title: Structure of two fragments of the third cytoplasmic loop of the rat angiotensin II AT1A receptor. Implications with respect to receptor activation and G-protein selection and coupling.
abstract: The structural bases that render the third intracellular loop (i3) of the rat angiotensin II AT1A receptor one of the cytoplasmic domains responsible for G-protein coupling are still unknown. The three-dimensional structures of two overlapping peptides mapping the entire i3 loop and shown to differently interact with purified G-proteins have been obtained by simulated annealing calculations, using NMR-derived constraints collected in 70% water/30% trifluoroethanol solution. While the NH2-terminal half, Ni3, residues 213-231, adopts a stable amphipathic alpha-helix, extending over almost the entire peptide, a more flexible conformation is found for the COOH-terminal half, Ci3, residues 227-242. For this peptide, a cis-trans isomerization around the Lys6-Pro7 peptide bond generates two exchanging isomers adopting similar conformations, with an alpha-helix spanning from Asn9 to Ile15 and a poorly defined NH2 terminus. A quite distinct structural organization is found for the sequence EIQKN, common to Ni3 and Ci3. The data do suggest that the extension and orientation of the amphipathic alpha-helix, present in the proximal part of i3, may be modulated by the distal part of the loop itself through the Pro233 residue. A molecular model where this possibility is considered as a mechanism for G-protein selection and coupling is presented.
authors.affiliation: Institute of Biological Chemistry, University of Parma, 43100 Parma, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Franzoni
authors.forename: G
authors.initials: G
authors.lastname: Nicastro
authors.forename: T A
authors.initials: TA
authors.lastname: Pertinhez
authors.forename: E
authors.initials: E
authors.lastname: Oliveira
authors.forename: C R
authors.initials: CR
authors.lastname: Nakaie
authors.forename: A C
authors.initials: AC
authors.lastname: Paiva
authors.forename: S
authors.initials: S
authors.lastname: Schreier
authors.forename: A
authors.initials: A
authors.lastname: Spisni
mesh_terms: D000595:Amino Acid Sequence; D000804:Angiotensin II / Q000378:metabolism*; D000818:Animals; D002942:Circular Dichroism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D011485:Protein Binding; D017433:Protein Structure, Secondary; D051381:Rats; D011945:Receptors, Angiotensin / Q000737:chemistry* / Q000378:metabolism
chemical_list: D011945:Receptors, Angiotensin; D000804:Angiotensin II; D019204:GTP-Binding Proteins