Question 1

You are studying the responses of adipocytes in culture following their exposure to γ-irradiation. In comparison to normal adipocytes you find that when glucagon is added, the normal cells release free fatty acids to the medium but the irradiated cells do not. Binding studies show that both cell populations bind the same amount of glucagon with the same dissociation kinetics, indicating the receptors on the irradiated cells are unchanged. Given these results, which of the following proteins is most likely to be defective in the irradiated cells? A) a G_q-type G-protein B) a G_s-type G-protein C) phosphatidic acid phosphatase D) phosphatidylinositol-3-kinase (PI3K) E) protein kinase C (PKC)

Accepted Answer

The defect is likely in the G_s-type G-protein, which is involved in the signaling pathway for glucagon to stimulate lipolysis and release free fatty acids. Since the receptors are unchanged, the issue is likely downstream, affecting the G-protein that mediates the response.

Question 2

You are studying the responses of myocytes in culture following their exposure to γ-irradiation. In a comparison to normal myocytes you find that when insulin is added, the normal cells actively take up glucose but the irradiated cells do not. Binding studies show that both cell populations bind the same amount of insulin with the same dissociation kinetics, indicating the receptors on the irradiated cells are unchanged. Given these results, which of the following proteins is most likely to be defective in the irradiated cells? A) a G_q-type G-protein B) a G_s-type G-protein C) phosphatidic acid phosphatase D) phosphatidylinositol-3-kinase (PI3K) E) protein kinase A (PKA)

Accepted Answer

The defect is likely in the phosphatidylinositol-3-kinase (PI3K) pathway, which is crucial for insulin-mediated glucose uptake. Since insulin binding is normal, the issue is likely downstream, affecting the signaling pathway that facilitates glucose uptake.

Question 3

Lysophospholipids (LPLs) exhibit biological properties resembling those of extracellular growth factors or signaling molecules. One of the most important LPLs is sphingosine-1-phosphate (S1P). Which of the following proteins is responsible for transmitting the signal-transduction processes of S1P? A) a G_i-type G-protein coupled receptor (GPCR) B) a nuclear receptor C) a receptor serine/threonine kinase D) a receptor tyrosine kinase E) a receptor tyrosine phosphatase

Accepted Answer

Sphingosine-1-phosphate (S1P) signals through G-protein coupled receptors (GPCRs), specifically those of the G_i type, to mediate its effects.

Question 4

You are studying the signaling pathways suspected to be altered in a colonic carcinoma. You are particularly focused on the activity of the G-protein RAS in these cells. Your studies reveal that the RAS protein can bind GTP and become activated with the same efficiency as the RAS from normal cells. However, the rate at which the cancer cell RAS activity is decreased is 10 times slower than in normal cells. Which of the following proteins is most likely to be defective in the carcinoma cells, resulting in abnormal RAS signaling? A) a G_i-type G-protein B) a G_q-type G-protein C) a G_s-type G-protein D) a GTPase-activating protein E) a guanine nucleotide exchange factor

Accepted Answer

The GTPase-activating protein (GAP) is responsible for increasing the rate at which RAS hydrolyzes GTP to GDP, thereby inactivating RAS. If the rate of RAS inactivation is slower in cancer cells, it suggests a defect in GAP function.

Question 5

Steroid hormones are able to penetrate the plasma membrane following which they bind to specific receptors inside the cell termed intracellular or nuclear receptors. If a cell line that normally were able to respond to the presence of a particular steroid hormone sustained a mutation in the ligandbinding domain of the receptor, which of the following processes would most likely be impaired?
A) posttranscriptional processing of specific mRNAs
B) posttranslational processing of specific proteins
C) replication of DNA
D) transcription of specific genes
E) translation of specific mRNAs

Accepted Answer

The mutation in the ligand-binding domain of the receptor would impair the receptor's ability to bind the steroid hormone, which in turn would affect the transcription of specific genes that are normally regulated by the hormone-receptor complex.  Steroid hormones are lipophilic and hence freely penetrate the plasma membrane of all cells. Within target cells, steroid hormones interact with specific receptors. These receptor proteins are composed of 2 domains: a hormone-binding domain and a DNA-binding domain. Following hormonereceptor interaction, the complex is activated and enters the nucleus. The DNA-binding domain of the receptor interacts with specific nucleotide sequences termed hormone response elements (HREs). The binding of steroid-receptor complexes to HREs results in an altered rate of transcription of the associated gene(s). The effects of steroid-receptor complexes on specific target genes can be either stimulatory or inhibitory with respect to the rate of transcription.

