G Protein-Coupled Receptors: Conformational “Gatekeepers” of Transmembrane Signal Transduction and Diversification
Proteins in the cellular signaling machinery accomplish an amazing spectrum of functions necessary for the growth and survival of life by a network of signaling events separated in both space and time. Membrane proteins enable signal transduction across the cell membrane, which results in these signaling events inside the cell leading to a physiological response. G protein-coupled receptors (GPCRs) form the largest family of membrane proteins that process a very diverse set of extracellular signals and are capable of transducing multiple intracellular signaling pathways, mediated by G proteins and/or Arrestins, each with potentially different functional consequences. This “pleiotropic” nature of GPCRs is enabled by a high conformational flexibility of GPCRs, which allows for a unique ensemble of possible conformations depending on the state of the GPCR, whether it is in the apo form, or interacting with a ligand/antibody, or interacting with another protein. Each ligand can induce a different set of conformations in a GPCR, which can interact with G protein and Arrestin pathways in different ways, resulting in different physiological outcomes. This chapter provides an overview of how GPCRs use their conformational flexibility to perform a complex array of functions and how this can be used advantageously to bias signaling within the cell. A detailed understanding of the signaling pathways that are turned on by GPCRs, combined with the development of biased agonists and allosteric modulators to select specific outcomes, provides a promising avenue for developing therapeutics with minimal side-effects.