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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 20.6Second Messengers

In this section we consider in more detail the role of various second messengers in intracellular signaling (see Figure 20-4). As we saw earlier, hormone stimulation of Gs protein –  coupled receptors leads to activation of adenylyl cyclase and synthesis of cAMP. The resulting rise in the cAMP level produces markedly different effects in various types of cells. So far as is known, cAMP does not function in signaling pathways initiated by RTKs. However, other second messengers function in signaling pathways initiated by both GPCRs and RTKs.

cAMP and Other Second Messengers Activate Specific Protein Kinases

The diverse effects of cAMP are mediated through the action of cAMP-dependent protein kinases (cAPKs). These enzymes, also referred to as protein kinases A (PKAs), mod-ify the activities of target enzymes in various cell types by phosphorylating specific serine and threonine residues. Phosphorylation of many enzymes increases their catalytic activity, whereas phosphorylation of others decreases their activity.

The cAMP-dependent protein kinases are tetramers, consisting of two regulatory (R) subunits and two catalytic (C) subunits. In the tetrameric form, cAPK is enzymatically inactive. Binding of cAMP to the R subunits causes dissociation of the two C subunits, which then can phosphorylate specific acceptor proteins (see Figure 3-27a). Each R subunit has two distinct sites for binding cAMP, called A and B, each located in a different domain in the R subunit. Binding of cAMP to site B induces a conformational change that unmasks site A. Binding of cAMP to site A, in turn, leads to release of the C subunits. The binding of two cAMP molecules by a R subunit occurs in a cooperative fashion; that is, binding of the first cAMP molecule lowers the KD for binding of the second. Thus, small changes in the level of cytosolic cAMP can cause proportionately large changes in the amount of dissociated C subunits and, hence, in the activity of a cAPK.

As discussed later, other second messengers also activate protein kinases via a similar mechanism: in the absence of the second messenger, the kinase shows low activity; binding of the second messenger increases the kinase activity. Each type of second messenger – dependent kinase is inhibited by the binding of a peptide sequence, called a pseudosubstrate, to the active site. The pseudosubstrate sequence can be part of a distinct regulatory subunit, as in cAPKs, or it can be part of a regulatory domain within the same polypeptide chain that contains the active site. Binding of a second messenger to the inactive kinase induces a conformational change that leads to release of the part of the regulatory domain bound to the kinase active site or to dissociation of the regulatory subunit; in both cases, the active sites of the kinase are unmasked and their enzymatic activities activated.

The catalytic subunits of some second messenger –  dependent protein kinases are further activated by phosphorylation of specific residues within the phosphorylation lip. As in the case of MAP kinases, when the lip is unphosphorylated, it inhibits binding of ATP or protein substrates, or both. Phosphorylation induces a conformational change leading to effective binding of both ATP and protein substrates. X-ray crystallographic analyses have revealed that many different protein kinases share a common threedimensional structure similar to that of MAP kinase shown in Figure 20-30.

cAPKs Activated by Epinephrine Stimulation Regulate Glycogen Metabolism

The cAMP-dependent protein kinases (cAPKs) induce many effects depending on the particular substrate proteins that they phosphorylate. The first cAMP-mediated cellular response to be discovered — the release of glucose from glycogen — occurs in muscle and liver cells stimulated by epinephrine or agonists of β-adrenergic receptors.

Glycogen, a large glucose polymer, is the major storage form of glucose in animals. Like most polymers, glycogen is synthesized by one set of enzymes and degraded by another. Three enzymes convert glucose into uridine diphosphoglucose (UDP-glucose), the primary intermediate in glycogen synthesis. The glucose residue of UDP-glucose is transferred by glycogen synthase to the free hydroxyl group on carbon 4 of a glucose residue at the end of a growing glycogen chain (Figure 20-34a). Degradation of glycogen involves the stepwise removal of glucose residues from the same end by a phosphorolysis reaction, catalyzed by glycogen phosphorylase, yielding glucose 1-phospate (Figure 20-34b). In both muscle and liver cells, glucose 1-phosphate produced from glycogen is converted by phosphoglucomutase to glucose 6-phosphate. In muscle cells, this metabolite enters the glycolytic pathway (see Figure 16-3) and is metabolized to generate ATP for use in powering muscle contraction. Unlike muscle cells, liver cells contain a phosphatase that hydrolyzes glucose 6-phospate to glucose. Thus glycogen stores in the liver are primarily broken down to free glucose, which is immediately released into the blood and transported to other tissues, particularly the muscles and brain.

