Structure of two fragments of the third cytoplasmic loop of the rat angiotensin II AT(1A) receptor

The structural bases that render the third intracellular loop (i3) of the r at angiotensin II AT,, 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 differe ntly interact with purified G-proteins have been obtained by simulated anne aling 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 en tire peptide, a more flexible conformation is found for the COOH-terminal h alf, Ci3, residues 227-242, For this peptide, a cis-trans isomerization aro und the Lys(6)-Pro(7) peptide bond generates two exchanging isomers adoptin g similar conformations, with an ct-helix spanning from Asn(9) to Ile(15) a nd a poorly defined NH, 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 cu-helix, present in the proximal part of i3, may be modulated by the distal part of the loop i tself through the Pro(233) residue. A molecular model where this possibilit y is considered as a mechanism for G-protein selection and coupling is pres ented.