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Membrane Function Signal Transduction. I. Introduction to Receptors & Signal Transduction.

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Presentation on theme: "Membrane Function Signal Transduction. I. Introduction to Receptors & Signal Transduction."— Presentation transcript:

1 Membrane Function Signal Transduction

2 I. Introduction to Receptors & Signal Transduction

3 The Players l Signaling molecules l Receptors l G-proteins l Second messenger systems l Effector proteins

4 Signaling Molecules l Neurotransmitters l Hormones l Growth factors l Drugs l Other nomenclature  Ligand  Agonist / Antagonist

5 Receptors l Receptors are proteins associated with cell membranes l Receptors “recognize” signaling molecules by binding to them. l Binding of receptors by signaling molecules ---> Cell behavior change

6 Figure 1: Overview of Signaling Tyrosine Kinase mRNA Synthesis Protein Synthesis Second Messangers Protein Kinases Ion Channels Hormones: Steroids Thyroid Growth Factors Transmitters Hormones

7 Neurotansmitters: Biogenic Amines. l Catecholamines  Epinephrine  Norepinephrine  Dopamine l Esters:Acetylcholine l Indolamines  Histamine  5-HT

8 Neurotransmitters: Peptides l Substance P l Neuropeptide Y (NPY) l Enkephalins l Somatostatin l VIP

9 Neurotransmitters: Amino Acids l Excitatory  Glutamate  Aspartate l Inhibitory   -aminobutyric acid (GABA)  Glycine

10 Neurotranmitters: Other l Nitric Oxide l Arachadonic acid l Carbon Monoxide l PAF l Zinc

11 The G-Proteins l Involved in most signaling processes l Link receptor proteins to effector proteins. Trimeric proteins composed of , , and  -subunits.

12 Figure 2: G-Protein Cycling Adenylate Cyclase Phospholipase C Ion Channels Phospholipase A 2 Phosphodiesterase             A A A A R R R R GTP (GTPase) -P i GTP GDP

13 Functional G-Protein Units GTP-activated  -subunit  produce second messenger  and/or opens ion channels.  -complexes  Initially thought to be inert.  Probably not inert  Exact role currently ill-defined.

14 Second messengers produced by G-protein activation. l Adenylate Cyclase  cAMP l Phospholipase C (PLC)  Inositol triphosphate (IP 3 )  Diacylglycerol (DAG) l Ion Channel Activity

15 Families of G-proteins Unique structure of their  -subunits.  subunits appear to be similar across families. l Main families:  G  s  G  i  G  q

16 II. cAMP Second Messenger System

17 Figure 3: Adenylate Cyclase Adenylate Cyclase R1R1 R2R2 AsAs GsGs GiGi AiAi GTP GDP GTP GDP PDE AMP cAMP ATP-Mg ++ Reg C C C C Protein Protein-P Protein Kinase A (PKA) PKA

18 Summary of Adenylate Cyclase Activation l Receptors which associate with G s -type G-protein  Stimulates adenylate cyclase.  Increases cAMP l Receptors which associate with G i -type G-protein  Inhibit adenylate cyclase.  Decreases cAMP

19 Summary of cAMP action l cAMP exerts its effect by activating protein kinase A (PKA) l PKA phosphorylates proteins  Enzymes, pumps, and channels  Phosphorylation can either increase or decrease activity depending on the protein.

20 Adenylate Cyclase l Family of membrane spanning enzymes. l Types I through IV have been well characterized.  Additional types probably exist. l Types differ with respect to activity modulation by other second messenger systems

21 Adenylate Cyclase Activity and Other Messenger Systems l Kinases (PKA, PKC, other) can phosphorylate adenylate cyclase in some cells. l Binding of adenylate cyclase by:   -subunits of other G-proteins  Ca ++ /calmodulin complexes l Allows other second messenger systems to interact with cAMP system

22 III. The Phospholipase C Second Messenger System: IP 3 and DAG

23 Figure 4: Phospholipase C System R Ca ++ PKC Ca ++ Endoplasmic Reticulum GqGq PLC Protein Protein-P A DAG IP 3 PIP 2

24 Summary of the Phospholipase C Messengers l Agonist binds receptor l Occupied Receptor ---> activation of PLC (G q -mediated) l PLC Produces second messengers: IP 3 and DAG l PLC activation associated with Ca ++ -channel activation

25 Action of IP 3 l IP 3 binds to IP 3 -receptors on the endoplasmic reticulum l Releases intracellular Ca ++ stores.

26 Action of DAG l Remains membrane associated. l Activates Protein kinase C (PKC) which translocates from the cytosol to the membrane l Activated PKC phosphorylates other proteins and alters their function state.