Figure 20-34. Synthesis and degradation of glycogen.

Figure 20-34

Synthesis and degradation of glycogen. (a) Incorporation of glucose from UDP-glucose into glycogen is catalyzed by glycogen synthase. (b) Removal of glucose units from glycogen is catalyzed by glycogen phosphorylase. Because two different enzymes catalyze (more...)

In liver and muscle cells, the epinephrine-stimulated elevation in the cAMP level enhances the conversion of glycogen to glucose 1-phosphate in two ways: by inhibiting glycogen synthesis and by stimulating glycogen degradation, as outlined in Figure 20-35a. The entire process is reversed when epinephrine is removed and the level of cAMP drops (Figure 20-35b). This reversal is mediated by phosphoprotein phosphatase, which removes the phosphate residues from the inactive form of glycogen synthase, thereby activating it, and from the active forms of glycogen phosphorylase kinase and glycogen phosphorylase, thereby inactivating them. The activity of phosphoprotein phosphatase also is regulated by cAMP, although indirectly. At high cAMP levels, cAPK phosphorylates an inhibitor of phosphoprotein phosphatase; the phosphorylated inhibitor then binds to phosphoprotein phosphatase, inhibiting its activity (Figure 20-36). At low cAMP levels, the inhibitor is not phosphorylated and phosphoprotein phosphatase is active. As a result, the synthesis of glycogen by glycogen synthase is enhanced and the degradation of glycogen by glycogen phosphorylase is inhibited.

Figure 20-35. Regulation of glycogen breakdown and synthesis by cAMP in liver and muscle cells.

Figure 20-35

Regulation of glycogen breakdown and synthesis by cAMP in liver and muscle cells. Active enzymes are highlighted in darker shades; inactive forms, in lighter shades. (a) An increase in cytosolic cAMP activates a cAMP-dependent protein kinase (cAPK) that (more...)

Figure 20-36. Regulation of phosphoprotein phosphatase activity by cAMP is mediated by an inhibitor protein.

Figure 20-36

Regulation of phosphoprotein phosphatase activity by cAMP is mediated by an inhibitor protein. At high levels of cAMP, a cAMP-dependent protein kinase (cAPK) phosphorylates an inhibitor protein (IP), which then binds to phosphoprotein phosphatase (PP), (more...)

The cAMP-dependent switch that regulates glycogen metabolism thus exhibits dual regulation: activation of the enzymes catalyzing glycogen degradation and inhibition of enzymes promoting glycogen synthesis. The coordinate regulation of stimulatory and inhibitory pathways provides an efficient mechanism for operating switches and is a common phenomenon in regulatory biology.

Kinase Cascades Permit Multienzyme Regulation and Amplify Hormone Signals

The cAMP-mediated stimulation of glycogen breakdown illustrates two important properties of a cascade, a series of reactions in which the enzyme catalyzing one step is activated (or inhibited) by the product of a previous step. Although such a cascade may seem overcomplicated, it has at least two advantages for the cell.

First, a cascade allows an entire group of enzyme-catalyzed reactions to be regulated by a single type of molecule. As we have seen, the three enzymes in the glycogenolysis cascade — cAMP-dependent protein kinase (cAPK), glycogen phosphorylase kinase (GPK), and glycogen phosphorylase (GP) — are all regulated, directly or indirectly, by cAMP (see Figure 20-35a). Other metabolic pathways also are regulated by hormone-induced cascades, some mediated by cAMP and some by other second messengers.

Second, a cascade provides a huge amplification of an initially small signal (Figure 20-37). For example, blood levels of epinephrine as low as 10−10 M can stimulate liver gly-cogenolysis and release of glucose, resulting in an increase of blood glucose levels by as much as 50 percent. An epinephrine stimulus of this magnitude generates an intracellular cAMP concentration of 10−6 M, an amplification of 104. Because three more catalytic steps precede the release of glucose, another 104 amplification can occur. In striated muscle, the concentrations of the three successive enzymes in the glycogenolytic cascade (i.e., cAPK, GPK, and GP) are in a 1:10:240 ratio, which dramatically illustrates the amplification of the effects of epinephrine and cAMP.

