Intracellularly Truncated Human α2B-Adrenoceptors: Stable and Functional GPCRs for Structural Studies

All three α₂-adrenoceptor subtypes have a long third intracellular loop (3i), which is conserved by overall size and charge-hydrophobic properties but not by amino acid sequence similarity. These properties must be relevant for function and structure, because they have been preserved during hundreds of millions of years of evolution. The contribution of different loop portions to agonist/antagonist binding properties and G protein coupling of the human α_2B-adrenoceptor (α_2B-AR) was investigated with a series of 3i truncated constructs (Δ 3i). We used a variety of agonists/antagonists in competition binding assays. We stimulated α_2B-AR Δ3i with various agonists and measured ³⁵S]GTPγS binding in isolated cell membranes with or without antagonist inhibition. We also evaluated the ability of oligopeptides, analogous to the amino and carboxyl terminal parts of 3i, to promote G protein activation, monitored with the ³⁵S]GTPγS assay. Our results reveal that the carboxyl end residues of 3i, R360(6.24) to V372(6.36), are important for G_i/G_o protein activation. Deletions in regions from G206(5.72) to R245(5.110) altered the binding of some α_2B-AR agonists, indicating that agonist binding is dependent on the conformation of the 3i domain, possibly through the involvement of G protein interactions. The truncated receptor constructs may be more stable on purification and thus be useful for structural characterization of α_2B-AR.