27 PLC System and Calcium l PLC causes the IP 3 -mediated Calcium l PLC also causes the influx of Ca ++. l Ca ++ binds one of a family of Ca ++ binding proteins (calmodulin). l Ca ++ /calmodulin complex  binds to yet other proteins and changes their functional activity.

28 IV. Guanylate Cyclase: cGMP and Nitric Oxide As Second Messengers

29 Figure 5: Nitric Oxide and cGMP cGMP NO Ca ++ GTP GMP Intracellular Ca ++ Stores Ca ++ Arginine + Citrulline GTP NO PDE Membrane Bound Guanylate Cyclase Soluble Guanylate Cyclase C.M. Ion Channels cGMP-Dependent PK PDEase Activity NO Synthetase

30 NO is Membrane Soluble. l Diffusion to nearby cells l Increase cGMP levels in nearby cells l Vascular endothelial cells and nearby smooth muscle cells.


32 Acetylcholine As a Neurotransmitter Acetylcholine As a Neurotransmitter l Both the central and peripheral nervous systems. l Binds two broad classes of receptors:  Nicotinic receptors  Muscarinic receptors.

33 Nicotinic Receptor Features l Composed of 5 subunits:  2 , ,  and . l Subunits are arranged to form a central cavity that extends across the membrane. l Nicotinic receptors are also channels l ACh-binding opens gates and allows ion fluxes across the channel

34 Figure 6: Nicotinic Receptor Channel Agonist Binding Site Gate

35 Subclasses of Nicotinic Receptors l Skeletal muscle (N 1 or N m )  Unique  and  subunits l Autonomic ganglia (N 2 or N g ). l Both N 1 and N 2 are gene-product families not single receptor types.

36 Other Ligand-Gated Channels l Structural and sequence similarity to nicotinic receptors. l Example agonists for these channels include:  Serotonin (5-HT)  Glutamate  GABA  Glycine

37 Muscarinic receptors l Muscarinic receptors are not channels. l Operate through G-proteins to alter second messenger systems. l 5 muscarinic subtypes have been cloned and sequenced (M 1, M 2, M 3, M 4, M 5 ).

38 Grouping Muscarinic Receptors l M 1, M 3, and M 5 receptors: Activate Phospholipase C through Gq.  PLC activation ---> increased IP 3 --> increased intracellular Ca ++  Increased intracellular Ca++ ---> Activation of Ca ++ -sensitive K + & Cl - channels.

39 Grouping Muscarinic Receptors l M 2 and M 4 receptors  G i -coupled inhibition of adenylate cyclase  G o or G i -coupled regulation of certain Ca ++ & K + channels.

40 VI. Signaling by Epinephrine and Norepinephrine and Coupling Through Adrenergic Receptors

41 Three Families of Adrenergic Receptors:  -receptors: Three subtypes      and  .   -receptors: Three subtypes   A    B  and   C   -receptors: Three subtypes   A     B  and   C

42 . All adrenergic receptors appear to be coupled to cellular processes through G-proteins

43 Occupation of  Adrenergic Receptors l G s -mediated stimulation of adenylate cyclase l Increased cAMP l Increased PKA activity.

44 Occupation of   -Adrenergic Receptors l Mechanistic details sketchy l Possibly G q -mediated PLC activation  Increases IP 3 and DAG for some subtypes (1B)?  Activates Ca ++ -channels for other subtypes (1A)?

45 Occupation of   -Adrenergic Receptors l G i -mediated inhibition of adenylate cyclase. l Decreased cAMP l Decreased PKA activity.

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