Circular dichroism studies of the interaction between synthetic peptides corresponding to intracellular loops of beta-adrenergic receptors and phospholipid vesicles.

We previously showed that peptides corresponding to the N-terminal parts of the third intracellular loops of turkey and hamster beta-adrenergic receptors (tu beta I3N and ha beta I3N, respectively) can activate the GS protein (one of the GTP-binding regulatory proteins which couples to the beta-adrenergic receptor) reconstituted in phospholipid vesicles, and also that such activation can be greatly enhanced by a modification which increases the hydrophobicity of the peptides. These observed phenomena suggest that the interaction with phospholipid membranes is important for the activity of these peptides; hence, in the present study we employed circular dichroism to analyze the interaction of the synthetic peptides corresponding to the intracellular loops of G protein-coupled receptors with phosphatidylserine/phosphatidylcholine mixed vesicles. The tu beta I3N and ha beta I3N peptides were subsequently found to take on an alpha-helical conformation upon binding with the vesicles, whereas those corresponding to the intracellular loops of m1 and m2 muscarinic acetylcholine receptors in contrast did not interact with the vesicles. The positions of several side chains of the membrane-bound loop peptides were also determined. Our results show for the first time the interaction occurring between the intracellular loops of beta-adrenergic receptors and a phospholipid membrane.