homework 12
Which of the following alterations to the signaling pathway would lead to increased transcription by the CREB protein? A. inhibition of cAMP breakdown B. inhibition of GTP hydrolysis C. inhibition of entry of PKA into the nucleus D. inhibition of adenylyl cyclase
- inhibition of cAMP breakdown - inhibition of GTP hydrolysis Explanation: When GTP hydrolysis is blocked, Gα will remain active and continually stimulate adenylyl cyclase to produce cAMP. Directly blocking cAMP breakdown will have the same effect. In both cases, cAMP remains high and PKA remains active and able to phosphorylate CREB, leading to transcription. Blocking adenylyl cyclase activity or PKA entry to the nucleus will both inhibit the signaling pathway and block activation of CREB.
Alterations in signaling in the pituitary gland can lead to human disease. The GH-releasing hormone (GHRH) stimulates release of growth hormone (GH) from the pituitary gland by binding to GHRH receptors, which are G-protein-coupled receptors. Excessive activity of the GHRH signaling pathway leads to excessive release of growth hormone, which can lead to acromegaly, a form of gigantism. Some patients can reach more than 8 feet tall as they continue to grow even in adulthood. Consider steps that could be taken to reduce GH release. Sort each of the following interventions into the proper category. -Inhibit interaction of Gα with receptor -Block hydrolysis of GTP -Block ligand binding to receptor -Activate phosphorylation of the receptor by GRK kinase -Block binding of arresting to the receptor
Decreases GH: -Inhibit interaction of Gα with receptor -Block ligand binding to receptor -Activate phosphorylation of the receptor by GRK kinase Increases GH Release: -Block binding of arresting to the receptor -Block hydrolysis of GTP Explanation: Any alteration that would lessen GHRH signaling could possibly reduce GH release. Blocking the ligand from binding to GHRH receptor would block the signaling from even beginning and lead to less GH release. Blocking the receptor from binding to Gα would also block the beginning of signaling. Additionally, increasing phosphorylation of the GHRH receptor by GRK would lead to rapid inactivation of the signaling pathway and would also lead to less GH release. Any alteration that would increase GHRH signaling could lead to increased GH release and would make the acromegaly worse. Blocking hydrolysis of GTP on Gα and blocking the binding of arrestin to the GHRH receptor would both increase the length of signaling by preventing inactivation of the signaling pathway. These would both lead to increased GH release and would worsen acromegaly.
How does diacylglycerol (DAG) function in the inositol phospholipid pathway? A. Along with the Ca2+, it recruits and activates PKC at the plasma membrane. B. It activates phospholipase C. C. It produces cyclic AMP. D. It binds to and opens Ca2+ channels in the plasma membrane, allowing extracellular Ca2+ to enter the cytosol. E. It produces IP3.
Explanation: Two messenger molecules are produced when a membrane inositol phospholipid is hydrolyzed by activated phospholipase C: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. Diacylglycerol remains in the plasma membrane and, together with Ca2+ released by the action of IP3, helps activate the enzyme protein kinase C (PKC), which is recruited from the cytosol to the cytosolic face of the plasma membrane. Once activated, PKC phosphorylates a set of intracellular proteins that varies depending on the cell type.
Which of the following represents the correct order of signaling events that might be triggered by a GPCR that activates expression of a target gene via the production of cyclic AMP? A. adenylyl cyclase → cyclic AMP → G protein → PKA → transcription regulator B. G protein → adenylyl cyclase → cyclic AMP → PKC → transcription regulator C. G protein → adenylyl cyclase → cyclic AMP → PKA → transcription regulator D. G protein → cyclic AMP → adenylyl cyclase → PKA → transcription regulator E. G protein → PKA → cyclic AMP → adenylyl cyclase → transcription regulator
G protein → adenylyl cyclase → cyclic AMP → PKA → transcription regulator Explanation: When an extracellular signal molecule binds to a GPCR, the receptor activates a G protein located on the other side of the plasma membrane. G proteins, in turn, can activate membrane-bound enzymes, including adenylyl cyclase, which produces cyclic AMP. This small messenger molecule exerts most of its effects by activating the enzyme cyclic-AMP-dependent protein kinase (PKA). The binding of cyclic AMP causes PKA to release a regulatory protein that normally inhibits its activity. Activated PKA then catalyzes the phosphorylation of particular serines or threonines on intracellular target proteins. In some cases, PKA phosphorylates transcription regulators, proteins that activate the transcription of selected genes. For example, an increase in cyclic AMP in certain neurons in the brain controls the production of proteins involved in some forms of learning.
Which of the following steps are required in the activation of the G-protein signaling pathway? A. Gα exchanges GDP for GTP. B. Activated Gα influences target proteins. C. Ligand binds to the G-protein-coupled receptor. D. The activated receptor induces interaction between Gα and Gβγ.
