When lipid mediators bind to G-protein-coupled receptors (GPCRs), the ligand first enters the lipid bilayer, then diffuses laterally in the cell membrane to make hydrophobic contact with the receptor protein, and finally enters the receptor's binding pocket. In this process, the location of the hydrophobic contact point on the surface of the receptor has been little discussed even in cases in which the crystal structure has been determined, because the ligand binding pocket is buried inside the transmembrane (TM) domains. Here, we coupled an activator ligand to a series of membrane phospholipid surrogates, which constrain the depth of entry of the ligand into the lipid bilayer. Consequently, via measurement of the receptor-activating activity as a function of the depth of entry into the membrane, these surrogates can be used as molecular rulers to estimate the location of the hydrophobic contact point on the surface of GPCR. We focused on lysophosphatidylserine (LysoPS) receptor GPR34 and prepared a series of simplified membrane-lipid-surrogate-conjugated lysophospholipid analogues by attaching alkoxy amine chains of varying lengths to the hydrophobic tail of a potent GPR34 agonist. As expected, the activity of these lipid-conjugated LysoPS analogues was dependent on chain length. The predicted contact position matches the position of the terminal benzene ring of a nonlipidic ligand that protrudes between TMs 4 and 5 of the receptor. We further found that the nature of the terminal hydrophilic functional group of the conjugated membrane lipid surrogate strongly influences the activity, suggesting that lateral hydrophilic contact of LysoPS analogues with the receptor's surface is also crucial for ligand-GPCR binding.
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