Figure 20-37. Intracellular transduction of an extracellular signal via a cascade of sequential reactions produces a large amplification of the signal.

Figure 20-37

Intracellular transduction of an extracellular signal via a cascade of sequential reactions produces a large amplification of the signal. In this example, binding of a single epinephrine molecule to one receptor molecule induces synthesis of a large number (more...)

Cellular Responses to cAMP Vary among Different Cell Types

The effects of cAMP on the synthesis and degradation of glycogen are confined mainly to liver and muscle cells, which store glycogen. However, cAMP also mediates the intracellular responses of many other cells to a variety of hormones that stimulate Gs protein – coupled receptors. In virtually all eukaryotic cells studied, the action of cAMP appears to be mediated by one or more cAPKs, but the nature of the metabolic response varies widely among different cells, as indicated in Table 20-3. The effects of cAMP on a given cell type depend, in part, on the specificity of the particular cAPK and on the cAPK substrates that it expresses.

Table 20-3. Metabolic Responses to Hormone-Induced Rise in cAMP in Various Tissues.

Table 20-3

Metabolic Responses to Hormone-Induced Rise in cAMP in Various Tissues.

For instance, in adipocytes, elevation of cAMP activates a cAPK that stimulates the production of fatty acids by controlling the activity of lipases. These fatty acids are released and taken up as an energy source by cells in other tissues such as the kidney, heart, and muscles. Likewise, the hormone- induced stimulation of cAPK in ovarian cells promotes the synthesis of estradiol and progesterone, two steroid hormones crucial to the development of female sex characteristics. cAMP acting through cAMP-dependent protein kinases also plays a critical role in mediating the communication between cells critical for the formation of specific tissues during development. The cAPK in nerve cells modulates the activity of specific ion channels important in short-term learning (Chapter 21) and can produce long-term changes in neurons (memory) through changes in the activity of specific transcription factors.

Anchoring Proteins Localize Effects of cAMP to Specific Subcellular Regions

In many cell types, a rise in the cAMP level may produce a response that is required in one part of the cell but is unwanted, perhaps deleterious, in another part. For instance, in migrating cells specific cAMP-dependent signals help regulate membrane cytoskeletal dynamics underlying motility at the leading edge of the cell, but similar cytoskeletal changes in other parts of the cell may be harmful. Recent biochemical and cell biological experiments have identified a family of anchoring proteins that localize inactive cAPKs to specific subcellular locations, thereby restricting cAMP-dependent responses to these locations.

This family of proteins, referred to as cAMP kinase –  associated proteins (AKAPs), have a bipartite structure with one region conferring a specific subcellular location and another that binds to the regulatory subunit of cAPKs. One such protein, AKAP250, is localized to filopodia and presumably functions to integrate cAMP-dependent signals regulating the structure of the actin-based cytoskeleton (see Figure 18-1). Specific anchoring proteins may also function to localize other signaling proteins including other kinases and phosphatases, and thus may play an important role in integrating information from multiple signaling pathways to provide local control of specific cellular processes.

Modification of a Common Phospholipid Precursor Generates Several Second Messengers

In the rest of this section, we briefly discuss several other second messengers and the mechanisms by which they regulate various cellular activities. A number of these are derived from phosphatidylinositol (PI). The inositol group in this phospholipid, which extends into the cytosol adjacent to the membrane, can be [reversibly] phosphorylated at various positions by the combined actions of various kinases and phosphatases, yielding several different phosphoinositides, which are membrane bound (Figure 20-38). The levels of PIs in cells are dynamically regulated by extracellular signals. For instance, in unstimulated cells the levels of PIs phosphorylated in the D3 position (PI-3Ps) are very low. In response to some signals (e.g., PDGF), there is an acute rise in PIs phosphorylated at this position through the activation of PI-3 kinase. Both GPCRs and RTKs stimulate the activity of PI-3 kinases. Some PIs [e.g., PI(3,4,5)P3] are rapidly regulated in response to signal, and others are not [e.g., PI(3)P]. As we discuss later in this chapter, PIs act as membrane docking sites for signaling molecules and also, in some cases, stimulate catalytic activity. Proteins bind to PIs through a PH domain. Different PH domains show different phosphoinositide binding specificities.

