Structure of the C-terminal fragment 300-320 of the rat angiotensin II AT(1A) receptor and its relevance with respect to G-protein coupling

Angiotensin II AT(1A) receptor is coupled to G-protein, and the molecular mechanism of signal transduction is still unclear. The solution conformation of a synthetic peptide corresponding to residues 300-320 of the rat AT(1A) receptor, located in the C-terminal cytoplasmic tail and indicated by mutagenesis work to be critical for the G-protein coupling, has been investigated by circular dichroism (CD), nuclear magnetic resonance (NMR) and restrained molecular dynamics calculations. The CD data indicate that, in acidic water, at concentration below 0.8 mM, the peptide exists in a predominantly coil structure while at higher concentration it can form helical aggregates; addition of small amounts of trifluoroethanol induces a secondary structure, mostly due to the presence of helical elements. Using NMR-derived constraints, an ensemble of conformers for the peptide has been determined by restrained molecular dynamics calculations. Analysis of the converged three-dimensional structures indicates that a significant population of them adopts an amphipathic α-helical conformation that, depending upon experimental conditions, presents a variable extension in the stretch Leu⁶-Tyr²⁰. An equilibrium with nonhelical structured conformers is also observed. We suggest that the capability of the peptide to modulate its secondary structure as a function of the medium dielectric constant, as well as its ability to form helical aggregates by means of intermolecular hydrophobic interactions, can play a significant role for G-protein activation.