The suppression of tumorigenicity 2/IL-33 (ST2/IL-33) pathway has been implicated in several immune and inflammatory diseases. ST2 is produced as 2 isoforms. The membrane-bound isoform (ST2L) induces an immune response when bound to its ligand, IL-33. The other isoform is a soluble protein (sST2) that is thought to be a decoy receptor for IL-33 signaling. Elevated sST2 levels in serum are associated with an increased risk for cardiovascular disease. We investigated the determinants of sST2 plasma concentrations in 2,991 Framingham Offspring Cohort participants. While clinical and environmental factors explained some variation in sST2 levels, much of the variation in sST2 production was driven by genetic factors. In a genome-wide association study (GWAS), multiple SNPs within IL1RL1 (the gene encoding ST2) demonstrated associations with sST2 concentrations. Five missense variants of IL1RL1 correlated with higher sST2 levels in the GWAS and mapped to the intracellular domain of ST2, which is absent in sST2. In a cell culture model, IL1RL1 missense variants increased sST2 expression by inducing IL-33 expression and enhancing IL-33 responsiveness (via ST2L). Our data suggest that genetic variation in IL1RL1 can result in increased levels of sST2 and alter immune and inflammatory signaling through the ST2/IL-33 pathway.

Thioesterase activity is typically required for the release of products from polyketide synthase enzymes, but no such enzyme has been characterized in deep-sea bacteria associated with the production of polyunsaturated fatty acids. In this work, we have expressed and purified the Orf6 thioesterase from Photobacterium profundum. Enzyme assays revealed that Orf6 has a higher specific activity toward long-chain fatty acyl-CoA substrates (palmitoyl-CoA and eicosapentaenoyl-CoA) than toward short-chain or aromatic acyl-CoA substrates. We determined a high resolution (1.05 Å) structure of Orf6 that reveals a hotdog hydrolase fold arranged as a dimer of dimers. The putative active site of this structure is occupied by additional electron density not accounted for by the protein sequence, consistent with the presence of an elongated compound. A second crystal structure (1.40 Å) was obtained from a crystal that was grown in the presence of Mg(2+), which reveals the presence of a binding site for divalent cations at a crystal contact. The Mg(2+)-bound structure shows localized conformational changes (root mean square deviation of 1.63 Å), and its active site is unoccupied, suggesting a mechanism to open the active site for substrate entry or product release. These findings reveal a new thioesterase enzyme with a preference for long-chain CoA substrates in a deep-sea bacterium whose potential range of applications includes bioremediation and the production of biofuels.