Figure 20-38. Several second messengers are derived from phosphatidylinositol (PI).

Figure 20-38

Several second messengers are derived from phosphatidylinositol (PI). (a) Pathway for synthesis of DAG and IP3, two important second messengers. Each membrane-bound PI kinase places a phosphate on a specific hydroxyl group on the inositol ring, producing (more...)

Phosphoinositides can be cleaved by the membraneassociated enzyme phospholipase C (PLC) to generate yet other second messengers. These cleavage reactions produce 1,2-diacylglycerol (DAG), a lipophilic molecule that remains linked to the membrane, and free phosphorylated inositols, which can diffuse into the cytosol. The main pathway shown in Figure 20-38a generates DAG and inositol 1,4,5-trisphos-phate (IP3). Signaling pathways involving any of these second messengers sometimes are referred to as inositol-lipid pathways.

Activation of the β isoform of phospholipase C (PLCβ) is induced by binding of hormones to GPCRs containing either a Go or Gq α subunit. Pertussis toxin locks G, but not G, in the inactive, GDP-bound form, preventing activation of PLCβ even in the presence of hormone. The effect of pertussin toxin on G thus is opposite to the effect of cholera toxin on G discussed earlier (see Figure 20-17). Certain activated RTKs can increase the activity of the γ isoform of PLC. Thus hormone-induced stimulation of PLC activity and subsequent generation of DAG and phosphorylated inositols is mediated by both GPCRs and RTKs.

Hormone-Induced Release of Ca2+ from the ER Is Mediated by IP3

Most intracellular Ca2+ ions are sequestered in the mitochondria and endoplasmic reticulum (ER) or other vesicles. Cells employ various mechanisms for regulating the concentration of Ca2+ ions free in the cytosol, which usually is kept below 0.2 μM. Ca2+ ATPases pump cytosolic Ca2+ ions across the plasma membrane to the cell exterior or into the lumens of the endoplasmic reticulum or other intracellular vesicles that store Ca2+ ions (see Figure 15-11). As we discuss below, a small rise in cytosolic Ca2+ induces a variety of cellular responses.

Binding of many hormones to their cell-surface receptors on liver, fat, and other cells induces an elevation in cytosolic Ca2+ even when Ca2+ ions are absent from the surrounding medium. In this situation, Ca2+ is released into the cytosol from the endoplasmic reticulum (ER) and other intracellular vesicles. The mechanism by which a hormone-receptor signal on the cell surface is transduced to the ER became clear in the early 1980s, when it was shown that a rise in the level of cytosolic Ca2+ often is preceded by an increase in IP3.

Since it is water soluble, IP3 can diffuse within the cytosol carrying a hormone signal from the cell surface to the ER surface. Here, IP3 binds to a Ca2+-channel protein composed of four identical subunits, each containing an IP3-binding site in the large N-terminal cytosolic domain. IP3 binding induces opening of the channel allowing Ca2+ ions to exit from the ER into the cytosol (Figure 20-39). The resulting rise in the cytosolic Ca2+ level is only transient, however, because Ca2+ ATPases located in the plasma membrane and ER membrane actively pump Ca2+ from the cytosol to the cell exterior and ER lumen, respectively. Furthermore, within a second of its generation, IP3 is hydrolyzed to inositol 1,4-bisphosphate, which does not stimulate Ca2+ release from the ER.

Figure 20-39. Elevation of cytosolic Ca2+ via the inositol-lipid signaling pathway.

Figure 20-39

Elevation of cytosolic Ca2+ via the inositol-lipid signaling pathway. This pathway can be triggered by ligand binding to RTKs or to GPCRs, as illustrated here. Binding of a hormone to its receptor leads to activation of the G protein (Gq), which in turn (more...)

Without some means for replenishing depleted stores of intracellular Ca2+, a cell would soon be unable to increase the cytosolic Ca2+ level. Elegant patch-clamping studies have revealed that certain plasma-membrane Ca2+ channels, called store-operated channels (SOCs) open in response to depletion of intracellular Ca2+ stores (see Figure 20-39). Although the specific signal that promotes opening of SOCs has not yet been identified, opening of these channels is critical to cellular responses induced by elevated cytosolic Ca2+.

