Insight into the role of urotensin II-related peptide tyrosine residue in UT activation

While sharing common biological activity, the two endogenous ligands of the G protein-coupled receptor UT, e.g. urotensin II (UII) and urotensin II-related peptide (URP), also exhibit distinct effects that could be explained by distinct interactions with their cognate receptor (UT). Accordingly, introduction of a similar substitution at the intracyclic Tyr residue in UII and URP led to compounds with divergent pharmacologic profiles. Hypothesizing that the Tyr6 residue of URP is a key-element to understand the specific activation of UT by URP, we undertook a study of the structure-activity relationship in which this particular residue was replaced by non-natural and constrained amino acids. Each compound was evaluated for its ability to bind UT, to induce rat aortic ring contraction and to activate Gq and G12 signaling pathways. We identified [Pep6]URP, that binds UT with an affinity similar to that of URP, but behaves as a biased ligand. Used as an antagonist, this peptide is also able to selectively reduce the maximal aortic contraction of URP but not UII. Our results suggest that the orientation of the Tyr residue can stabilize at least two different conformations of UT, leading to biased signaling and a probe-dependent allosteric effect.