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 molecu lar mechanism of signal transduction is still unclear, The solution co nformation of a synthetic peptide corresponding to residues 300-320 of the rat AT(1A) receptor, located in the C-terminal cytoplasmic tail a nd indicated by mutagenesis work to be critical for the G-protein coup ling, has been investigated by circular dichroism (CD), nuclear magnet ic 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 call form helical aggregates; addition of small amou nts of trifluoroethanol induces a secondary structure, mostly due to t he presence of helical elements, Using MMR-derived constraints, an ens emble of conformers for the peptide has been determined by restrained molecular dynamics calculations, Analysis of the converged three-dimen sional structures indicates that a significant population of them adop ts an amphipathic alpha-helical conformation that, depending upon expe rimental conditions, presents a variable extension in the stretch Leu( 6)-Tyr(20). An equilibrium with nonhelical structured conformers is al so observed, We suggest that the capability of the peptide to modulate its secondary structure as a function of the medium dielectric consta nt, as well as its ability to form helical aggregates by means of inte rmolecular hydrophobic interactions, can play a significant role for G -protein activation.