When various phosphorylated inositols found in cells are added to preparations of ER vesicles, only IP3 causes release of Ca2+ ions from the vesicles. This simple experiment demonstrates the specificity of the IP3 effect. In addition, not all cells respond identically to IP3. This differential sensitivity may reflect expression of different isoforms of the IP3-sensitive Ca2+ channel in the ER membrane and/or variation in the Ca2+ content of the ER itself. Because of this variability, different types of cells may exhibit very different responses to the same extracellular signal.

Opening of the IP3-sensitive Ca2+ channels is potentiated by cytosolic Ca2+ ions, which increases the affinity of the receptor for IP3, resulting in greater release of stored Ca2+. High concentrations of cytosolic Ca2+, however, inhibit IP3-induced release of Ca2+ from intracellular stores by decreasing the affinity of the receptor for IP3. The complex regulation of the IP3 receptor in ER membranes can lead to rapid oscillations in the cytosolic Ca2+ level when the IP3 pathway in cells is stimulated. For example, stimulation of hormone-secreting cells in the pituitary by luteinizing hormone – releasing factor causes rapid, repeated spikes in the cytosolic Ca2+ level; each spike is associated with a burst in secretion of luteinizing hormone (LH). The purpose of the fluctuations of Ca2+, rather than a sustained rise in cytosolic Ca2+, is not understood. One possibility is that a sustained rise in Ca2+ might be toxic to cells.

Opening of Ryanodine Receptors Releases Ca2+ Stores in Muscle and Nerve Cells

In addition to IP3-sensitive Ca2+ channels, muscle cells and neurons possess other Ca2+ channels called ryanodine receptors (RYRs), because of their sensitivity to the plant alkaloid ryanodine. In skeletal muscle cells, these receptors are located in the membrane of the sarcoplasmic reticulum (SR) and associate with the cytoplasmic domain of the dihydropyridine receptor, a voltage-sensing protein in the plasma membrane. A change in potential across the plasma membrane induces a conformational change in the RYR, so that Ca2+ ions are released from the sarcoplasmic reticulum into the cytosol (Figure 20-40).

Figure 20-40. Release of Ca2+ stores mediated by ryanodine receptors (RYRs) in skeletal muscle.

Figure 20-40

Release of Ca2+ stores mediated by ryanodine receptors (RYRs) in skeletal muscle. Voltagesensing dihydropyridine receptors in the plasma membrane contact ryanodine receptors located in the membrane of the sarcoplasmic reticulum. In response to a change (more...)

While the IP3-induced increase in Ca2+ requires activation of SOCs in the plasma membrane to sustain Ca2+-mediated cellular responses, release of Ca2+ from internal stores in muscle cells is sufficient to induce muscle contraction. Like the IP3 receptor, the ryanodine receptor is stimulated by a small rise in cytosolic Ca2+, although it is not inhibited at high Ca2+ concentrations. Thus, Ca2+ feeds forward to activate RYRs. Repolarization of the muscle membrane closes the RYR channels and renders them insensitive to cytosolic Ca2+.

In smooth muscle cells, RYRs are found within the SR membrane; in neurons, these receptors are located in the ER membrane. In both cases, the RYRs are closely associated with Ca2+ channels in the plasma membrane. It is unclear how the opening of these Ca2+ channels in response to a voltage change or neurotransmitter leads to opening of RYRs. Recent studies indicate that a newly identified intracellular signaling molecule, cyclic ADP ribose, promotes release of Ca2+ via RYRs from internal stores in neurons.

The precise spatial control of Ca2+ release from intracellular stores also plays an important role in cell physiology. For instance, in arterial smooth muscle cells, the local release of Ca2+ from the regions of the sarcoplasmic reticulum in close apposition to the plasma membrane leads to muscle relaxation. This local release is mediated by RYRs in the SR membrane and leads to opening of K+ channels in the plasma membrane; as a result K+ flows out of the cell, hyperpolarizing it. This hyperpolarization serves to inhibit Ca2+-dependent contraction. In contrast, global release of intracellular Ca2+ throughout the muscle leads to contraction. Spatial control of Ca2+ release is also important for the control of IP3-mediated Ca2+ channels.

