The long-term objectives of the project are to develop a thorough understanding of the functional conformations of the opioid peptides beta-endorphin and dynorphin A(1 -17), and the selectivities of these ligands for binding and activating mu- delta- kappa- and epsilon-opioid receptors. This-knowledge will be useful in the design of opioid peptides and mimetic with novel receptor selectivities, enhanced potencies and specificities, and varied agonist-antagonist character. A model for the functional conformations of beta-endorphin and dynorphin has been developed, based on the expectation-that interface-induced, amphiphilic structures in these peptides will bind to the hydrophilic/lipophilic interface of the cell surface. The first receptor (a delta receptor) has also recently been cloned, and tentative models for its gross structural features, including seven transmembrane alpha- helices, are also already proposed. In this project period, opioid peptide-receptor interactions will be probed in three ways, in order to further develop, test and possibly combine both the ligand and the receptor models: (i) A new approach, involving the incorporation of peptide ligands into surface loops on a large globular protein by genetic engineering, will be developed and applied to opioid receptors. In this method, the """"""""guest"""""""" -peptide ligand effectively ties its """"""""host"""""""" protein near to the ligand binding site. Steric and electrostatic information concerning the receptor surface is then obtained by monitoring receptor affinity as the host protein structure is altered. (2) Synthetic peptide segments of the delta-opioid receptor, and others as they are cloned and sequenced, will be used to test for protein folding interactions and disulfide bridging between (a) the proposed transmembrane helices in model lipid environments, and (b) the extracellular loops in aqueous solution. For the extracellular loops, detailed synthetic models incorporating loop-mimicking conformational constraints and helix initiating structures will-be developed, and interactions with peptide ligands will also be explored. (3) Synthetic chemistry developed earlier in the project will be applied to the design of covalent bridges linking residue pairs in the i-th and (i+7)-th positions in the amphiphilic alpha-helix proposed as a membrane-binding and receptor-activating element in (beta-endorphin. Such bridges, if successful for helix stabilization, should be generally applicable in peptide mimetic design.
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