Cell and Molec Exam 4

Intracellular steroid hormone receptors have ending sites for a signaling molecule and a DNA sequence. How is it that the same steroid hormone receptor, which binds to a specific DNA sequence, can regulate different genes in different cell types?

The specific genes regulated in response to an activated steroid hormone receptor depends not only on the genes having the appropriate DNA sequence for binding the receptor but also on a variety of other nuclear proteins that influence gene expression, some of which cary between different cell types.

Can signaling via a steroid hormone receptor lead to amplification of the original signal? If so, how?

Because the interactions of the signal molecule with its receptor and of the activated receptor with its gene are both one-to-one , there is no amplification in this part of the signaling pathway. The signal can, however, be amplified when the target genes are transcribed, because each activated gene produces multiple copies of mRNA, each of which is used to make multiple copies of the protein that the gene encodes.

Which of the following statements is true? a) Extracellular signal molecules that are hydrophilic must bind to a cell surface receptor so as to signal a target cell to change its behavior. b) To function, all extracellular signal molecules must be transported by their receptor across the plasma membrane into the cytosol. c) A cell-surface receptor capable of binding only one type of signal molecule can mediate only one kind of cell response. d) Any foreign substance that binds to a receptor for a normal signal molecule will always induce the same response that is produced by that signal molecule on the same cell type.

a) Extracellular signal molecules that are hydrophilic myst bind to a cell surface receptor so as to signal a target cell to change its behavior.

Acetylcholine binds to a GPCR on heart muscle, making the heart beat more slowly. The activated receptor stimulates a G protein, which opens a K+ channel in the plasma membrane, as shown in Figure Q16-32. Which of the following would enhance this effect of the acetylcholine? (a) addition of a high concentration of a nonhydrolyzable analog of GTP (b) addition of a drug that prevents the α subunit from exchanging GDP for GTP (c) mutations in the acetylcholine receptor that weaken the interaction between the receptor and acetylcholine (d) mutations in the acetylcholine receptor that weaken the interaction between the receptor and the G protein

(a) addition of a high concentration of a nonhydrolyzable analog of GTP

The following happens when a G-protein-coupled receptor activates a G protein. (a) The β subunit exchanges its bound GDP for GTP. (b) The GDP bound to the α subunit is phosphorylated to form bound GTP. (c) The α subunit exchanges its bound GDP for GTP. (d) It activates the α subunit and inactivates the βγ complex.

(c) The α subunit exchanges its bound GDP for GTP.

Given the generic signaling pathway in Figure Q16-12, write the number corresponding to the item on the line next to the descriptor below.

2- Receptor Protein 4- Effector Proteins 3- Intracellular Signaling Proteins 1- Ligand

Rank the following types of cell signaling from 1-4 , with 1 representing the type of signaling in which the signal molecule travels the least distance and 4 the type of signaling in which the signal molecule travels the largest distance. -Paracrine Signaling -Contact-Dependent Signaling -Neuronal Signaling -Endocrine Signaling

3- Paracrine Signaling 1- Contact-Dependent Signaling 2-Neuronal Signaling 4-Endocrine Signaling

Which of the following statements about molecular switches is false? a) Phosphatases remove the phosphate from GTP on GTP-binding proteins, turning them off. b) Protein kinases transfer the terminal phosphate from ATP onto a protein. c) Serine/threonine kinases are the most common types of protein kinase. d) A GTP-binding protein exchanges its bound GDP for GTP to become activated.

a) Phosphatases remove the phosphate from GTP on GTP-binding proteins, turning them off.

Name the three main classes of cell-surface receptor.

-Ion-channel-coupled receptors -G-protein-coupled receptors -Enzyme-coupled receptors

The local mediator nitric oxide stimulates the intracellular enzyme guanylyl cyclase by ______.

c) diffusing into cells and stimulating the cyclase directly.

All members of the steroid hormone receptor family _____.

d) interact with signal molecules that diffuse through the plasma membrane.

Match the class of cell-surface receptor with the best description of its function. Not all descriptors will be used. -G-protein-coupled receptors (D) -Ion-channel-coupled receptors (A) -Enzyme-coupled receptors (E)

D. All receptors of this class are polypeptides with seven transmembrane domains. A. Alter the membrane potential directly by changing the permeability of the plasma membrane. E. Discovred for their role in responding to growth factors in animal cells.