Ca2+-Calmodulin Complex Mediates Many Cellular Responses

Localized increases in the cytosolic level of free Ca2+ are critical to its function as a second messenger. In secretory cells, such as the insulin-producing β cells in the pancreatic islets, a rise in Ca2+ triggers the exocytosis of secretory vesicles and the release of insulin. In smooth or striated muscle cells, a rise in Ca2+ triggers contraction; in both liver and muscle cells, an increase in Ca2+ activates the degradation of glycogen to glucose 1-phosphate. As discussed already, small increases in cytosolic Ca2+ and the associated cellular responses often are preceded by a hormone-induced rise in IP3 (Table 20-4).

Table 20-4. Cellular Responses to Hormone-Induced Rise in Inositol 1,4,5-Trisphosphate (IP3) and Subsequent Rise in Cytosolic Ca2+ in Various Tissues.

Table 20-4

Cellular Responses to Hormone-Induced Rise in Inositol 1,4,5-Trisphosphate (IP3) and Subsequent Rise in Cytosolic Ca2+ in Various Tissues.

A small cytosolic protein called calmodulin, which is ubiquitous in eukaryotic cells, mediates many cellular effects of Ca2+ ions. Each calmodulin molecule binds four Ca2+ ions (Figure 20-41). Binding of Ca2+ causes calmodulin to undergo a conformational change that enables the Ca2+-calmodulin complex to bind to and activate many enzymes, such as myosin light-chain kinase, which regulates the activity of myosin. Because Ca2+ binds to calmodulin in a cooperative fashion, a small change in the level of cytosolic Ca2+ leads to a large change in the level of active calmodulin.

Figure 20-41. Structure of calmodulin, a cytosolic protein of 148 amino acids that binds Ca2+ ions.

Figure 20-41

Structure of calmodulin, a cytosolic protein of 148 amino acids that binds Ca2+ ions. (a) The backbone of the calmodulin molecule, deduced from crystallographic analysis of the Ca2+-calmodulin complex. The four bound Ca2+ ions are represented by blue (more...)

One well-studied enzyme activated by the Ca2+-calmodulin complex is cAMP phosphodiesterase; as noted earlier, this enzyme degrades cAMP and terminates its effects. This reaction thus links Ca2+ and cAMP, one of many examples in which these two second messengers interact to finetune certain aspects of cell regulation. As with other second messengers, the Ca2+-calmodulin complex also activates several protein kinases that, in turn, phosphorylate transcription factors, thereby modifying their activity and regulating gene expression.

DAG Activates Protein Kinase C, Which Regulates Many Other Proteins

After its formation by hydrolysis of PIP2 or other phosphoinositides, DAG remains associated with the membrane (see Figure 20-38). The principal function of DAG is to activate a family of plasma-membrane protein kinases collectively termed protein kinase C. In the absence of hormone stimulation, protein kinase C is present as a soluble cytosolic protein that is catalytically inactive. A rise in the cytosolic Ca2+ level causes protein kinase C to bind to the cytosolic leaflet of the plasma membrane, where it can be activated by the membrane-associated DAG. Thus activation of protein kinase C depends on both Ca2+ ions and DAG, suggesting an interaction between the two branches of the inositol-lipid signaling pathway (see Figure 20-39).

The activation of protein kinase C in different cells results in a varied array of cellular responses, indicating that it plays a key role in many aspects of cellular growth and metabolism. In liver cells, for instance, protein kinase C helps regulate glycogen metabolism by phosphorylating glycogen synthase, yielding the inactive form of this enzyme. Protein kinase C also phosphorylates various transcription factors; depending on the cell type, these induce or repress synthesis of certain mRNAs.

