BIO-333 Chapter 16.1-2 WS

*Figure 16-21* A Gi protein directly couples receptor activation to the opening of K+ channels in the plasma membrane of heart pacemaker cells.

*A Gi protein directly couples receptor activation to the opening of K+ channels in the plasma membrane of heart pacemaker cells.* (A) Binding of the neurotransmitter acetylcholine to its GPCR on the heart cells results in the activation of the G protein, Gi. (B) The activated βγ complex directly opens a K+ channel in the plasma membrane, increasing its permeability to K+ and thereby making the membrane harder to activate and slowing the heart rate. (C) Inactivation of the α subunit by hydrolysis of its bound GTP returns the G protein to its inactive state, allowing the K+ channel to close.

*Figure 16-26* A rise in intracellular cyclic AMP can activate gene transcription.

*A rise in intracellular cyclic AMP can activate gene transcription.* Binding of a signal molecule to its GPCR can lead to the activation of adenylyl cyclase and a rise in the concentration of cytosolic cyclic AMP. The increase in cyclic AMP activates PKA, which then moves into the nucleus and phosphorylates specific transcription regulators. Once phosphorylated, these proteins stimulate the transcription of a whole set of target genes (Movie 16.3). This type of signaling pathway controls many processes in cells, ranging from hormone synthesis in endocrine cells to the production of proteins involved in long-term memory in the brain. Activated PKA can also phosphorylate and thereby regulate other proteins and enzymes in the cytosol (as shown in Figure 16-25).

*Figure 16-36* Activated Akt promotes cell survival

*Activated Akt promotes cell survival.* One way it does so is by phosphorylating and inactivating a protein called Bad. In its unphosphorylated state, Bad promotes apoptosis (a form of cell death) by binding to and inhibiting a protein, called Bcl2, which otherwise suppresses apoptosis. When Bad is phosphorylated by Akt, Bad releases Bcl2, which now blocks apoptosis, thereby promoting cell survival.

*Figure 16-32* Activation of an RTK stimulates the assembly of an intracellular signaling complex.

*Activation of an RTK stimulates the assembly of an intracellular signaling complex.* Typically, the binding of a signal molecule to the extracellular domain of an RT K causes two receptor molecules to associate into a dimer. The signal molecule shown here is itself a dimer and thus can physically cross-link two receptor molecules; other signal molecules induce a conformational change in the RT Ks, causing the receptors to dimerize (not shown). In either case, dimer formation brings the kinase domains of each cytosolic receptor tail into contact with the other; this activates the kinases to phosphorylate the adjacent tail on several tyrosines. Each phosphorylated tyrosine serves as a specific docking site for a different intracellular signaling protein, which then helps relay the signal to the cell's interior; these proteins contain a specialized interaction domain—in this case, a module called an SH2 domain—that recognizes and binds to specific phosphorylated tyrosines on the cytosolic tail of an activated RT K or on another intracellular signaling protein.

*Figure 16-25* Adrenaline stimulates glycogen breakdown in skeletal muscle cells.

*Adrenaline stimulates glycogen breakdown in skeletal muscle cells.* The hormone activates a GPCR, which turns on a G protein (Gs) that activates adenylyl cyclase to boost the production of cyclic AMP. The increase in cyclic AMP activates PKA, which phosphorylates and activates an enzyme called phosphorylase kinase. This kinase activates glycogen phosphorylase, the enzyme that breaks down glycogen. Because these reactions do not involve changes in gene transcription or new protein synthesis, they occur rapidly.

*Figure 16-39* Akt stimulates cells to grow in size by activating the serine/threonine kinase Tor.

*Akt stimulates cells to grow in size by activating the serine/threonine kinase Tor.* The binding of a growth factor to an RT K activates the PI-3-kinase-Akt signaling pathway (as shown in Figure 16-35). Akt then indirectly activates Tor by phosphorylating and inhibiting a protein that helps to keep Tor shut down (not shown). Tor stimulates protein synthesis and inhibits protein degradation by phosphorylating key proteins in these processes (not shown). The anticancer drug rapamycin slows cell growth by inhibiting Tor. In fact, the Tor protein derives its name from the fact that it is a target of rapamycin.

*Figure 16-18* All GPCRs possess a similar structure.

*All GPCRs possess a similar structure.* The polypeptide chain traverses the membrane as seven α helices. The cytoplasmic portions of the receptor bind to a G protein inside the cell. (A) For receptors that recognize small signal molecules, such as adrenaline or acetylcholine, the ligand usually binds deep within the plane of the membrane to a pocket that is formed by amino acids from several transmembrane segments. (B) Shown here is the structure of a GPCR that binds to adrenaline (red). Stimulation of this receptor by adrenaline makes the heart beat faster. Receptors that recognize signal molecules that are proteins usually have a large extracellular domain that, together with some of the transmembrane segments, binds the protein ligand (not shown).

*Figure 16-19* An activated GPCR activates G proteins by encouraging the α subunit to expel its GDP and pick up GTP.

