Plasticity in Ligand Recognition at Somatostatin Receptors

Abstract

Somatostatin is a signaling peptide that plays a pivotal and wide-ranging role in physiologic processes relating to metabolism and growth through its actions at somatostatin receptors (SSTRs). Members of the somatostatin receptor subfamily, particularly SSTR2, are key drug targets for neuroendocrine neoplasms, with several synthetic peptide agonists currently in clinical use. Here, we show the cryogenic electron microscopy (cryo-EM) structures of active-state SSTR2 in complex with heterotrimeric Gi protein and either the endogenous ligand SST14 or the FDA-approved drug octreotide. Complemented by biochemical assays and molecular dynamics simulations, these structures reveal key details of ligand recognition, receptor activation, and subtype-selectivity at somatostatin receptors. We find that SSTR ligand recognition is highly diverse, as demonstrated by ligand-induced conformational changes in ECL2, substantial sequence divergence across subtypes in extracellular regions, and loss of ligand binding upon several structurally homologous substitutions between subtypes. Despite this complexity, we rationalize several previously described sources of SSTR subtype selectivity and identify an additional key interaction for SSTR2/3/5 specific binding. These results shed light on the basis of ligand recognition by somatostatin receptors and provide valuable insights for structure-based drug discovery at these targets.