Isolation of the Melanocortin 5 Receptor

Abstract

The family of melanocortin receptors (MCR) specifically binds peptide hormones derived from propiomelanocortin, including adrenocorticotropic hormone (ACTH) and alpha-melanocyte stimulating hormone (αMSH). These receptors are generally characterized as integral proteins having seven membrane-spanning domains and a short carboxyl tail on the cytosolic side, and encoded by intronless genes. Currently five different MCRs have been identified and cloned (1). The ligand-binding specificity and equilibrium-binding constants for a variety of natural and synthetic analogues of the melanocortins have been determined for each MCR. In general, this information has been obtained by measuring ligand binding to cell lines transfected to overexpress receptor mRNA. Successful transfection has generally been determined by reverse transcriptase-polymerase chain reaction (RT-PCR). A follow-up experiment invariably tests for ligand-specific stimulation of adenylyl cyclase activity to confirm that hormone binding stimulates a G-protein coupled receptor to dissociate from a G_αs-GTP. These experiments have produced important information about binding and have begun the process toward developing both agonists and antagonists to MCR mediated activities. In addition, earlier cloning work has produced a valuable source of information concerning the inferred amino acid sequence based on the cDNA and information about probable tissue expression of these receptors.