Purification and structural analysis of a soluble human chorionogonadotropin hormone-receptor complex

Receptors for the luteotropin/human chorionogonadotropin hormone belong to the G-protein-coupled receptor family by their membrane-anchoring domains. They also possess a large extracellular domain (ECD) responsible for most o f the hormone-receptor interactions. Structure-function studies identified several contacts between hormone and receptor ECD, but the precise topology of the complex is still unknown because of the lack of suitable heterologo us expression means. Receptor ECDs exhibit leucine repeats and have been mo delized on the basis of the three-dimensional structure of the porcine ribo nuclease inhibitor, the first structurally known leucine-rich repeats prote in. Here we report overexpression (up to 20 mg per liter) and purification to homogeneity of a soluble human chorionogonadotropin-ECD receptor complex secreted by stably cotransfected Chinese hamster ovary cells. Biochemical analysis and surface plasmon resonance data were in favor of a unique dimer with a 1:1 ligand-receptor stoichiometry. Immunopurified complex was submi tted to circular dichroism characterization; CD spectra deconvolution indic ated more than 25% alpha helices contributed by the receptor, in agreement with the porcine ribonuclease inhibitor-based modelization.