The lab you work in has discovered a previously unidentified extracellular signal molecule called QGF, a 75,000-dalton proteins. You add purified QGF to different types of cells to determine its effect on these cells. When you add QGF to heart muscle cells, you observe an increase in cell contraction. When you add it to fibroblasts, they undergo cell division. When you add it to nerve cells, they die. When you add it to glial cells, you do not see any effect on cell division or survival. Given these observations, which of the following statements is most likely to be true? a) Because it acts on so many diverse cell types. QGF probably diffuses across the plasma membrane into the cytoplasm of these cells. b) Glial cells do not have a receptor for QCF. c) QCF activates different intracellular signaling pathways in heart muscle cells, fibroblasts, and nerve cells to produce the different responses observed. d) Heart muscle cells, fibroblasts, and nerve cells must all have the same receptor for QGF.

c) QGF activates different intracellular signaling pathways in heart muscle cells, fibroblasts, and nerve cells to produce the different responses observed.

Foreign substances like nicotine, morphine, and menthol exert their initial effects by _____. a) killing cells immediately, exerting their physiological effects by causing cell death. b) diffusing through cell plasma membrane and binding to transcription factors to change gene expression. c) interacting with cell-surface receptors , causing the receptors to transduce signal inappropriately in the absence of the normal stimulus. d) removing cell-surface receptors from the plasma membrane.

c) interacting with cell-surface receptors , causing the receptors to transduce signal inappropriately in the absence of the normal stimulus.

Which of the following statements about G-protein-coupled receptors (GPCRs) is false? (a) GPCRs are the largest family of cell-surface receptors in humans. (b) GPCRs are used in endocrine, paracrine, and neuronal signaling. (c) GPCRs are found in yeast, mice, and humans. (d) The different classes of GPCR ligands (proteins, amino acid derivatives, or fatty acids) bind to receptors with different numbers of transmembrane domains.

(d) The different classes of GPCR ligands (proteins, amino acid derivatives, or fatty acids) bind to receptors with different numbers of transmembrane domains.

The length of time a G protein will signal is determined by _______. (a) the activity of phosphatases that turn off G proteins by dephosphorylating Gα. (b) the activity of phosphatases that turn GTP into GDP. (c) the degradation of the G protein after Gα separates from Gβγ. (d) the GTPase activity of Gα.

(d) the GTPase activity of Gα.

Explain why the signal molecules used in neuronal signaling work at a longer range than those used in contact-dependent signaling.

The neurotransmitter released from a neuron in neuronal signaling must diffuse across the synaptic cleft to reach receptors on the target cell. In contrast, in contact-dependent signaling, the signal molecule is attached to the plasma membrane of the signaling cell and interacts with receptors located on the plasma membrane of the receiving cell, thus, the cells must be in direct contact for this type of signaling to occur.

When a signal needs to be sent to most cells throughout a multicellular organism, the signal most suited for this is a _______.

b) Hormone

Figure Q16-18 shows the pathway through which nitric oxide (NO) triggers smooth muscle relaxation in a blood-vessel wall. Which of the following situations would lead to relaxation of the smooth muscle cells in the absence of acetylcholine?

b) a muscle cell that has a defect in guanylyl cyclase such that it constitutively converts GTP to cyclic GMP.

An extracellular signal molecule can act to change a cell's behavior by acting through cell-surface ____ that control intracellular signaling proteins. These intracellular signaling proteins ultimately change the activity of ____ proteins that bring about cell responses. Intracellular signaling proteins can ____ the signal received to evoke a strong response from just a few extracellular signal molecules. A cell that receives more than one extracellular signal at the same time can ____ this information using intracellular signaling proteins. _____ proteins can act as molecular switches letting a cell know that signal has been received. Enzymes that phosphorylate proteins, termed ____, can also serve as molecular switches; the actions of these enzymes are countered by the activity of _____.

receptors, effector, amplify, integrate, GTP-binding, protein kinases, protein phosphatases

