Interaction of Angiotensin II with the C-terminal 300-320 fragment of the rat Angiotensin II Receptor AT1a monitored by NMR

Interaction between angiotensin II (Ang II) and the fragment peptide 300–320 (fCT300–320) of the rat angiotensin II receptor AT1a was demonstrated by relaxation measurements, NOE effects, chemical shift variations, and CD measurements. The correlation times modulating dipolar interactions for the bound and free forms of Ang II were estimated by the ratio of the nonselective and single-selective longitudinal relaxation rates. The intermolecular NOEs observed in NOESY spectra between HN protons of 9LysfCT and 6Hisang, 10PhefCT and 8Pheang, HN proton of 3TyrfCT and Hα of 4Tyrang, 5PhefCTHδ and Hα of 4Tyrang indicated that Ang II aromatic residues are directly involved in the interaction, as also verified by relaxation data. Some fCT300–320 backbone features were inferred by the CSI method and CD experiments revealing that the presence of Ang II enhances the existential probability of helical conformations in the fCT fragment. Restrained molecular dynamics using the simulated annealing protocol was performed with intermolecular NOEs as constraints, imposing an α-helix backbone structure to fCT300–320 fragment. In the built model, one strongly preferred interaction was found that allows intermolecular stacking between aromatic rings and forces the peptide to wrap around the 6Leu side chain of the receptor fragment.