Gα exchanges GDP for GTP. Activated Gα influences target proteins. Ligand binds to the G-protein-coupled receptor. Explanation: When ligand binds to the G-protein-coupled receptor, the receptor changes conformation and induces the Gα subunit to release GDP and bind GTP, as shown in the following figure. The activated GTP-bound Gα then releases from the Gβγ subunits and both Gα and Gβγ can regulate downstream targets.
When activated phospholipase C cleaves an inositol phospholipid, what happens to the small signaling molecules the enzyme produces? A. Both inositol 1,4,5-trisphosphate (IP3) and diacylglycerol are released into the cytosol. B. Inositol 1,4,5-trisphosphate (IP3) remains in the membrane, while diacylglycerol is released into the cytosol. C. Both inositol 1,4,5-trisphosphate (IP3) and diacylglycerol are retained in the membrane. D. Inositol 1,4,5-trisphosphate (IP3) is released into the cytosol, while diacylglycerol is retained in the membrane. E. Both inositol 1,4,5-trisphosphate (IP3) and Ca2+ are released into the cytosol.
Inositol 1,4,5-trisphosphate (IP3) is released into the cytosol, while diacylglycerol is retained in the membrane. Explanation: The cleavage of a membrane inositol phospholipid by phospholipase C generates two second messenger molecules: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 is a water-soluble sugar phosphate that is released into the cytosol; there, it binds to and opens Ca2+ channels that are embedded in the endoplasmic reticulum (ER) membrane. Diacylglycerol is a lipid that remains embedded in the plasma membrane; there, it helps recruit and activate a protein kinase called PKC, which translocates from the cytosol to the plasma membrane.
Which of the following statements is true? A. Many ion-channel-coupled receptors have an intrinsic catalytic domain on the cytosolic side of the plasma membrane. B. All enzyme-coupled receptors have an intrinsic catalytic domain on the cytosolic side of the plasma membrane. C. Ion-channel-coupled receptors can rapidly alter the membrane potential in response to signal binding. D. Each extracellular signal molecule interacts with a single class of cell-surface receptor.G-protein-coupled receptors are GTP-binding proteins.
Ion-channel-coupled receptors can rapidly alter the membrane potential in response to signal binding. Explanation: All cell-surface receptor proteins bind to an extracellular signal molecule and transduce their message into one or more intracellular signaling molecules that alter the cell's behavior. Most of these receptors belong to one of three large classes, which differ in the transduction mechanism they use. 1. Ion-channel-coupled receptors change the permeability of the plasma membrane to selected ions, thereby altering the membrane potential and, if the conditions are right, producing an electrical current. 2. G-protein-coupled receptors activate membrane-bound, trimeric GTP-binding proteins (G proteins), which then activate (or inhibit) an enzyme or an ion channel in the plasma membrane, initiating an intracellular signaling cascade. 3. Enzyme-coupled receptors either act as enzymes or associate with enzymes inside the cell; when stimulated, the enzymes can activate a wide variety of intracellular signaling pathways. The number of different types of receptors in each of these three classes is even greater than the number of extracellular signals that act on them. This is because for many extracellular signal molecules, there is more than one type of receptor, and these may belong to different receptor classes. Acetylcholine, for example, acts on salivary gland cells through a G-protein-coupled receptor, whereas in skeletal muscle cells it acts via an ion-channel-coupled receptor. These two types of receptors generate different intracellular signals and thus enable the two types of cells to react to acetylcholine in different ways.
How does IP3 function in the inositol phospholipid pathway? A. It generates a hole in the endoplasmic reticulum (ER), through which Ca2+ can enter the cytosol. B. It produces diacylglycerol. C. It opens Ca2+ channels that are embedded in the ER membrane, allowing Ca2+ to enter the cytosol. D. It directly activates phospholipase C (PLC). E. It directly activates protein kinase C (PKC).
It opens Ca2+ channels that are embedded in the ER membrane, allowing Ca2+ to enter the cytosol. Explanation: Two messenger molecules are produced when a membrane inositol phospholipid is hydrolyzed by activated phospholipase C: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. 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.
Signaling via a GPCR ceases when which condition occurs? A. The G protein associates with an activated GPCR. B. The α subunit hydrolyzes its bound GTP. C. The α subunit exchanges GDP for GTP. D. The G protein dissociates from the activated GPCR. E. The α subunit dissociates from the βγ complex.
