Lecture: CHAPTER 13 Signal Transduction Pathways

Transcription

1 Lecture: CHAPTER 13 Signal Transduction Pathways

2 Chapter 13 Outline

3 Signal transduction cascades have many components in common: 1. Release of a primary message as a response to a physiological circumstance. 2. Reception of the primary message by a receptor, usually an integral membrane protein. 3. Relay of the detection of the primary message to the cell interior by the generation of a intracellular second message. 4. Activation of effector molecules by the second messenger that result in a physiological response. 5. Termination of the signal cascade.

4 Principles of signal transduction An environmental signal is first received by interaction with a cellular component, most often a cell surface receptor. The information that the signal has arrived is then converted into other chemical forms, or transduced. The transduction process often comprises many steps. The signal is often amplified before evoking a response. Feedback pathways regulate the entire signaling process.

5 Common second messengers Second messengers are intracellular molecules that change in concentration in response to environmental signals. That change in concentration conveys information inside the cell.

6 13.2 Receptor Proteins Transmit Information into the Cell There are three major classes of membrane receptors: 1. Seven transmembrane (7TM) receptors associated with heterotrimeric G proteins. G protein coupled receptors (GPRC). 2. Dimeric membrane receptors that recruit protein kinases. 3. Dimeric protein receptors that are protein kinases.

7 Seven transmembranehelix (7TM) receptors mediate a host of biological functions by responding to a variety of signal molecules (ligands), including hormones, tastants*, and even photons. The binding of a ligand outside the cell induces a structural or conformational change in the receptor that can be detected inside the cell. *Tastants are tasteprovoking chemical molecules that are dissolved in ingested liquids or saliva. Tastants stimulate the sense of taste.

8 The structure of 7TM receptors The β adrenergic receptor is activated by binding to epinephrine, also called adrenaline. Upon binding of epinephrine, the cytoplasmic aspect to the β adrenergic receptor activates a heterotrimeric G protein. The unactivated G protein is a heterotrimer consisting of an α subunit, bound to GDP, and β and γ subunits. (A)

9 Upon activation by the receptor, the α subunit dissociates from the βγ dimer and exchanges GDP for GTP. The GTP bound α subunit transmits the signal to other cellular components. Because 7TM receptors are always associated with G proteins, they are often called G protein coupled receptors (GPCR). In the case of the β adrenergic receptor signal transduction pathway, the activated G protein, termed G αs, stimulates the integral membrane enzyme, adenylate cyclase. Activation of the cyclase leads to the synthesis of the second message, cyclic adenosine monophosphate (camp).

10 The activation of protein kinase A by a G protein pathway Adenylate cyclase activation βγ dimer acts as inhibitor for subunit two large intracellular domains Hormone binding to a 7TM receptor initiates a signaltransduction pathway that acts through a G protein and camp to activate protein kinase A.

11 Cyclic AMP activates protein kinase A. Protein kinase A consists of two pairs of subunits: 2 catalytic (C) subunits and 2 regulatory (R) subunits. Binding of camp by the regulatory (R) subunits dissociates these subunits from the complex, resulting in activation of the 2 C subunits. The activated C subunits continue the epinephrine signal transduction pathway by phosphorylating protein targets that alter physiological functions of the cell. The regulation of protein kinase A. The binding of four molecules of camp activates protein kinase A by dissociating the inhibited holoenzyme (R 2 C 2 ) into a regulatory subunit (R 2 ) and two catalytically active subunits (C)

12 The ways by which the epinephrine imitated pathway is shut down 1. G α has inherent GTPase activity that cleaves the bound GTP to GDP. The G α bound to GDP spontaneously reassociates with the βγ subunits, terminating the activity of the G protein. 2. Cyclic AMP phosphodiesterase converts camp to AMP, which does not activate protein kinase A. 3. Epinephrine β adrenergic receptor interaction is reversible. Once the concentration of epinephrine falls, the receptor will no longer be active. Resetting G. On hydrolysis of the bound GTP by the intrinsic GTPase activity of G α, G α reassociates with the βγ dimer to form the heterotrimeric G protein, thereby terminating the activation of adenylate cyclase.

13 Excess cortisol secretion results in Cushing syndrome, a constellation of diseases with a variety of symptoms including muscle weakness, thin, easily bruised skin and osteoporosis. In some cases, the syndrome is caused by a constitutively active protein kinase A. In these patients, the catalytic subunit does not bind the regulatory subunit and is thus never inhibited. Cholera is an acute bacterial disease that produces life threatening diarrhea. Choleragen, the bacterial toxin, modifies a G αs protein such that it is trapped in the active GTP bound form. The net result is a loss of NaCl and water into the intestine. Pertussis toxin, the cause of whopping cough, also modifies a G protein. In the case of pertussis toxin, the G protein, G αi, is trapped in the inactive form. G αi, which normally inhibits a host of biochemical targets, is thus rendered inactive.

