Classification, Evolution, Pharmacology and Structure of G protein-coupled Receptors

Abstract: G protein-coupled receptors (GPCR) are integral membrane proteins with seven ?-helices that translate a remarkable diversity of signals into cellular responses. The superfamily of GPCRs is among the largest and most diverse protein families in vertebrates.We have searched the human genome for GPCRs and show that the family includes approximately 800 proteins, which can divided into five main families; Glutamate, Rhodopsin, Adhesion, Frizzled/Taste2 and Secretin. This study represents one of the first overall road maps of the GPCR family in a mammalian genome. Moreover, we identified eight novel members of the human Adhesion family which are characterized by long N-termini with various domains. We also investigated the GPCR repertoire of the chicken genome, where we manually verified a total of 557 chicken GPCRs. We detected several specific expansions and deletions that may reflect some of the functional differences between human and chicken.Substantial effort has been made over the years to find compounds that can bind and activate the melanocortin 4 receptor (MC4R). This receptor is involved in food intake and is thus an important target for antiobesity drugs. We used site-directed mutagenesis to insert micromolar affinity binding sites for zinc between transmembrane (TM) regions 2 and 3. We generated a molecular model of the human MC4R which suggests that a rotation of TM3 is important for activation of the MC4R.Furthermore, we present seven new vertebrate prolactin releasing hormone receptors (PRLHRs) and show that they form two separate subtypes, PRLHR1 and PRLHR2. We performed a pharmacological characterization of the human PRLHR which showed that the receptor can bind neuropeptide Y (NPY) related ligands. We propose that an ancestral PRLH peptide has coevolved with a redundant NPY binding receptor, which then became PRLHR. This suggests how gene duplication events can lead to novel peptide ligand/receptor interactions and hence spur the evolution of new physiological functions.