Principles of cell signaling - UT Southwestern

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39057_ch14_cellbio.qxd 8/28/06 5:11 PM Page 606 to catalyze protein phosphorylation and dephosphorylation; using adenylyl cyclase to create cAMP while using phosphodiesterases to hydrolyze it or anion transporters to pump it out of the cell; or using GTP/GDP exchange factors (GEFs) to activate G proteins and GTPaseactivating proteins (GAPs) to deactivate them. Depending on stoichiometry and detailed mechanism, these strategies can convey either additive or nonadditive inputs while maintaining fine control over the kinetics of activation and deactivation of a signaling pathway. The use of distinct reactions for activation and deactivation is analogous to the use of distinct anabolic and catabolic enzymes in reversible metabolic pathways. 14.13 Cellular signaling uses both allostery and covalent modification Key concepts • Allostery refers to the ability of a molecule to alter the conformation of a target protein when it binds noncovalently to that protein. • Modification of a protein’s chemical structure is also frequently used to regulate its activity. Cellular signaling uses almost every imaginable mechanism for regulating the activities of intracellular proteins, but most can be described as either allosteric or covalent. Individual signaling proteins typically respond to multiple allosteric and covalent inputs. Allostery refers to the ability of a molecule to alter the conformation of a target protein when it binds noncovalently to that protein. Because a protein’s activity reflects its conformation, the binding of any molecule that alters conformation can change the target protein’s activity. Any molecule can have allosteric effects: protons or Ca2+, small organic molecules, or other proteins. Allosteric regulation can be both inhibitory or stimulatory. Covalent modification of a protein’s chemical structure is also frequently used to regulate its activity. The change in the protein’s chemical structure alters its conformation and, thus, its activity. Most regulatory covalent modification is reversible. The classic and most common regulatory covalent event is phosphorylation, in which a phosphoryl group is transferred from ATP to the protein, most often to the hydroxyl group of serine (Ser), threonine(Thr), or tyrosine (Tyr). Enzymes that phosphorylate proteins are known as protein kinases. Their actions are opposed by phosphoprotein phosphatases, which catalyze the hydrolysis of the phosphoryl group to yield free phosphate and restore the unmodified hydroxyl residue. Other forms of covalent modification are also common and will be addressed throughout the chapter. 14.14 Second messengers provide readily diffusible pathways for information transfer Key concepts • Second messengers can propagate signals between proteins that are at a distance. • cAMP and Ca2+ are widely used second messengers. Signaling pathways make use of both proteins and small molecules according to their distinctive attributes. A small molecule used as an intracellular signal, or second messenger, has a number of advantages over a protein as a signaling intermediary. Small molecules can be synthesized and destroyed quickly. Because they can be made readily, they can act at high concentrations so that their affinities for target proteins can be low. Low affinity permits rapid dissociation, such that their signals can be terminated promptly when free second messenger molecules are destroyed or sequestered. Because second messengers are small, they also can diffuse quickly within the cell, although many cells have developed mechanisms to spatially restrict such diffusion. Second messengers are, thus, superior to proteins in mediating fast responses, particularly at a distance. Second messengers are also useful when signals have to be addressed to large numbers of target proteins simultaneously. These advantages often overcome their lack of catalytic activity and their inability to bind multiple molecules simultaneously. FIGURE 14.14 lists intracellular second messengers developed through evolution. This number is surprisingly low. Several are nucleotides synthesized from major metabolic nucleotide precursors. They include cAMP, cyclic GMP, ppGppp, and cyclic ADP-ribose. Other soluble second messengers include a sugar phosphate, inositol-1,4,5-trisphosphate (IP 3 ), a divalent metal ion Ca2+, and a free radical gas nitric oxide (NO • ). Lipid second messengers include diacylglycerol and phosphatidylinositol-3,4,5-trisphosphate, 606 CHAPTER 14 Principles of cell signaling
