STING, stimulator of interferon genes, is an innate immune sensor of cyclic dinucleotides that regulates the induction of type I interferons. STING C-terminal domain forms a V-shaped dimer and binds a c-di-GMP molecule at the dimer interface through direct and solvent-mediated hydrogen bonds. The guanine bases of c-di-GMP stack against the phenolic rings of a conserved tyrosine residue. Mutations at the c-di-GMP binding surface reduce nucleotide binding and affect signaling.

Prolidases, metalloproteases that catalyze the cleavage of Xaa-Pro dipeptides, are conserved enzymes found in prokaryotes and eukaryotes. In humans, prolidase is crucial for the recycling of collagen. To further characterize the essential elements of this enzyme, we utilized the Escherichia coli prolidase, PepQ, which shares striking similarity with eukaryotic prolidases. Through structural and bioinformatic insights, we have extended previous characterizations of the prolidase active site, uncovering a key component for substrate specificity. Here we report the structure of E. coli PepQ, solved at 2.0 Å resolution. The structure shows an antiparallel, dimeric protein, with each subunit containing N-terminal and C-terminal domains. The C-terminal domain is formed by the pita-bread fold typical for this family of metalloproteases, with two Mg(II) ions coordinated by five amino-acid ligands. Comparison of the E. coli PepQ structure and sequence with homologous structures and sequences from a diversity of organisms reveals distinctions between prolidases from Gram-positive eubacteria and archaea, and those from Gram-negative eubacteria, including the presence of loop regions in the E. coli protein that are conserved in eukaryotes. One such loop contains a completely conserved arginine near the catalytic site. This conserved arginine is predicted by docking simulations to interact with the C-terminus of the substrate dipeptide. Kinetic analysis using both a charge-neutralized substrate and a charge-reversed variant of PepQ support this conclusion, and allow for the designation of a new role for this key region of the enzyme active site.

AIM2 (absent in melanoma 2) is an innate immune receptor for cytosolic double-stranded DNA (dsDNA). The engagement of dsDNA by AIM2 activates the AIM2 inflammasome, resulting in the cleavage of pro-interleukin-1 by caspase-1. The DNA-binding HIN-200 domain of mouse AIM2 bound to a 15 bp dsDNA and to an 18 bp dsDNA was purified and crystallized. The AIM2 HIN-200 domain in complex with the 15 bp DNA crystallized in the cubic space group I23 or I2 1 3, with unit-cell parameter a = 235.60 Å . The complex of the AIM2 HIN-200 domain and the 18 bp DNA crystallized in a similar unit cell. Diffraction data for the two complexes were collected to about 4.0 Å resolution. Mutagenesis and DNA-binding studies suggest that mouse AIM2 uses a similar surface to human AIM2 to recognize DNA.