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 α-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 b...