1 / 55

Signalling at Cell Surface

Signalling at Cell Surface. 2 April 2007. Receptors. Classification of receptors. Intracellular receptors (for lipid soluble messengers) function in the nucleus as transcription factors to alter the rate of transcription of particular genes.

miracle
Download Presentation

Signalling at Cell Surface

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Signalling at Cell Surface 2 April 2007

  2. Receptors

  3. Classification of receptors • Intracellular receptors (for lipid soluble messengers) • function in the nucleus as transcription factors to alter the rate of transcription of particular genes. • Plasma membrane receptors (for lipid insoluble messengers) • Receptors function as ion channels • receptors function as enzymes or are closely associated with cytoplasmic enzymes • receptors that activate G proteins which in turn act upon effector proteins, either ion channels or enzymes, in the plasma membrane.

  4. Cell Surface Receptors • May work both fast and slow • Always use “second messengers”

  5. Cell-Surface Receptors Belong to Four Major Classes • G proteincoupled receptors : epinephrine, serotonin, and glucagon. • Ion-channel receptors: acetylcholine receptor at the nerve-muscle junction. • Tyrosine kinase linked receptors: cytokines, interferons, and human growth factor. • Receptors with intrinsic enzymatic activity

  6. Four classes of ligand-triggered cell-surface receptors

  7. RECEPTORION CHANNELS • multi-subunit, transmembrane protein complexes • complex is both the receptor and ion channel • stimuli: chemical, stretch or voltage • stimulus induces conformational change to open or close ion channel • two types: 1) ligand-gated ion channel 2) voltage-gated ion channel

  8. LIGAND-GATED ION CHANNELS • chemical stimuli bind to receptor and open or close ion channel • stimuli can be extracellular or intracellular • EXTRACELLULAR STIMULI: (neurotransmitters) • e.g. acetylcholine, dopamine, GABA, glutamate • INTRACELLULAR STIMULI: (second messengers) • e.g. IP3, cAMP, cGMP, Ca2+

  9. LIGAND-GATED ION CHANNEL AT THE SYNAPSE • occurs at gap (synaspe) between nerve and target cell • acetylcholine (ACh) released into synapse • ACh binds to ion channel on target cell, opens channel, influx of Na+ • enzyme acetylcholinesterase released into synapse to breakdown ACh

  10. ACETYLCHOLINE ANTAGONISTS • very potent neurotoxins • bind to receptor and prevent opening of Na+ channel • e.g. cobratoxin from Indian cobra • atropine from deadly nightshade • S. American arrow poison (curare) - very fast acting so shot animals don’t run too far

  11. VOLTAGE GATED ION CHANNELS • ion channel undergoes conformational change folllowing electrical stimulus • this “depolarization” opens the channel • leads to flow of Na+ into cell • constitutes an “action potential” • channel recloses

  12. Signaling pathways downstream from G protein coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs)

  13. Structural formulas of four common intracellular second messengers.

  14. Intracellular proteins • Two groups of evolutionary conserved proteins function in signal transduction • 1. GTPase switch proteins • Conversion from GDP bound inactive state to GTP-bound active state is mediated by guanine nucleotide exchange factors (GEFs) • Intrinsic GTPase activity hydrolyzes bound GTP to GDP + Pi

  15. GTP hydrolysis is accelerated by GTPase accelerating protein (GAPs) • Two classes of GTPase switch proteins: • Trimeric (large) G proteins • Directly bind to receptors • Monomeric (small) G proteins • Linked to receptors via adapter proteins and GEFs

  16. Common intracellular signaling proteins

  17. 2. Protein kinases and phosphatases • Human genome encodes 500 PKs and 100 PPs • Two types of PK • Those that P* OH group on Tyr residue • Those that P* OH group on Ser or Thr residues • PK is activated • By other kinases • By direct binding to other proteins • By second messengers

  18. Regulation of signaling • External signal decreases • Degradation of second mesenger • Desensitization to prolonged signaling • Receptor endocytosis • Modulation of receptor activity • Phosphorylation • Binding to other proteins

  19. G Protein-Coupled Receptors • A very large family of receptors coupled to trimeric G proteins • Activate or inhibit adenylyl cyclase • All have seven membrane spanning region • Ligands include: • Hormones, neurotransmitters, light activated receptors (rhodopsins), thousands of odorant receptors

  20. GPCRs and G proteins are involved in the regulation of many important physiological functions

  21. Signal transducing G protein has 3 subunits • G, Gß and G • G is the GTPase switch protein and modulates the activity of an effector protein • Effector proteins are either membrane bound ion channels or enzymes generating second messengers

  22. GPCR-mediated dissociation of trimeric G proteins has been demonstarted in fluorescence energy transfer experiments

  23. The activation/deactivation cycle of G proteins Agonist-receptor complex     + GTP   GDP GTP GDP  Inactive effector Active effector    Pi   Active effector GDP GTP

  24. G proteins can be linked to: • adenylate cyclase • produces cyclic AMP (cAMP) • guanyl cyclase • produces cyclic GMP (cGMP) • phospholipase C • produces inositol trisphosphate (IP3) • and diacyl glycerol (DAG) • ion channels

  25. G-Protein-Activated Enzymes

  26. first messenger receptor transducer amplifier second messenger

  27. Activity of bg subunits • Activation of K+ Channels

  28. G-Protein-Activated Enzymes • Generate new molecules - “second messengers

  29. G proteins and cAMP

  30. cAMP vs PKA

  31. cAMP and gene transcription

  32. Epinephrine case • Mediates body’s response to stress, when all tissues need glucose and fatty acids to produce ATP • ß-adrenergic receptors • Heart muscle: contraction • Smooth muscle cells of intestine: relax • 2-adrenergic receptors • Smooth muscle cells of endothelium, skin, kidney and intestine: constrict

  33. ß1 and ß2 adrenergic receptors are coupled to stimulatory G protein (Gs) • Actvates adenylyl cyclase • 1 adrenergic receptor is coupled to inhibitory G protein (Gi) • Inhibits adenylyl cyclase • 2 adrenergic receptor is coupled to Gq that activates another effector enzyme

  34. Bacterial toxins • Vibrio cholera • Catalyzes chemical modification of Gs that prevents hydrolysis of GTP to GDP • Active state • Bordetella pertussis • Catalyzes chemical modification of Gi that prevents release of GDP • Inactive state

  35. Critical domain of GPCR resides in C3 loop according to chimeric receptor expression experiments

More Related