*An activated GPCR activates G proteins by encouraging the α subunit to expel its GDP and pick up GTP.* (A) In the unstimulated state, the receptor and the G protein are both inactive. Although they are shown here as separate entities in the plasma membrane, in some cases at least, they are associated in a preformed complex. (B) Binding of an extracellular signal molecule to the receptor changes the conformation of the receptor, which in turn alters the conformation of the bound G protein. The alteration of the α subunit of the G protein allows it to exchange its GDP for GTP. This exchange triggers an additional conformational change that activates both the α subunit and a βγ complex, which dissociate to interact with their preferred target proteins in the plasma membrane (Movie 16.2). The receptor stays active as long as the external signal molecule is bound to it, and it can therefore catalyze the activation of many molecules of G protein. Note that both the α and γ subunits of the G protein have covalently attached lipid molecules (red) that help anchor the subunits to the plasma membrane.

*Figure 16-27* Phospholipase C activates two signaling pathways.

*Phospholipase C activates two signaling pathways.* Two small messenger molecules are produced when a membrane inositol phospholipid is hydrolyzed by activated phospholipase C. Inositol 1,4,5-trisphosphate (IP3) diffuses through the cytosol and triggers the release of Ca2+ from the ER by binding to and opening special Ca2+ channels in the ER membrane. The large electrochemical gradient for Ca2+ across this membrane causes Ca2+ to rush out of the ER and into the cytosol. Diacylglycerol remains in the plasma membrane and, together with Ca2+, helps activate the enzyme protein kinase C (PKC), which is recruited from the cytosol to the cytosolic face of the plasma membrane. PKC then phosphorylates its own set of intracellular proteins, further propagating the signal. At the start of the pathway, both the α subunit and the βγ subunit of the G protein Gq are involved in activating phospholipase C.

*Figure 16-33* RTKs activate Ras.

*RTKs activate Ras.* An adaptor protein docks on a particular phosphotyrosine on the activated receptor (the other signaling proteins that are shown bound to the receptor in Figure 16-32 are omitted for simplicity). The adaptor recruits a Ras guanine nucleotide exchange factor (Ras-GEF) that stimulates Ras to exchange its bound GDP for GTP. The activated Ras protein can now stimulate several downstream signaling pathways, one of which is shown in Figure 16-34. Note that the Ras protein contains a covalently attached lipid group (red) that helps anchor the protein to the inside of the plasma membrane.

*Figure 16-35* RTKs activate the PI-3-kinase-Akt signaling pathway.

*RTKs activate the PI-3-kinase-Akt signaling pathway.* An extracellular survival signal, such as IGF, activates an RT K, which recruits and activates PI 3-kinase. PI 3-kinase then phosphorylates an inositol phospholipid that is embedded in the cytosolic side of the plasma membrane. The resulting phosphorylated inositol phospholipid then attracts intracellular signaling proteins that have a special domain that recognizes it. One of these signaling proteins, Akt, is a protein kinase that is activated at the membrane by phosphorylation mediated by two other protein kinases (here called protein kinases 1 and 2); protein kinase 1 is also recruited by the phosphorylated lipid docking sites. Once activated, Akt is released from the plasma membrane and phosphorylates various downstream proteins on specific serines and threonines (not shown).

*Figure 16-34* Ras activates a MAP-kinase signaling module.

*Ras activates a MAP-kinase signaling module.* The Ras protein activated by the process shown in Figure 16-33 activates a three-kinase signaling module, which relays the signal. The final kinase in the module, MAP kinase, phosphorylates various downstream signaling or effector proteins.

*Figure 16-20* The G-protein α subunit switches itself off by hydrolyzing its bound GTP to GDP.

*The G-protein α subunit switches itself off by hydrolyzing its bound GTP to GDP.* When an activated α subunit interacts with its target protein, it activates that target protein (or in some cases inactivates it; not shown) for as long as the two remain in contact. Normally the α subunit hydrolyzes its bound GTP to GDP within seconds. This loss of GTP inactivates the α subunit, which dissociates from its target protein and—if the α subunit had separated from the βγ complex (as shown)—reassociates with a βγ complex to re-form an inactive G protein. The G protein is now ready to couple to another activated receptor, as in Figure 16-19B. Both the activated α subunit and the activated βγ complex can interact with target proteins in the plasma membrane. See also Movie 16.2.

*Figure 16-31* The light-induced signaling cascade in rod photoreceptor cells greatly amplifies the light signal.

*The light-induced signaling cascade in rod photoreceptor cells greatly amplifies the light signal.* When rod photoreceptors are adapted for dim light, signal amplification is enormous. The intracellular signaling pathway from the G protein transducin uses components that differ from the ones in previous figures. The cascade functions as follows. In the absence of a light signal, the small messenger molecule cyclic GMP is continuously produced by an enzyme in the cytosol of the photoreceptor cell. The cyclic GMP then binds to cation channels in the photoreceptor cell plasma membrane, keeping them open. Activation of rhodopsin by light results in the activation of transducin α subunits. These turn on an enzyme called cyclic GMP phosphodiesterase, which breaks down cyclic GMP to GMP (much as cyclic AMP phosphodiesterase breaks down cyclic AMP; see Figure 16-23). The sharp fall in the cytosolic concentration of cyclic GMP causes the bound cyclic GMP to dissociate from the cation channels, which therefore close. Closing these channels decreases the influx of Na+, thereby altering the voltage gradient (membrane potential) across the plasma membrane and, ultimately, the rate of neurotransmitter release, as described in Chapter 12. The red arrows indicate the steps at which amplification occurs, with the thickness of the arrow roughly indicating the magnitude of the amplification.