Electrostatic and hydrophobic forces tether the proximal region of the angiotensin II receptor (AT_1a) carboxyl terminus to anionic lipids

The carboxyl terminus of the type-1 angiotensin II receptor (AT_1A) is a focal point for receptor activation and deactivation. Synthetic peptides corresponding to the membrane-proximal, first 20 amino acids of the carboxyl terminus adopt an α-helical conformation in organic solvents, suggesting that the secondary structure of this region may be sensitive to hydrophobic environments. Using surface plasmon resonance, immobilized lipid chromatography, and circular dichroism, we examined whether this positively charged, amphipathic α-helical region of the AT_1A receptor can interact with lipid components in the cell membrane and thereby modulate local receptor attachment and structure. A synthetic peptide corresponding to the proximal region of the AT_1A receptor carboxyl terminus (Leu³⁰⁵ to Lys³²⁵) was shown by surface plasmon resonance to bind with high affinity to the negatively charged lipid, dimyristoyl L-α-phosphatidyl-DL-glycerol (DMPG), but poorly to the zwitterionic lipid, dimyristoyl L-α-phosphatidylcholine (DMPC). In contrast, a peptide analogue possessing substitutions at four lysine residues (corresponding to Lys^{307,308,310,311}) displayed poor association with either lipid, indicating a crucial anionic component to the interaction. Circular dichroism analysis revealed that both the wild-type and substituted peptides possessed α-helical propensity in methanol and trifluoroethanol, while the wild-type peptide also adopted partially inserted helical structure in DMPG and DMPC liposomes. In contrast, the substituted peptide exhibited spectra that suggested the presence of β-sheet and α-helical structure in both liposomes. Immobilized lipid chromatography was used to characterize the hydrophobic component of the membrane interaction, and the results demonstrated that hydrophobic and electrostatic interactions mediated the binding of the wild-type peptide but that the substituted peptide bound to the model membranes mainly via hydrophobic forces. We propose that, in intact AT_1A receptors, the proximal carboxyl terminus associates with the cytoplasmic face of the cell membrane via a high-affinity, anionic phospholipid-specific tethering that serves to increase the amphipathic helicity of this region. Such associations may be important for receptor function and common for G protein-coupled receptors.