, the researchers said.

Three studies published by Nature that describe this work. Two studies reveal the first full-length structure of the elusive STING protein attached to different molecules in the pathway. STING is a key member of an important pathway in innate immunity - the body's first line of defense against infections. The third study gives insight into the primitive function of the cGAS-STING pathway and underscores the importance of understanding its protein structure.

cGAS enzyme. a DNA sensor, launches the body's immune defense against infections and cancers in a pathway in which the STING protein is pivotal. The cGAS enzyme patrols the cell's interior and triggers an immune response when it encounters foreign DNA. It can also trigger autoimmunity when it finds self-DNA in areas of the cell where that genetic material should not exist. cGAS produces a small molecule called cGAMP (cyclic GMP-AMP), which binds to STING and launches an immune response.

"The cGAS-cGAMP-STING pathway leads to interferon production in mammals in response to DNA invasion," the senior author said. Interferons - signaling proteins - tell cells to heighten their defenses against foreign invaders.

One of the two structural studies reports the first structure of STING in complex with TBK1 (Tank-Binding Kinase 1), the molecule that activates it and leads to production of specific interferons and other signaling molecules. The experiments, which used both human and chicken STING proteins, also reveal the tail end of STING, which was invisible in previous structures.

The structure reveals that the C-terminal tail of STING adopts a β-strand-like conformation and inserts into a groove between the kinase domain of one TBK1 subunit and the scaffold and dimerization domain of the second subunit in the TBK1 dimer. In this binding mode, the phosphorylation site Ser366 in the STING tail cannot reach the kinase-domain active site of bound TBK1, which suggests that STING phosphorylation by TBK1 requires the oligomerization of both proteins. Mutational analyses validate the interaction mode between TBK1 and STING and support a model in which high-order oligomerization of STING and TBK1, induced by cGAMP, leads to STING phosphorylation by TBK1.

The structures also show that the transmembrane and cytoplasmic regions interact to form an integrated, domain-swapped dimeric assembly. Closure of the ligand-binding domain, induced by cGAMP, leads to a 180° rotation of the ligand-binding domain relative to the transmembrane domain. This rotation is coupled to a conformational change in a loop on the side of the ligand-binding-domain dimer, which leads to the formation of the STING tetramer and higher-order oligomers through side-by-side packing. This model of STING oligomerization and activation is supported by the structure-based mutational analyses.

That tail is lacking in STING from sea anemones, the subject of the third paper. Sea anemones are part of a group of invertebrates (creatures that lack a backbone) thought to exist for hundreds of millions of years before the first humans did. That study finds that STING in sea anemones can activate an immune response through autophagy. Often called cellular housekeeping, autophagy clears away unwanted molecules - in this case pathogens - within the cell by breaking them down for recycling. Humans have the c-GAS-STING-to-autophagy immune pathway as well as the c-GAS-STING-to-interferon pathway that sea anemones lack.

That finding suggest that, as in a scorpion, the "STING is in the tail." More seriously, the author said the study suggests that autophagy is a primordial (primitive) pathway for immunity that developed earlier than the interferon pathway found in humans.

. https://www.nature.com/articles/s41586-019-1000-2

https://www.nature.com/articles/s41586-019-0998-5

https://www.nature.com/articles/s41586-019-1006-9