Classical ligands bind to the extracellular surface of their cognate receptors and activate signaling pathways without crossing the plasma membrane barrier. We selectively targeted the intracellular receptor–G protein interface by using cell-penetrating membrane-tethered peptides. Attachment of a palmitate group to peptides derived from the third intracellular loop of protease-activated receptors-1 and -2 and melanocortin-4 receptors yields agonists and/or antagonists of receptor–G protein signaling. These lipidated peptides—which we have termed pepducins—require the presence of their cognate receptor for activity and are highly selective for receptor type. Mutational analysis of both intact receptor and pepducins demonstrates that the cell-penetrating agonists do not activate G proteins by the same mechanism as the intact receptor third intracellular loop but instead require the C-tail of the receptor. Construction of such peptide–lipid conjugates constitutes a new molecular strategy for the development of therapeutics targeted to the receptor–effector interface
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