G-protein-coupled receptor (GPCRs) all have a similar structure with ___ transmembrane domains. When a GPCR binds an extracellular signal, an intracellular G protein, composed of ___ subunits, becomes activated. ____ of the G-protein subunits are tethered to the plasma membrane by short lipid tails. When unstimulated, the α subunit is bound to ___, which is exchanged for ___ on stimulation. The intrinsic ___ activity of the α subunit is important for inactivating the G protein. ____ inhibits this activity of the α subunit, thereby keeping the subunit in an active state.

seven, three, two, GDP, GTP, GTPase, choleratoxin

Cells can signal to each there in various ways. A signal that must be relayed to the entire body is out efficiently sent by _______ cells, which produce hormones that are carried through the body through the bloodstream. On the other hand,______ methods of cell signaling do not require the release of a secreted molecule and are used for very localized signaling events. During ______ signaling, the signal remains in the neighborhood of the secreting cell and thus acts as a local mediator on nearby cells. Finally, _______ signaling involves the conversion of electrical impulses into a chemical signal. Cells receive signals through a ______, which can be an integral membrane protein or can reside inside the cell.

endocrine, contact-dependent, paracrine, neuronal, receptor

Receipt of extracellular signals can change cell behavior quickly (for example, in seconds or less) or much more slowly (for example, in hours). A. What kind of molecular changes could cause quick changes in cell behavior? B. What kind of molecular changes could cause slow changes in cell behavior? C. Explain why the response you named in A results in a quick change, whereas the response you named in B results in a slow change.

A. Any answer that involves the modification of existing cell components is correct. Protein phosphorylation, protein dephosphorylation, protein ubiquitylation, lipid phosphorylation, and lipid cleavage are all examples of correct answers. B. Responses that involve altercations in gene expression occur slowly. C. Modification of existing cell components can happen quickly, whereas responses that depend on changes in gene expression that take much longer, because the genes will need to be transcribed the mRNAs will need to be translated, and the proteins need to accumulate to high-enough level to instigate change.

Your friend is studying a dement of a newly discovered virus that carries an enhancer of gene expression that confers responsiveness to glucocorticoid on genes that are linked to it. He constructs two versions of a reporter gene: one has only a minimal promoter linked to it: the other reporter gene has both this minimal promoter plus the viral enhancer attached to it. The reporter gene allows him to measure the amount of transcription that occurs from each construct. Your friend puts each of these constructs into two different cell lines and examines the expression of the reporter gene in each cell line, as shown in Figure Q16-25. He is puzzled by these findings and asks for your help in interpreting them. A. From the data, can you tell whether both cell lines contain glucocorticoid receptors? Why? B. What might account for the difference in the transcription of the reporter gene in cell lines 1 and 2 after introduction of the construct containing the viral enhancer in the presence of glucocorticoid?

A. Both cell lines seem to contain the glucocorticoid receptor, because both cell lines demonstrate increases in reporter gene expression in response to the addition of glucocorticoid. With the introduction of the construct containing the glucocorticoid-responsive viral enhancer, cell line 1 showed a 1000-fold increase and cell line 2 showed an 80-fold increase. B. There are four reasonable explanations for this difference i glucocorticoid responses in the two cell lines. 1. Cell line 1 contains a protein that does not exist in cell line 2. This protein cooperates with the glucocorticoid receptor to activate the transcription of the reporter gene, leading to higher levels of the reporter gene in cell line 1 than in cell line 2. 2. Cell line 2 contains a slightly defective glucocorticoid receptor that does not bind DNA as strongly as the normal glucocorticoid receptor in cell line 1, thus leading to lower levels of reporter fine activity in cell line 2 than in cell line 1. 3. Cell line 1 contains a mutant glucocorticoid receptor that binds DNA much more strongly than the normal glucocorticoid receptor found in cell line 2. Therefore, higher levels of reporter fine activity are seen in cell line 1 than in cell line 2. 4. Cell line 2 contains a protein that does not exist in cell line 1. This protein acts as an antagonist to the glucocorticoid receptor to decrease the level of transcription from the reporter gene, leading to lower levels of reporter gene expression in cell line 2 than in cell line 1.