Question 6

In an examination of the signaling events in a myocyte cell line in culture, you find that addition of an α₁-adrenergic agonist does not lead to the expected release of stored intracellular calcium into the cytoplasm. You ascertain that the receptors for the agonist are present and that the agonist does indeed bind to the receptor. Given these results, which of the following proteins is most likely to be defective in these cells? A) a G_s-type G-protein B) a G_q-type G-protein C) phosphatidic acid phosphatase D) phosphatidylinositol-3-kinase (PI3K) E) protein kinase A (PKA)

Accepted Answer

The α1-adrenergic receptor typically signals through a Gq-type G-protein, which activates phospholipase C, leading to the release of intracellular calcium. If calcium release is not occurring, the Gq protein may be defective.

Question 7

In a comparative study of 2 related cell lines, you find that one responds normally to insulin while the other has an impaired response. You discover that both cell lines bind insulin with equal affinity, but that the impaired response is manifest in an inability to recruit the insulin response substrate-1 (IRS-1) protein to the receptor. This would most likely be due to which of the following?
A) inability of the receptor to phosphorylate the RAS G-protein
B) loss of activation of phospholipase C-β (PLC-β)
C) mutation in the tyrosine phosphate recognition site of IRS-1
D) serine phosphorylation of the insulin receptor preventing IRS-1 binding
E) tyrosine phosphorylation of the insulin receptor, leading to the loss of the IRS-1-binding site

Accepted Answer

A mutation in the tyrosine phosphate recognition site of IRS-1 would prevent IRS-1 from being recruited to the insulin receptor, leading to an impaired insulin response despite normal insulin binding.  Binding of insulin to its receptors triggers activation of the intrinsic tyrosine kinase activity of the receptor. In addition to the incorporation of phosphate on tyrosine residues of several target proteins, the receptor autophosphorylates tyrosine residues on itself. These receptor-associated tyrosine phosphates serve as docking site for several signaling proteins including IRS-1. A loss in the ability of IRS-1 to bind to the phosphorylated tyrosine due to a mutation in the recognition motif would impair insulin-mediated signaling through IRS-1.

Question 8

You are examining the characteristics of a cell line isolated from tumor tissue and comparing it to normal tissue cells. You find that addition of a particular growth factor to the normal cells results in increased gluconeogenesis, but that in the tumorderived cells this effect is absent. You suspect that there is a defect in receptor-mediated signaling and surmise that there is altered G-protein-coupled activity in the tumor cells. Which type of G-protein is likely to be defective? A) G₁₂ B) G_i C) G_q D) G_s

Accepted Answer

G_s is typically involved in stimulating adenylate cyclase, which increases cAMP levels and can lead to increased gluconeogenesis. If the tumor cells do not respond to the growth factor with increased gluconeogenesis, it suggests a defect in the G_s protein pathway.

Question 9

Examination of cells from a tumor demonstrates that there is a defect in receptor-mediated activation of protein kinase C, PKC. You assume that this defect is due to the receptor-coupled G-protein. Which of the following G-protein classes is likely to be defective? A) G₁₂ B) G_i C) G_q D) G_s

Accepted Answer

G_q is the G-protein class that is likely to be defective because it is responsible for activating PKC.

Question 10

The frizzled proteins are a class of G-protein-coupled receptor (GPCR) that serves as receptors for the Wnt family of growth factors. Which of the following signal-transduction proteins is downstream of the frizzled receptors? A) β-catenin B) mitogen-activated protein kinase (MAPK) C) phospholipase C-β (PLC-β) D) protein kinase C (PKC) E) SMAD₄

Accepted Answer

β-catenin is a key component of the Wnt signaling pathway, which is activated downstream of frizzled receptors.

Question 11

The regulatory subunit of a mutant protein kinase A (PKA) contains an amino acid substitution that prevents the binding of cAMP. As a consequence of this mutation, which of the following hormone responses is most likely to be decreased? A) 1,25-dihydroxycholecalciferol B) estrogen C) glucagon D) insulin E) thyroxine (T₄)

Accepted Answer

Glucagon exerts its effects by increasing cAMP levels, which activate PKA. A mutation preventing cAMP binding to PKA would decrease glucagon's ability to activate PKA, thus reducing its response.

Question 12

Phospholipase-catalyzed hydrolysis of which of the following compounds results in an increased cytosolic concentration of calcium ions in response to ligand binding to members of serotonin receptor family?
A) phosphatidylcholine
B) phosphatidylethanolamine
C) phosphatidylgylcerol
D) phosphatidylinositol bisphosphate
E) phosphatidylserine

Accepted Answer

Phospholipase C hydrolyzes phosphatidylinositol bisphosphate (PIP2) to produce inositol trisphosphate (IP3), which then triggers the release of calcium ions from the endoplasmic reticulum into the cytosol. Several members of the serotonin receptor family are coupled to the G_q class of G-proteins that activate membrane-associated PLC-β. Active PLC-β hydrolyzes membrane-associated phosphatidylinositol- 4,5-bisphosphate, PIP₂), resulting in increased production of the intracellular messengers IP₃ and DAG. IP₃ binds to a receptor on the ER membrane and triggers the release of stored calcium ion into the cytosol. The calcium ions and the DAG serve to act as second messengers in the activation of PKC.