39057_ch14_cellbio.qxd 8/28/06 5:11 PM Page 607 Second messenger 3':5'-cyclic AMP (cAMP) RNA polymerase Magic spot (ppGpp, ppGppp) ObgE transcription arrest detector Cyclic di-GMP phosphodiesterase Inositol-1,3,5- trisphosphate (IP 3 ) Diacylglycerol (DAG) Phosphatidyl- inositol-4,5- bisphosphate (PIP 2 ) 3':5'-Cyclic GMP (cGMP) Cyclic ADP-ribose Nitric oxide (NO. ) Ca2+ Cyclic diguanosinemonophosphate Phosphatidyl- inositol-3,4,5- trisphosphate Adenylyl Protein kinase A cyclase Bacterial transcription factors Cation channel Cyclic nucleotide phosphodiesterase Rap GDP/GTP exchange factor (Epac) IP 3 -gated Ca 2+ channel Protein kinase C Trp cation channel Ion channel Transporters Protein kinase G Ca2+ channel Various two component system proteins Guanylyl cyclase Numerous calmodulin Akt (protein kinase B) Second messengers Targets Other PH domains/proteins Synthesis/ Release Rel1A SpoT Cation channel Cyclic nucleotide phosphodiesterase Phospholipase C Phospholipase C PIP 5-kinase Guanylyl cyclase ADP-ribose cyclase Diguanylate cyclase PI 3-kinase GTP PIP 2 PIP 2 PI-4-P GTP NAD GTP Stored Ca2+ PIP 2 phosphatidylinositol-4,5-diphosphate, sphingosine-1-phosphate and phosphatidic acid. The first signaling compound to be described as a second messenger was cAMP. The name arose because cAMP is synthesized in animal cells as a second, intracellular signal in response to numerous extracellular hormones, the first messengers in the pathway. cAMP is used by prokaryotes, fungi, and animals to convey information to a variety of regulatory proteins. (Its occurrence in higher plants has still not been proved.) Adenylyl cyclases, the enzymes that synthesize cAMP from ATP, are regulated in various ways depending on the organism in which they occur. In animals, adenylyl cyclase is an integral protein of the plasma membrane whose multiple isoforms are stimulated by diverse agents (see Figure 14.13). In animal cells, adenylyl cyclase is generally stimulated by G s , which was originally discovered as an adenylyl cyclase regulator. Some fungal adenylyl cyclases are also stimulated by G proteins. Bacterial cyclases are far more diverse in their regulation. cAMP is removed from cells in two ways. It may be extruded from cells by an ATP-driven anion pump but is more often hydrolyzed to 5′- AMP by members of the cyclic nucleotide phosphodiesterase family, a large group of proteins that are themselves under multiple regulatory controls. The prototypical downstream regulator for cAMP in animals is the cAMP-dependent protein kinase, but a bacterial cAMP-regulated transcription factor was discovered shortly thereafter, and other effectors are now known (Figure 14.14). The cAMP system remains the prototypical eukaryotic signaling pathway in that its components exemplify almost all of the recognized varieties of signaling molecules and their interactions: hormone, receptor, G protein, adenylyl cyclase, protein kinase, phosphodiesterase, and extrusion pump. The second messenger-stimulated protein kinase PKA is a tetramer composed of two catalytic (C) subunits and two regulatory (R) subunits, as illustrated in FIGURE 14.15. The R subunit binds to the catalytic subunit in the substratebinding region, maintaining C in an inhibited state. Each R subunit binds two molecules of cAMP, four cAMP molecules per PKA holoenzyme. When these sites are filled, the R subunit dimer dissociates rapidly, leaving two free catalytic subunits with high activity. The difference in affinity of R for C in the presence and absence of cAMP is ~10,000-fold. The strongly cooperative binding of cAMP generates a very steep activation curve with an apparent threshold below which no significant activation of PKA occurs, as illustrated in Figure 14.15. PKA activity, thus, increases dramatically over a narrow range of cAMP concentrations. PKA is also regulated Precursor Removal Phosphodiesterase ATP Organic anion transporter SpoTcatalyzed hydrolysis Phosphatase Diacylglycerol kinase Diacylglycerol lipase Phospholipase C Phosphatase Phosphodiesterase Hydrolysis NO . synthase arginine Reduction Release from storage organelles or plasma membrane channels Reuptake and extrusion pumps Phosphatase FIGURE 14.14 Major second messengers, some of the proteins that they regulate, their sources and their disposition. 14.14 Second messengers provide readily diffusible pathways for information transfer 607
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Principles of cell signaling - UT Southwestern

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