Chapter 16 cell biology

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.

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. (a) endocrine (b) paracrine (c) neuronal (d) contact-dependent

d. contact dependent

All members of the steroid hormone receptor family __________________. (a) are cell-surface receptors. (b) do not undergo conformational changes. (c) are found only in the cytoplasm. (d) interact with signal molecules that diffuse through the plasma membrane.

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

When Ras is activated, cells will divide. A dominant-negative form of Ras clings too tightly to GDP. You introduce a dominant-negative form of Ras into cells that also have a normal version of Ras. Which of the following statements is true? (a) The cells you create will divide less frequently than normal cells in response to the extracellular signals that typically activate Ras. (b) The cells you create will run out of the GTP necessary to activate Ras. (c) The cells you create will divide more frequently compared to normal cells in response to the extracellular signals that typically activate Ras. (d) The normal Ras in the cells you create will not be able to bind GDP because the dominant-negative Ras binds to GDP too tightly.

(a) The cells you create will divide less frequently than normal cells in response to the extracellular signals that typically activate Ras.

These intracellular signaling proteins ultimately change the activity of __________________ proteins that bring about cell responses.

Effectors

Cells can signal to each other in various ways. A signal that must be relayed to the entire body is most efficiently sent by __________________ cells, which produce hormones that are carried throughout the body through the bloodstream.

Endocrine

Some intracellular signaling pathways involve chains of protein kinases that phosphorylate each other, as seen in the __________________ signaling module.

MAP Kinase

Finally, __________________ signaling involves the conversion of electrical impulses into a chemical signal

Neuronal

Cells signal to one another in various ways. Some use extracellular signal molecules that are dissolved gases, such as __________________, which can diffuse easily into cells.

Nitric Oxide

During __________________ signaling, the signal remains in the neighborhood of the secreting cell and thus acts as a local mediator on nearby cells.

Paracrine

Enzymes that phosphorylate proteins, termed ___________, can also serve as molecular switches; the actions of these enzymes are countered by the activity of __________________.

Protein Kinases; Protein Phosphates

Cells receive signals through a __________________, which can be an integral membrane protein or can reside inside the cell.

Receptor

Which of the following statements is true? (a) MAP kinase is important for phosphorylating MAP kinase kinase. (b) PI 3-kinase phosphorylates a lipid in the plasma membrane. (c) Ras becomes activated when an RTK phosphorylates its bound GDP to create GTP. (d) STAT proteins phosphorylate JAK proteins, which then enter the nucleus and activate gene transcription.

b. PI 3-kinase phosphorylates a lipid in the plasma membrane.

Which of the following statements is true? (a) Because endocrine signals are broadcast throughout the body, all cells will respond to the hormonal 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 into 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

Ca2+ can trigger biological effects in cells because an unstimulated cell has an extremely __________________ concentration of free Ca2+ in the cytosol, compared with its concentration in the __________________ space and in the __________________, creating a steep electrochemical gradient.

low; extracellular; endoplasmic reticulum

When Ca2+ enters the cytosol, it interacts with Ca2+-responsive proteins such as __________________, which also binds diacylglycerol, and __________________, which activates CaM-kinases.

protein kinase C; calmodulin

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.

three; two

Intracellular signaling proteins can __________________ the signal received to evoke a strong response from just a few extracellular signal molecules.

Amplify

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.

Contact-Dependent

When unstimulated, the α subunit is bound to __________________, which is exchanged for __________________ on stimulation.

GDP; GTP

__________________ proteins can act as molecular switches, letting a cell know that a signal has been received

GTP-binding

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.

GTPase; Cholera toxin

A cell that receives more than one extracellular signal at the same time can __________________ this information using intracellular signaling proteins.

Integrate

Cytokine receptors have no intrinsic enzyme activity but are associated with cytoplasmic tyrosine kinases called __________________s, which become activated on the binding of cytokine to its receptor and go on to phosphorylate and activate cytoplasmic transcriptional regulators called __________________s.

JAKs; STATS

An extracellular signal molecule can act to change a cell's behavior by acting through cell-surface __________________ that control intracellular signaling proteins.