Question 13

Which of the following intracellular signal molecules regulates the catalysis of the conversion of GTP to cGMP?
A) cAMP
B) ceramide
C) diacylglycerol
D) inositol trisphosphate
E) nitric oxide

Accepted Answer

Nitric oxide (NO) is a signaling molecule that activates guanylate cyclase, which catalyzes the conversion of GTP to cGMP.  Within smooth muscle cells, nitric oxide (NO) reacts with the heme moiety of a soluble guanylyl cyclase, resulting in activation of the latter and a consequent conversion of GTP to cGMP. The net effect is the activation of cGMP-dependent protein kinase (PKG) and the phosphorylation of substrates leading to smooth muscle cell relaxation.

Question 14

You are studying the signal-transduction pathways in a cell line derived from a breast cancer. Your studies identify a novel protein that specifically binds a phosphorylated tyrosine residue on the cytoplasmic aspect of a protein kinase-linked receptor. This binding anchors the protein near the cell surface. Which of the following domains is most likely to be responsible for this binding property?
A) α-helix
B) helix-loop-helix
C) β-pleated sheets
D) SH2
E) zinc finger

Accepted Answer

The SH2 (Src Homology 2) domain is known for binding specifically to phosphorylated tyrosine residues on receptor tyrosine kinases, which aligns with the described function of the novel protein.  Many receptors that have intrinsic tyrosine kinase activity as well as the tyrosine kinases that are associated with cell surface receptors contain tyrosine residues that, upon phosphorylation, interact with other proteins of the signal-transduction cascade. These other proteins contain a domain of amino acid sequences that are homologous to a domain first identified in the SRC proto-oncogene. These domains are termed SH2 domains (SRC homology domain 2).

Question 15

The pathogen, Vibrio cholera, secretes cholera toxin, which in turn leads to G-protein-mediated activation of adenylate cyclase in intestinal enterocytes, resulting in massive elevation in the kinase activity of protein kinase A (PKA). The phosphorylation of which of the following proteins, by PKA, results in the induction of watery diarrhea in individuals infected with this pathogen?
A) glucagon receptor
B) inositol 1,4,5-trisphosphate receptor
C) phosphorylase
D) plasma membrane chloride channel
E) tyrosine kinase

Accepted Answer

PKA phosphorylates the plasma membrane chloride channel, specifically the CFTR channel, leading to increased chloride ion secretion into the intestinal lumen, which causes watery diarrhea. The toxin protein from Vibrio cholera catalyzes the ADP ribosylation of the α-subunit of G_s G-proteins in the intestines. The ADP ribosylation causes the G_as subunit to lose its catalytic activity in hydrolyzing GTP, so it remains activated longer than normal. Increased G_as activation leads to increased adenylate cyclase activity, which increases the intracellular concentration of cAMP to more than 100-fold over the normal and overactivates PKA. The activated PKA then phosphorylates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel proteins, which leads to ATP-mediated efflux of chloride ions and leads to secretion of H₂O and other ions into the intestinal lumen.

Question 16

You are studying the activation of the enzyme adenylate cyclase in response to the addition of glucagon to myocytes in culture. You find that this activation involves the activity of a member of the G-protein family. Which of the following most closely describes the properties of this G-protein?
A) integral membrane protein
B) protein associated with the mitochondrial membrane
C) protein localized on the inner surface of the plasma membrane
D) soluble protein within the cytoplasm
E) transmembrane protein that binds a ligand at the outer surface of the plasma membrane

Accepted Answer

G-proteins are typically localized on the inner surface of the plasma membrane where they interact with receptors and adenylate cyclase to transduce signals. The G_s family of G-proteins stimulates the activity of adenylate cyclase, resulting in increased production of cAMP from ATP. These G-proteins are localized to the inner surface of the plasma membrane in close proximity to G-protein-coupled receptors that activate the G_s protein in response to ligand binding.

