G protein-coupled receptors

(noun)

These comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. Any adrenergic effects on cells are generally mediated by G protein-coupled receptors.

Related Terms

  • adrenoreceptor
  • adrenergic receptor

Examples of G protein-coupled receptors in the following topics:

  • Ionotropic and Metabotropic Receptors

    • Although both ionotropic and metabotropic receptors are activated by neurotransmitters, ionotropic receptors are channel-linked while metabotropic receptors initiate a cascade of molecules via G-proteins.
    • Two types of membrane-bound receptors are activated with the binding of neurotransmitters: ligand-gated ion channels (LGICs) inotropic receptors and metabotropic G- protein coupled receptors.
    • Metabotropic receptors are a subtype of membrane receptors that do not form an ion channel pore but use signal transduction mechanisms, often G proteins, to activate a series of intracellular events using second messenger chemicals.
    • The metabotropic G protein-coupled receptors have seven hydrophobic transmembrane domains.
    • When the neurotransmitter binds to the receptor, there is an activation via the G-protein that later activates the  secondary messengers.
  • Adrenergic Neurons and Receptors

    • The adrenergic receptors (or adrenoceptors) are a class of metabotropic G protein-coupled receptors that are targets of the catecholamines, especially norepinephrine or noradrenaline, and epinephrine (adrenaline).
    • There are two main groups of adrenergic receptors, α and β, with several subtypes. α receptors have the subtypes α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor).
    • All three are linked to Gs proteins (although β2 also couples to Gi), which in turn are linked to adenylate cyclase.
    • α1-adrenergic receptors are members of the G protein-coupled receptor superfamily.
    • On activation, a heterotrimeric G protein, Gq, activates phospholipase C (PLC).
  • Direct Gene Activation and the Second-Messenger System

    • Most hormone receptors are G protein-coupled receptors.
    • Upon hormone binding, the receptor undergoes a conformational change and exposes a binding site for a G-protein.
    • The G-protein is bound to the inner membrane of the cell and consists of three sub-units: alpha, beta, and gamma.
    • Upon binding to the receptor, it releases a GTP molecule, at which point the alpha sub-unit of the G-protein breaks free from the beta and gamma sub-units and is able to move along the inner membrane until it contacts another membrane-bound protein: the primary effector. 
    • G-protein is activated and produces an effector. 3.
  • Olfaction: The Nasal Cavity and Smell

    • Each functions using cilia, small hair-like projections that contain olfactory receptor proteins.
    • These proteins carry out the transduction of odorants into electrical signals for neural processing.
    • In mammals, olfactory receptors have been shown to signal via G protein.
    • This is a similar type of signaling of other known G protein-coupled receptors (GPCR).
    • The binding of an odorant particle on an olfactory receptor activates a particular G protein (Gαolf), which then activates adenylate cyclase, leading to cAMP production. cAMP then binds and opens a cyclic nucleotide-gated ion channel.
  • Types of Receptors

    • G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein.
    • All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
    • Cell signaling using G-protein-linked receptors occurs as a cyclic series of events.
    • Once the G-protein binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP.
    • When a signaling molecule binds to a G-protein-coupled receptor in the plasma membrane, a GDP molecule associated with the α subunit is exchanged for GTP.
  • Reception and Transduction

    • Humans have about 12 million olfactory receptors distributed among hundreds of different receptor types that respond to different odors.
    • The sensory receptors on the cilia are proteins.
    • When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated.
    • Sour tastants are acids which belong to the thermoreceptor protein family.
    • Sweet, bitter, and umami tastants require a G-protein-coupled receptor.
  • Gustation: Taste Buds and Taste

    • Bitter, sweet, and umami tastes use similar mechanisms based on a G protein-coupled receptor, or GPCR.
    • The salt receptor, NaCl, is arguable the simplest of all the receptors found in the mouth.
    • Synthetic sweeteners such as saccharin activate a separate set of GPCRs, initiating a similar but different process of protein transitions.
    • There are three different receptor proteins at work in a sour taste.
    • It is thought that umami receptors act similarly to bitter and sweet receptors (involving GPCRs), but very little is known about their actual function.
  • Plasma Membrane Hormone Receptors

    • When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated.
    • G proteins are proteins separate from receptors that are found in the cell membrane.
    • When a hormone is not bound to the receptor, the G protein is inactive and is bound to guanosine diphosphate, or GDP.
    • When a hormone binds to the receptor, the G protein is activated by binding guanosine triphosphate, or GTP, in place of GDP.
    • The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase.
  • Binding Initiates a Signaling Pathway

    • Cell-surface receptors, also known as transmembrane receptors, are membrane-anchored (integral) proteins that bind to external ligand molecules.
    • There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
    • G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein.
    • The activated G-protein then interacts with either an ion channel or an enzyme in the membrane.
    • All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
  • Mapping Protein-Protein Interactions

    • Mapping protein-protein interactions gives us a better understanding of molecular mechanisms inside the cell.
    • The protein complexes formed could be stable (proteins interact for a prolonged period of time) or transient (proteins interact for a brief period of time).
    • The most widely employed tools are the yeast two-hybrid system and affinity purification coupled to mass spectrometry.
    • If the two proteins do interact the bait recruits the prey to a specific cellular location where it can stimulate a detectable output (e.g., gene activation).
    • Affinity purification of protein complexes coupled to mass spectrometry is carried out as follows: a specific protein (the bait) is manipulated to express an affinity tag.
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