14 Death s dispensary. An 1866 cartoon illustrating that contaminated water is a frequent source of cholera infection

15 Some G protein coupled receptors activate the phosphoinositide pathway. This pathway involves the G αq protein as a component of the trimeric G protein complex. G αq activates phospholipase C, which cleaves the membrane lipid phosphatidylinositol bisphosphate into two second messengers: (a) inositol 1, 4, 5 trisphosphate (IP 3 ) and (b) diacylglycerol (DAG). IP 3 binds to the IP 3 gated channel (IP 3 receptor) in the endoplasmic reticulum, allowing an influx of Ca 2+ ions into the cytoplasm. The Ca 2+ ions regulate a host of cellular functions. DAG, in conjunction with Ca 2+, activates protein kinase C, a serine/threonine kinase.

16 The phospholipase C reaction Phospholipase C cleaves the membrane lipid phosphatidylinositol 4,5 bisphosphate into two second messengers: diacylglycerol, which remains in the membrane, and inositol 1,4,5 trisphosphate, which diffuses away from the membrane.

17 The cleavage of phosphatidylinositol 4,5 bisphosphate (PIP 2 ) into diacylglycerol (DAG) and inositol 1,4,5 trisphosphate (IP 3 ) results in the release of calcium ions (owing to the opening of the IP 3 receptor ion channels) and the activation of protein kinase C (owing to the binding of protein kinase C to free DAG in the membrane). Calcium ions bind to protein kinase C and help facilitate its activation. The phosphoinositide cascade

18 Human growth hormone receptor is a monomeric integral membrane protein with an extracellular and an intracellular domain joined by an intramembrane α helix. Upon hormone binding, the receptor dimerizes. Sultan Kosen

19 Dimerization of the extracellular domains of the receptor brings together the intracellular domains, which are associated with Janus kinase 2 (JAK2). Each JAK phosphorylates its partner on a tyrosine reside, activating the two kinases. The activated kinases then phosphorylate other targets, including a regulator of gene expression called signal transducer and activator of transcription 5 (STAT5). STAT5 further propagates the signal by altering gene expression. The cross phosphorylation of two molecules of JAK2 induced by receptor dimerization.

20 Some growth factors and hormone receptors, such as the epidermal growth factor and insulin, bind to receptors that are tyrosine kinases, called receptor tyrosine kinases (RTK). Upon growth factor or hormone binding, these receptor form dimers. Receptor dimerization leads to cross phosphorylation and activation of the two intracellular kinase domains. The phosphorylated kinases form docking platforms for other components of the signal transduction pathway. Mutations in these receptors in humans cause a variety pathologies. The EGF signaling pathway

21 A key component of the EGF pathway, as well as other signal transduction pathways, is the protein Ras. Ras is a member of the family of signal proteins called small G proteins or small GTPases. The small G proteins are monomeric. Members of this family control a variety of cellular processes. Like the G α protein, Ras is active when bound to GTP and inactive when bound to GDP. Ras also has inherent GTPase activity, to control signal duration. Other proteins function to modulate the GTPase activity of Ras.

22 The polypeptide hormone insulin is secreted when the blood is rich in glucose. Insulin is the biochemical signal for the fed state. The insulin receptor is a receptor tyrosine kinase Insulin structure. Notice that insulin consists of two chains (shown in blue and yellow) linked by two interchain disulfide bonds. The chain (blue) also has an intrachain disulfide bond. The insulin receptor. The receptor consists of two units, each of which consists of an α subunit and a β subunit linked by a disulfide bond. The α subunit lies outside the cell and two α subunits come together to form a binding site for insulin. Each β subunit lies primarily inside the cell and includes a protein kinase domain.

23 13.4 Metabolism in Context: Insulin Signaling Regulates Metabolism The activated kinase of the insulin receptor phosphorylates insulinreceptor substrates (IRSs). The phosphorylated IRSs are adaptor proteins to convey the insulin signal. Phosphoinositide 3 kinase binds IRS and then phosphorylates phosphatidylinositol 4,5 bisphophate (PIP 2 ) to form phosphatidylinositol 3,4,5 trisphosphate (PIP 3 ). PIP 3 activates PIP 3 dependent kinase, which in turn, phosphorylates and activates the kinase AKT. AKT phosphorylates glucose transporter (GLUT4), increasing glucose uptake by the cells, as well a enzymes that convert glucose into glycogen.

24 Insulin Signaling Pathway The action of a lipid kinase in insulin signaling PI3K Protein phosphatases remove phosphates from the activated proteins in the insulin signal transduction pathway, terminating the insulin signal. Lipid phosphatases such as PTEN contribute to signal termination by converting PIP 3 into PIP 2.

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26 Ca 2+ is an important second messenger in eukaryotic signal transduction pathways. The protein calmodulin is a common Ca 2+ sensor. Calmodulin, with four Ca 2+ binding sites called EF hands, is activated upon binding Ca 2+. The Ca 2+ calmodulin complex activates a variety of biochemical targets, including pumps, such as the plasma membrane Ca 2+ ATPase, and the calmodulin dependent protein kinase (CaM kinase). Recall the Ca 2+ also plays a role in the phosphoinositide cascade.

27 Genes that control cell growth are called proto oncogenes. A mutated proto oncogene leading to unrestrained growth is called an oncogene. Genes that normally restrain cancer are called tumor suppressor genes. Chronic myelogenous leukemia (CML) can be caused by overexpression of a tyrosine kinase. A chromosomal translocation results in a fusion protein, bcr abl, which results in overexpression of the kinase activity of abl. CML can be treated by kinase inhibitors that specifically target bcr abl.