Question 17

You are studying the activity of the G-protein RAS in a cell line derived from a colonic adenoma. You find that the signal-transduction pathway induced by the activation of RAS is continuously active in these cells and that this correlates to a mutation in the RAS gene. This mutation in RAS most likely results in the inhibition of which of the following activities of this G-protein?
A) GTPase activity
B) guanylate cyclase activity
C) guanine nucleotide binding
D) phosphoprotein phosphatase activity
E) protein kinase activity

Accepted Answer

The mutation in the RAS gene likely inhibits its GTPase activity, preventing RAS from hydrolyzing GTP to GDP, thus keeping it in an active state and continuously signaling.  The activity state of G-proteins is regulated both by the rate of GTP exchange for GDP and by the rate at which the GTP is hydrolyzed to GDP. The activity of G-proteins with respect to GTP hydrolysis is regulated by a family of proteins termed GTPase-activating proteins, GAPs. The inability to hydrolyze GTP bound to a G-protein, such as RAS, results in constant activation of the G-protein leading to inappropriate continuous activation of the growth signals transmitted by that G-protein. This can result in aberrant cell growth associated with cancer.

Question 18

Activation of protein kinase C by epidermal growth factor requires a product of the reaction catalyzed by which of the following enzymes?
A) adenylate cyclase
B) guanylate cyclase
C) phospholipase C-γ
D) phosphoprotein phosphatase
E) protein kinase A

Accepted Answer

Phospholipase C-γ catalyzes the hydrolysis of PIP2 to produce IP3 and DAG, the latter of which activates protein kinase C. When the EGF binds to its cell surface receptor, it triggers activation of the phospholipase C-γ (PLC-γ) isoform of PLC. PLC-γ hydrolyzes membrane-associated phosphatidylinositol-4,5-bisphosphate (PIP₂) into inositol trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ binds to a receptor on the ER membrane and triggers the release of stored calcium ion into the cytosol. The calcium ions and the DAG serve to act as second messengers in the activation of PKC.

Question 19

T-cells are activated when antigen-MHC complexes bind to the T-cell receptor on the cell membrane. This process activates second messengers that in turn activate protein kinase C (PKC), leading to induction of transcription factors and subsequent gene expression. Which of the following most likely represents the second messenger of this signal transduction?
A) calcium ions
B) cAMP
C) cGMP
D) diacylglycerol
E) interleukin-2 (IL-2)

Accepted Answer

Diacylglycerol (DAG) is a second messenger that is produced in response to the binding of antigen-MHC complexes to the T-cell receptor. DAG activates protein kinase C (PKC), which in turn activates transcription factors and subsequent gene expression. The activation of the T-cell receptor results in activation of the phospholipase C-γ (PLC-γ) isoform of PLC. PLC-γ hydrolyzes membrane-associated phosphatidylinositol-4,5-bisphosphate (PIP₂) into inositol trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ binds to a receptor on the ER membrane and triggers the release of stored calcium ion into the cytosol. The calcium ions and the DAG serve to act as second messengers in the activation of PKC.

Question 20

Cyclic AMP (cAMP) activates protein kinase A (PKA) by doing which of the following?
A) binding noncovalently to the regulatory subunit
B) binding to the receptor protein that moves to the nucleus
C) causing conformational changes in nuclear histones
D) modification of enzyme conformation by adenylation of a tyrosine residue
E) promoting proper conformation of the substrate

Accepted Answer

cAMP activates PKA by binding noncovalently to the regulatory subunits of PKA, causing them to release the catalytic subunits, which then become active.  cAMP binds to the regulatory subunits of PKA, resulting in release of the catalytic subunits.

Quiz 40: Mechanisms of Signal Transduction

Lysophospholipids (LPLs) exhibit biological properties resembling those of extracellular growth factors or signaling molecules. One of the most important LPLs is sphingosine-1-phosphate (S1P) . Which of the following proteins is responsible for transmitting the signal-transduction processes of S1P?

Examination of cells from a tumor demonstrates that there is a defect in receptor-mediated activation of protein kinase C, PKC. You assume that this defect is due to the receptor-coupled G-protein. Which of the following G-protein classes is likely to be defective?

The frizzled proteins are a class of G-protein-coupled receptor (GPCR) that serves as receptors for the Wnt family of growth factors. Which of the following signal-transduction proteins is downstream of the frizzled receptors?

The regulatory subunit of a mutant protein kinase A (PKA) contains an amino acid substitution that prevents the binding of cAMP. As a consequence of this mutation, which of the following hormone responses is most likely to be decreased?

Phospholipase-catalyzed hydrolysis of which of the following compounds results in an increased cytosolic concentration of calcium ions in response to ligand binding to members of serotonin receptor family?

Which of the following intracellular signal molecules regulates the catalysis of the conversion of GTP to cGMP?

You are studying the activation of the enzyme adenylate cyclase in response to the addition of glucagon to myocytes in culture. You find that this activation involves the activity of a member of the G-protein family. Which of the following most closely describes the properties of this G-protein?

Activation of protein kinase C by epidermal growth factor requires a product of the reaction catalyzed by which of the following enzymes?

Cyclic AMP (cAMP) activates protein kinase A (PKA) by doing which of the following?