Receptors

G-protein-coupled receptors (GPCRs) all have a similar structure with __________________ transmembrane domains

Seven

You are interested in cell-size regulation and discover that signaling through a GPCR called ERC1 is important in controlling cell size in embryonic rat cells. The G protein downstream of ERC1 activates adenylyl cyclase, which ultimately leads to the activation of PKA. You discover that cells that lack ERC1 are 15% smaller than normal cells, while cells that express a mutant, constitutively activated version of PKA are 15% larger than normal cells. Given these results, which of the following treatments to embryonic rat cells should lead to smaller cells? (a) addition of a drug that causes cyclic AMP phosphodiesterase to be hyperactive (b) addition of a drug that prevents GTP hydrolysis by Gα (c) addition of a drug that activates adenylyl cyclase (d) addition of a drug that mimics the ligand of ERC1

a. addition of a drug that causes cyclic AMP phosphodiesterase to be hyperactive

Which of the following mechanisms is not directly involved in inactivating an activated RTK? (a) dephosphorylation by serine/threonine phosphatases (b) dephosphorylation by protein tyrosine phosphatases (c) removal of the RTK from the plasma membrane by endocytosis (d) digestion of the RTK in lysosomes

a. dephosphorylation by serine/threonine phosphatases

The activation of the serine/threonine protein kinase Akt requires phosphoinositide 3-kinase (PI 3-kinase) to _________. (a) activate the RTK. (b) create phosphorylated lipids that serve as docking sites that localize Akt to the plasma membrane. (c) directly phosphorylate Akt. (d) to create DAG.

b. create phosphorylated lipids that serve as docking sites that localize Akt to the plasma membrane

When a signal needs to be sent to most cells throughout a multicellular organism, the signal most suited for this is a ___________. (a) neurotransmitter. (b) hormone. (c) dissolved gas. (d) scaffold.

b. hormone

The lab you work in has discovered a previously unidentified extracellular signal molecule called QGF, a 75,000-dalton protein. 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 QGF. (c) QGF 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.

b.Glial cells do not have a receptor for QGF.

The growth factor RGF stimulates proliferation of cultured rat cells. The receptor that binds RGF is a receptor tyrosine kinase called RGFR. Which of the following types of alteration would be most likely to prevent receptor dimerization? (a) a mutation that increases the affinity of RGFR for RGF (b) a mutation that prevents RGFR from binding to RGF (c) changing the tyrosines that are normally phosphorylated on RGFR dimerization to alanines (d) changing the tyrosines that are normally phosphorylated on RGFR dimerization to glutamic acid

b.a mutation that prevents RGFR from binding to RGF

The growth factor Superchick stimulates the proliferation of cultured chicken cells. The receptor that binds Superchick is a receptor tyrosine kinase (RTK), and many chicken tumor cell lines have mutations in the gene that encodes this receptor. Which of the following types of mutation would be expected to promote uncontrolled cell proliferation? (a) a mutation that prevents dimerization of the receptor (b) a mutation that destroys the kinase activity of the receptor (c) a mutation that inactivates the protein tyrosine phosphatase that normally removes the phosphates from tyrosines on the activated receptor (d) a mutation that prevents the binding of the normal extracellular signal to the receptor

c. a mutation that inactivates the protein tyrosine phosphatase that normally removes the phosphates from tyrosines on the activated receptor

The local mediator nitric oxide stimulates the intracellular enzyme guanylyl cyclase by ________________. (a) activating a G protein. (b) activating a receptor tyrosine kinase. (c) diffusing into cells and stimulating the cyclase directly. (d) activating an intracellular protein kinase.

c. diffusing into cells and stimulating the cyclase directly.

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 membranes 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.

A protein kinase can act as an integrating device in signaling if it ___________________. (a) phosphorylates more than one substrate. (b) catalyzes its own phosphorylation. (c) is activated by two or more proteins in different signaling pathways. (d) initiates a phosphorylation cascade involving two or more protein kinases

c. is activated by two or more proteins in different signaling pathways.

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 (d) the GTPase activity of Gα

d. the GTPase activity of Gα

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.

Lipids can also relay signals in the cell, as we observe when phospholipase C cleaves the sugar-phosphate head off a lipid molecule to generate the two small messenger molecules __________________ (which remains embedded in the plasma membrane) and __________________ (which diffuses into the cytosol).

diacylglycerol; IP3