The α subunit hydrolyzes its bound GTP. Explanation: The activated parts of the G protein—the α subunit and the βγ complex—can each interact directly with target proteins in the plasma membrane, which in turn may relay the signal to other destinations in the cell. The longer these target proteins remain bound to an α subunit or a βγ complex, the more prolonged the relayed signal will be.The amount of time that the α subunit and βγ complex remain "switched on"—and hence available to relay signals—also determines how long a response lasts. This timing is controlled by the behavior of the α subunit. The α subunit has an intrinsic GTPase activity, and it eventually hydrolyzes its bound GTP to GDP, returning the whole G protein to its original, inactive conformation. GTP hydrolysis and inactivation usually occur within seconds after the G protein has been activated. The inactive G protein is then ready to be reactivated by another activated receptor.
GPCRs are sometimes referred to as "seven-pass transmembrane receptors" for which reason? A. They bind to seven extracellular signal molecules before passing a message to the cell interior. B. Their polypeptide chain crosses the lipid bilayer seven times. C. They are composed of seven distinct subunits arranged in a ring. D. They form seven pores through which signal molecules can cross the lipid bilayer. E. There are seven different G proteins with which they interact to amplify signals.
Their polypeptide chain crosses the lipid bilayer seven times. Explanation: Despite the diversity of the signal molecules that bind to them, all GPCRs have a similar structure: each is made of a single polypeptide chain that threads back and forth across the lipid bilayer seven times. The cytoplasmic portions of the receptor bind to a G protein inside the cell.For receptors that recognize small signal molecules, such as acetylcholine or epinephrine, the ligand (red) usually binds deep within the plane of the membrane to a pocket that is formed by amino acids from several transmembrane segments. Receptors that recognize signal molecules that are proteins usually have a larger extracellular domain that, together with some of the transmembrane segments, binds the protein ligand.
Which statement regarding G proteins is true? A. When a G protein is inactive, GDP is bound to its α subunit. B. The GTPase activity resides in the βγ complex. C. The GTPase activity resides in the β subunit. D. The β subunit is tethered to the membrane by a short lipid tail. E. Only the α subunit interacts with target proteins.
When a G protein is inactive, GDP is bound to its α subunit. Explanation: An activated GPCR activates G proteins by encouraging the α subunit to expel its GDP and pick up GTP. 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.
IP3 signaling helps regulate sweating, which is important for regulating body temperature. Anhidrosis, the inability to sweat normally, can be caused by genetic and environmental factors. A rare mutation has been identified in a family with several children suffering from anhidrosis. The mutation inactivates the protein that IP3 binds on the ER membrane. Suppose cells were isolated from affected family members and exposed to different treatments. Which of the following treatments would be able to repair the signaling defect in cells isolated from these patients? A. activation of phospholipase Cβ in the cell B. addition of high amounts of Ca2+ in the cytosol C. addition of PKC to the cell D. addition of high amounts of IP3 in the cell
addition of high amounts of Ca2+ in the cytosol Explanation: The five children in this family who suffered from anhidrosis all shared a single point mutation in the ITPR2 gene, which codes for the IP3 receptor calcium channel on the ER membrane in the skin cells that produce sweat. Adding any protein or second messenger such as IP3 or phospholipase Cβ to the cells will have no effect since the IP3 receptor is defective. Because the IP3 receptor is a Ca2+ channel that releases Ca2+ into the cytosol from the ER, adding Ca2+ to the cytosol will bypass the need for the Ca2+ channel and restore activation of PKC in isolated cells. This is not a practical treatment for the people themselves. There is currently no cure for this form of anhidrosis, but this was an important step in understanding the underlying cause of the disease. The study title "Abolished InsP3R2 function inhibits sweat secretion in both humans and mice" was published in 2014 in the Journal of Clinical Investigation (124:4773-4780).
When activated by the binding of Ca2+, calmodulin relays the Ca2+ signal onward by doing what action? A. transferring its bound Ca2+ to various intracellular proteins, thereby activating them B. destroying its bound Ca2+ C. binding to cyclic AMP D. binding to Ca2+/calmodulin-dependent protein kinases E. binding to various extracellular proteins and directly activating them
binding to Ca2+/calmodulin-dependent protein kinases Explanation: The effects of Ca2+ are mediated through the ion's interaction with various kinds of Ca2+-responsive proteins. The most widespread and common of these is calmodulin, which is present in the cytosol of all eukaryotic cells that have been examined, including those of plants, fungi, and protozoa. When Ca2+ binds to calmodulin, the protein undergoes a conformational change that enables it to interact with a wide range of target proteins in the cell, altering their activities. One particularly important class of targets for calmodulin is the Ca2+/calmodulin-dependent protein kinases (CaM-kinases). When these kinases are activated by binding to calmodulin complexed with Ca2+, they influence other processes in the cell by phosphorylating selected proteins. In the mammalian brain, for example, a neuron-specific CaM-kinase is thought to play an important part in some forms of learning and memory. Mutant mice that lack the kinase show a marked inability to remember where things are.
