Conformational Sampling and Binding Site Assessment of Suppression of Tumorigenicity 2 Ectodomain

Suppression of Tumorigenicity 2 (ST2), a member of the interleukin-1 receptor (IL-1R) family, activates type 2 immune responses to pathogens and tissue damage via binding to IL-33. Dysregulated responses contribute to asthma, graft-versus-host and autoinflammatory diseases and disorders. To study ST2 structure for inhibitor development, we performed the principal component (PC) analysis on the crystal structures of IL1-1R1, IL1-1R2, ST2 and the refined ST2 ectodomain (ST2(ECD)) models, constructed from previously reported small-angle X-ray scattering data. The analysis facilitates mapping of the ST2(ECD) conformations to PC subspace for characterizing structural changes. Extensive coverage of ST2(ECD) conformations was then obtained using the accelerated molecular dynamics simulations started with the IL-33 bound ST2(ECD) structure as instructed by their projected locations on the PC subspace. Cluster analysis of all conformations further determined representative conformations of ST2(ECD) ensemble in solution. Alignment of the representative conformations with the ST2/IL-33 structure showed that the D3 domain of ST2(ECD) (containing D1-D3 domains) in most conformations exhibits no clashes with IL-33 in the crystal structure. Our experimental binding data informed that the D1-D2 domain of ST2(ECD) contributes predominantly to the interaction between ST2(ECD) and IL-33 underscoring the importance of the D1-D2 domain in binding. Computational binding site assessment revealed one third of the total detected binding sites in the representative conformations may be suitable for binding to potent small molecules. Locations of these sites include the D1-D2 domain ST2(ECD) and modulation sites conformed to ST2(ECD) conformations. Our study provides structural models and analyses of ST2(ECD) that could be useful for inhibitor discovery.