Multisite interactions of receptors and G proteins: enhanced potency of dimeric receptor peptides in modifying G...

Synthetic peptides that activate or inhibit G proteins reveal structural determinants of receptor-G protein interactions and show promise as potential therapeutic agents. A cysteine-containing peptide from the carboxyl-terminal part of the third cytoplasmic loop of the alpha 2-adrenergic receptor (peptide Q) uncouples alpha 2-adrenergic receptors from Gi. Peptide Q readily forms disulfide-linked dimers (Qdimer), as detected by high performance liquid chromatography and mass spectrometry. Qdimer is > 100-fold more potent than monomeric Q peptide in inhibiting p-[125I] iodoclonidine binding to the human alpha 2a-adrenergic receptor in platelet membranes and transfected Chinese hamster ovary cells. In addition, Qdimer is 10-20 times more potent than monomeric Q peptide in inhibiting alpha 2 agonist-stimulated GTPase in cell membranes and in directly stimulating G(o)/Gi GTPase in lipid vesicles. The effect of Qdimer is reversible and not mimicked by cystine. Formylation of both tryptophans greatly reduces the potency of the dimer but a single formyl group is well tolerated, indicating an asymmetric interaction of the dimer with Gi in membranes. A mixed dimer of peptides from the amino- and carboxyl-terminal ends of the third cytoplasmic loop of the alpha 2-adrenergic receptor is most potent in all measures of G protein interactions, suggesting that the dimer of Q peptides mimics multiple intracellular portions of the alpha 2-adrenergic receptor with the G protein. These data confirm the importance of multiple receptor regions in G protein activation and suggest a strategy for examining the role of physically separated regions in protein-protein interactions.