What is required for PKC activation? A. binding to DAG and continuing presence of Ca2+ B. binding to DAG C. continuing presence of Ca2+ D. binding to Gq E. binding to Gq and DAG
binding to DAG and continuing presence of Ca2+ Explanation: Signaling through the IP3 pathway leads to cleavage of PIP2 to IP3 and DAG. IP3 release triggers Ca2+ release from the endoplasmic reticulum. The Ca2+ and DAG together activate PKC, which then phosphorylates further cellular targets.
Which of the following would increase phosphorylation of CREB by PKA? A. blocking nuclear entry of PKA B. blocking cAMP binding to the regulatory subunits of PKA C. blocking binding of the regulatory subunits to the catalytic subunits of PKA D. blocking ATP binding to the active site of PKA
blocking binding of the regulatory subunits to the catalytic subunits of PKA Explanation: The regulatory subunits of PKA keep the catalytic kinase subunits inactive in the absence of cAMP. Binding of cAMP to the regulatory subunits causes them to release the catalytic subunits that are now active. The active catalytic subunits are transported into the nucleus, where they can use ATP to phosphorylate targets such as CREB. Loss of the regulatory subunits would activate the PKA catalytic subunits even in the absence of cAMP.
Many drugs act by binding to what? A. cyclic AMP B. extracellular signal molecules C. plasma membrane D. nuclear receptors E. cell-surface receptors
cell-surface receptors Explanation: Cell-surface receptors provide targets for many foreign substances that interfere with our physiology, from heroin and nicotine to tranquilizers and chili peppers. These substances either block or overstimulate the receptor's natural activity. Many drugs and poisons act in this way, and a large part of the pharmaceutical industry is devoted to producing drugs that will exert a precisely defined effect by binding to a specific type of cell-surface receptor. In some cases, these drugs keep another ligand from binding to the receptor—an effect called competitive inhibition. The drug naloxone, used to treat heroin overdoses, displaces the drug from opioid receptors.
The enzyme cyclic AMP phosphodiesterase helps terminate a response mediated by an increase in cyclic AMP by doing what? A. inactivating adenylyl cyclase B. converting cyclic AMP to ATP C. converting ATP to cyclic AMP D. activating an inhibitory G protein, Gi E. converting cyclic AMP to AMP
converting cyclic AMP to AMP Explanation: Adenylyl cyclase synthesizes cyclic AMP from ATP, which is always present in the cell. To help terminate the signal, a second enzyme, called cyclic AMP phosphodiesterase, rapidly converts cyclic AMP to ordinary AMP. One way that caffeine acts as a stimulant is by inhibiting this phosphodiesterase in the nervous system, blocking cyclic AMP degradation and thereby keeping the concentration of this second messenger high.
When activated by a GPCR, what does adenylyl cyclase do? A. converts cAMP to AMP B. converts AMP to cAMP C. converts ATP to cAMP D. inhibits cyclic AMP phosphodiesterase E. binds to protein kinase A
converts ATP to cAMP Explanation: Many extracellular signals acting via GPCRs affect the activity of the enzyme adenylyl cyclase and thus alter the intracellular concentration of the second messenger molecule, cyclic AMP. Most commonly, the activated G protein α subunit switches on the adenylyl cyclase, causing a dramatic and sudden increase in the synthesis of cyclic AMP from ATP (which is always present in the cell). To help terminate the signal, a second enzyme, called cyclic AMP phosphodiesterase, rapidly converts cyclic AMP to ordinary AMP. One way that caffeine acts as a stimulant is by inhibiting this phosphodiesterase in the nervous system, blocking cyclic AMP degradation and thereby keeping the concentration of this second messenger high.
Which type of cell-surface receptor(s), when activated, catalyze(s) a reaction inside the cell? A. enzyme-coupled receptors B. ion-channel-coupled receptors C. G-protein-coupled receptors D. enzyme-coupled receptors AND G-protein-coupled receptors
enzyme-coupled receptors Explanation: When bound to a signal, enzyme-coupled receptors become active enzymes that catalyze a reaction inside the cell. G-protein-coupled receptors activate G-proteins, which can activate enzymes but do so indirectly.
What are the small intracellular signaling molecules often called? A. extracellular signaling molecules B. transmitter-gated ion channels C. intracellular signaling proteins D. second messangers E. first messangers
second messangers Explanation: The small molecules generated by enzymes such as adenylyl cyclase and phospholipase C are often called second messengers—the "first messengers" being the extracellular signals that activated the enzymes in the first place. Once activated, the enzymes generate large quantities of second messengers, which rapidly diffuse away from their source, thereby amplifying and spreading the intracellular signal.