Circle the phrase in each pair that is likely to occur more rapidly in response to an extracellular signal. A. changes in cell secretion/ increased cell division B. changes in protein phosphorylation/ changes in proteins being synthesized C. changes in mRNA levels/ changes in membrane potential

A. Changes in cell secretion B. Changes in protein phosphorylation C. Changes in membrane potential

Yes or no? A. An extracellular signaling molecule binds and activates a GPCR. B. The activated GPCRs cause Gα to separate from Gβ and Gγ. C. Adenylyl cyclase produces cyclic AMP. D. cAMP activates protein kinase A. E. Protein kinase A phosphorylates target proteins.

A. No. Each signaling molecule activates only one receptor molecule. B. Yes. Each activated GPCR activates many G-protein molecules. C. Yes. Each activated adenylyl cyclase molecule can generate many molecules of cAMP. D. No. In unstimulated cells, proteins kinase A is held inactive in a protein complex. Binding of cAMP to the complex induces a conformational change, releasing the active protein kinase A. Therefore, one cAMP cannot activate more than one molecule of protein kinase A. E. Yes. Each activated protein kinase A molecule can phosphorylate many molecules of each type of target protein.

When the neurotransmitter acetylcholine is applied to skeletal muscle cells, it binds the acetylcholine receptor and causes the muscle cells to contract. Succinycholine, which is a chemical analog of acetylcholine, binds to the acetylcholine receptor on skeletal muscle cells but causes the muscle cells to relax; it is therefore often used by surgeons as a muscle relaxant. Propose a model for why succinylcholine causes muscle relaxation. What might be the mechanism to explain the different activites of acetylcholine and succinylcholine on the acetylcholine receptor?

Although succinylcholine can bind to the acetylcholine receptor, it does not activate the receptor and therefore does not cause the muscle cell to contract. Instead, succinylcholine blocks the ability of acetylcholine to bind to the receptor and thereby prevents acetylcholine from stimulating muscle contraction.

Acetylcholine is a signaling molecule that elicits responses from heart muscle cells, salivary gland cells, and skeletal muscle cells. Which of the following statements is false? a) Heart muscle cells decrease their rate and force of contraction when they receive acetylcholine, whereas skeletal muscle cells contract. b) Heart muscle cells, salivary gland cells and skeletal muscle cells all express an acetylcholine receptor that belongs to the transmitter-gated ion channel family. c) Active acetylcholine receptors on salivary gland cells and heart muscle cells activate different intracellular signaling pathways. d) Heart muscle cells, salivary gland cells, and skeletal muscle cells all respond to acetylcholine within minutes of receiving the signal.

b) Heart muscle cells, salivary glad cells, and skeletal muscle cells all express an acetylcholine receptor that belongs to the transmitter-gated ion channel family.

Cell lines A and B both survive in tissue culture containing scrum but do not proliferate. Factor F is known to stimulate proliferation in cell line A. Cell line A produces a receptor protein (R) that cell line B does not produce. To test the role of receptor R, you introduce this receptor protein into cell line B, using recombinant DNA techniques. You then test all of your various cell lines in the presence of scum for their response to factor F, with the results summarized in Table Q16-1. Which of the following cannot be concluded from your results above?

b) Receptor R binds to factor F to induce cell proliferation in cell line A.

Which of the following statements is true? a) Because endocrine signals are broadcast throughout the body, all cells will respond to the hormone signal. b) The regulation of inflammatory responses at the site of an infection is an example of paracrine signaling. c) Paracrine signaling involves the secretion of signals in to the bloodstream for distribution throughout the organism. d) The axons of neurons typically signal target cells using membrane-bound signaling molecules that act on receptors in the target cells.

b) The regulation of inflammatory responses at the site of an infection is an example of paracrine signaling.

During nervous-system development in Drosophila, the membrane-bound protein Delta acts as an inhibitory signal to prevent neighboring cells from developing into neuronal cells. Delta is involved in ______ signaling.

d) Contact-Dependent

Which of the following statements is false? a) Nucleotides and amino acids can act as extracellular signal molecules. b) Some signal molecules can bind directly to intracellular proteins that bind DNA and regulate gene transcription. c) Some signal molecules are transmembrane proteins. d) Dissolved gases such as nitric oxide (NO) can act as signal molecules, but because they cannot interact with proteins they must act by affecting membrane lipids.

d) Dissolved gases such an nitric oxide (NO) can act as signal molecules, but because they cannot interact with proteins they must act by affecting membrane lipids.