Synthesis of cGMP Is Induced by Both Peptide Hormones and Nitric Oxide

Although cGMP was discovered more than thirty years ago, its role in as a second messenger has long been overshadowed by that of cAMP. cGMP regulates the activity of specific protein kinases and directly binds to and regulates ion channels in rod cells of the eye (Section 21.6). Synthesis of cGMP is catalyzed by two types of guanylate cyclase: a soluble cytosolic form and a transmembrane form, which constitutes part of the cytosolic domain of the cell-surface receptors for certain peptide hormones (e.g., atrial naturitic factor). Binding of ligand to the extracellular domain of these receptors promotes the activity of the intracellular guanylate cyclase catalytic domain, leading to formation of cGMP (see Figure 20-3d, left). Soluble guanylate cyclases are activated by a gas, nitric oxide (NO). These enzymes are heterodimers and contain a bound heme molecule that interacts with both subunits (Figure 20-42a). Binding of nitric oxide to the heme leads to a conformational change in the enzyme and stimulates its catalytic activity.

Figure 20-42. cGMP mediates local signaling by nitric oxide.

Figure 20-42

cGMP mediates local signaling by nitric oxide. (a) Schematic diagram of the structure of soluble guanylate cyclase. Binding of nitric oxide to the heme group stimulates the enzyme’s catalytic activity, leading to formation of cGMP from GTP. (b) (more...)

NO synthase, acting constitutively or in response to specific signals, catalyzes the formation of nitric oxide from arginine and O2. Once formed, nitric oxide diffuses only locally through tissues and is highly labile with a half-life of from 2 to 30 seconds. It plays an important role in mediating many local cellular interactions, as exemplified by the local control of arterial smooth muscle contractility (Figure 20-42b). Release of acetylcholine from adjacent tissues promotes influx of Ca2+ into endothelial cells lining blood vessels. After Ca2+ binds to calmodulin, the resulting complex stimulates the activity of NO synthase. The nitric oxide that is formed diffuses from the endothelial cell and into neighboring smooth muscle cells where it binds to and activates soluble guanylate cyclase. The subsequent increase in cGMP then leads to muscle relaxation and dilation of the vessel. Nitric oxide also helps control communication at certain synapses in the central nervous system (Section 21.7).


  •  Second messengers activate certain protein kinases. Binding of a second messenger leads to release of regulatory subunits or domains that mask the catalytic site(s), thereby activating the kinase activity. Phosphorylation of a specific region of the catalytic domain called the phosphorylation lip further activates these protein kinases.
  •  cAMP-dependent protein kinases (cAPKs) mediate the diverse effects of cAMP in different cells. The effect of cAMP in a cell depends largely on the particular cAPK and the protein substrates that it contains.
  •  In liver and muscle cells, hormone-induced activation of cAPK sets into motion a kinase cascade that both inhibits glycogen synthesis and stimulates glycogen breakdown (see Figure 20-35).
  •  Kinase cascades triggered by cAPK and other second messenger – controlled protein kinases can regulate multiple target proteins, which function together to mediate a specific cellular response. Such cascades also act to amplify the original signal.
  •  Localization of cAPK to specific regions of the cell by anchoring proteins restricts the effects of cAMP to particular subcellular locations.
  •  Signaling through GPCRs and RTKs stimulates PI-3 kinase to generate specific phosphoinositides. Signaling proteins containing PH domains bind to different phosphoinositides.
  •  Activation of both GPCRs and RTKs activate phospholipase C, which hydrolyzes PIP2 to the second messengers IP3, which diffuses into the cytosol, and DAG, which remains membrane bound (see Figure 20-38).
  •  Hormone stimulation of the inositol-lipid pathway leads to the IP3-mediated release of Ca2+ ions from the endoplasmic reticulum and activation of protein kinase C by DAG (see Figure 20-39). A sustained rise in cytosolic Ca2+ mediated by IP3 requires opening of store-operated Ca2+ channels in the plasma membrane.
  •  In skeletal muscle cells, activation of ryanodine receptors in the SR membrane releases enough Ca2+ into the cytosol to induce contraction.
  •  Ca2+ forms a complex with a multivalent Ca2+- binding protein called calmodulin. The Ca2+-calmodulin complex regulates the activity of many different proteins, including protein kinases that in turn regulate the activity of various transcription factors.
  •  Protein kinase C is coordinately regulated by Ca2+, which recruits it to the membrane, and DAG, which activates it.
  •  cGMP is produced by cell-surface receptors with guanylate cyclase activity, which are activated by peptide hormones, and by soluble guanylate cyclase, which is activated by binding of nitric oxide.
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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21705