Characterization of urotensin-II receptor structural domains involved in the recognition of U-II, URP, and urantide.

Urotensin-II (U-II) and urotensin-II-related peptide (URP) are potent vasoconstrictors, and this action is mediated through a G protein-coupled receptor identified as UT. This receptor is expressed abundantly in the mammalian cardiovasculature, and the effects of U-II and URP can be blocked with urantide, a selective antagonist. Thus, we carried out a study with the aim to characterize the conformational arrangement of the three extracellular loops of UT as well as the transmembrane domains III and IV. Secondary structures of the synthetic receptor fragments were determined using circular dichroism (CD) spectroscopy in a variety of solvent and micelle conditions. Spectra showed that all receptor segments but not the extracellular loop I exhibited a propensity for adopting the alpha-helix folding. Furthermore, using surface plasmon resonance (SPR) technology, we measured the binding affinities of the ligands, U-II, URP, and urantide toward the UT extracellular segments. SPR data showed that both U-II and URP bind extracellular loops II and III with similar affinities, whereas none of these two ligands were able to interact with the extracellular loop I. Moreover, the binding of urantide was observed only with the second extracellular loop. These results imply that U-II and URP but not urantide would bind to UT according to a common pattern. Also, the correlation of the CD spectral information with the affinity data suggested that the adoption of a helical geometry in UT, by extracellular loops II and III, might be essential for favoring the binding of ligands.