Receptor Signalling and GPCRs PDF

Department of Biochemistry HuBi 2001– Introduction to Biochemistry Receptor Signalling and intro to GPCRs Instructor: Renan Danielski  Learning objectives Know what receptor proteins are and what they do Become familiar with the 8 featu...

Department of Biochemistry HuBi 2001– Introduction to Biochemistry Receptor Signalling and intro to GPCRs Instructor: Renan Danielski  Learning objectives Know what receptor proteins are and what they do Become familiar with the 8 features of signal transduction pathways Know the general types of signal transducers Understand the types of extracellular chemical signaling Understand G-proteins and GPCRs  Receptor Proteins What type of information do animal cells exchange? Concentration of ions and glucose in extracellular fluids The interdependent metabolic activities taking place in different tissues The correct placement of cells during development  Receptor Proteins In cells.. Information comes in the form of signals Signals are captured by receptors and converted to cellular response This always involves a chemical process Signal transduction: how information (signal) is converted into chemical change (transduction) in all living cells  Receptor Proteins Receptor Proteins Not always in the cell membrane - may be in other structures, or in cytosol Have specific binding of ligand Usually induces a conformational change Also have effector specificity May mediate a variety of actions e.g., cell-cell signalling, adhesion, endocytosis, cytosolic signalling cascades, gene regulation, etc. often involves signal amplification Receptor Proteins Signal Transduction The Basics 1. A signal (ligand) interacts with the receptor 2. The activated receptor interacts with cellular machinery 3. A second signal/protein activity change is produced 4. Metabolic activity changes (target cell) 5. End of the transduction event 8 features of signal transduction pathways 8 features of signal transduction Signal Transduction Features in a pathways few words a) Specificity: ligand and receptor adapt to each other b) Sensitivity: high affinity c) Amplification: enzyme cascade d) Modularity: multiple domains, multiple enzymes, multiple functions e) Desensitization/Adaptation: Overwhelming signals lead to desensitization f) Integration: Multiple signals, one message g) Divergence: branching effect h) Localized response: confined regulation General types of signal transducers Extracellular chemical signaling Most extracellular ligands must interact with a membrane receptor They are too large/hydrophilic to move through membrane, so they act indirectly via receptors Many ligand types that vary in size, charge, hydrophobicity, etc. Small molecules (e.g., amino acids) Gases (e.g., nitric oxide) Soluble proteins (e.g., hormones) Membrane anchored factors (e.g., cell antigens) Extracellular chemical signaling Small and hydrophobic ligands: Are able to cross the cell membrane and bind to intracellular receptors in the nucleus or cytoplasm Binding to receptors is necessary for travelling through the bloodstream (hydrophilic environment) Examples: nitric oxide (gas) and steroid hormones Estradiol and testosterone Vitamin D  Receptor Proteins Extracellular Chemical Signaling Polar and charged ligands: Must bind on the outside of the cell Extracellular domains of cell-surface receptors Examples: peptide ligands such as growth factors, insulin, certain neurotransmitters Extracellular chemical signaling Cell-surface receptors Composed of three domains: Extracellular ligand-binding domain Hydrophobic domain that spans across the membrane Intracellular domain: transmits a signal Ligand-gated ion channels, G protein-coupled receptors, receptor tyrosine kinases Extracellular chemical signaling Cell-surface receptors Ligand-gated ion channels Ligand binding: ion channel opens Ions can bypass the hydrophobic core of the membrane Protein’s structural changes determine whether the channel will be open or closed Extracellular chemical signaling Cell-surface receptors Receptor tyrosine kinases Enzyme-linked receptors are located in the cell surface Their intracellular domains are associated with an enzyme (or is an enzyme) Receptor tyrosine kinases: transfer phosphate groups to the AA tyrosine More on them later…  GPCRs Large diverse family of receptors that respond to a variety of extracellular signals and regulating a variety of cellular activities Activate trimeric G proteins – a member of the guanosine nucleotide binding protein (G protein) family Function as On-Off switches for intracellular signaling pathways Activate or inactivate ion channels or effector enzymes that generate second messenger molecules  GPCRs Signal transduction through GPCRs 1. A plasma membrane receptor with 7 transmembrane helical segments 2. A G-protein that cycles between active (GTP-bound) and inactive (GDP-bound) forms 3. Effector enzyme (or ion channels) in the plasma membrane that is regulated by the activated G- protein.  GPCRs G-proteins G-protein coupled receptor (GPCR) Typical receptor structure with 7 membrane spanning regions C-terminal loop and tail has G- protein binding activity Three different families that bind ligands in different ways G-proteins GTP/GDP binding and hydrolysis Covalently attached to lipids (anchored in membrane) GPCRs G-proteins Monomeric type Usually involved in alteration of gene expression ON-OFF transition conformational changes GPCRs G-proteins Trimeric type (α, β, γ subunits) The “real G-proteins” 100 different members with thousands of different receptors and functions Subunits can dissociate from each other during signalling GPCRs GPCRs GPCRs Varying Roles of G-protein Subunits e.g., Gβγ may act as effector molecule (instead of Gα) GPCRs Activation of Effector Proteins