Review Questions for Exam #2

Which type of lipids are the most abundant in the plasma membrane? (a) phospholipids (b) glycolipids (c) sterols (d) triacylglycerides

(a) phospholipids

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. Choice (a) is false. GTP-binding proteins themselves hydrolyze their bound GTP to GDP, using their own intrinsic GTPase activity.

(a) Phosphatases remove the phosphate from GTP on GTP-binding proteins, turning them off because GTP-binding proteins themselves hydrolyze their bound GTP to GDP, using their own intrinsic GTPase activity.

11. 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 because hyperactivating cyclic AMP phosphodiesterase will degrade the cAMP, terminating the signal more quickly than usual. All other answers will lead to larger cells.

Membrane lipids are capable of many different types of movement. Which of these does not occur spontaneously in biological membranes? (a) switching between lipid layers (flip-flop) (b) lateral movement (c) rotation (d) flexing of hydrocarbon chains

(a) switching between lipid layers (flip-flop)

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.

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. PI 3-kinase activity causes the localization of Akt to the plasma membrane, where it is phosphorylated by another protein kinase.

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

Where does most new membrane synthesis take place in a eukaryotic cell? (a) in the Golgi apparatus (b) in the endoplasmic reticulum (c) in the plasma membrane (d) in the mitochondria (e) on ribosomes

(b) in the endoplasmic reticulum

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.

We can estimate the relative mobility of a population of molecules along the surface of a living cellby fluorescently labeling the molecules of interest, bleaching the label in one small area, and then measuring the speed of signal recovery as molecules migrate back into the bleached area. What is this method called? What does the abbreviation stand for? (a) SDS (b) SPT (c) GFP (d) FRAP

(d) FRAP

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.

Male cockroaches with mutations that strongly decrease the function of an RTK called RTKX are oblivious to the charms of their female comrades. This particular RTK binds to a small molecule secreted by sexually mature females. Most males carrying loss-of-function mutations in the gene for Ras protein are also unable to respond to females. You have just read a paper in which the authors describe how they have screened cockroaches that are mutant in RTKX for additional mutations that partly restore the ability of males to respond to females. These mutations decrease the function of a protein that the authors call Z. Which of the following types of protein could Z be? Explain your answer. (a) a protein that activates the Ras protein by causing Ras to exchange GDP for GTP (b) a protein that stimulates hydrolysis of GTP by the Ras protein (c) an adaptor protein that mediates the binding of the RTKX to the Ras protein (d) a transcriptional regulator required for the expression of the Ras gene

A protein that stimulates hydrolysis of GTP by the Ras protein Mutations that increase the activity of Ras should mimic the effect of stimulating RTKX in a receptor-independent fashion. Because the intracellular concentration of GTP is higher than that of GDP, some proportion of the Ras molecules is expected to be GTP-bound and active; ridding the cells of a protein that stimulates GTP hydrolysis will increase this pool of active Ra

Cholera toxin

Inhibits the activity of GTPase in the alpha subunit to keep the subunit in an active state

Activated GPCRs activate G proteins by reducing the strength of binding of GDP to the α subunit of the G protein, allowing GDP to dissociate and GTP (which is present at much higher concentrations in the cell than GDP) to bind in its place. How would the activity of a G protein be affected by a mutation that reduces the affinity of the α subunit for GDP without significantly changing its affinity for GTP?

The G protein would be constantly active since GTP would constantly bind and react with the alpha subunit. When GTP was hydrolyzed to GDP, GDP would quickly be released and GTP would bind again.

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

protein tyrosine phosphatase that normally removes the phosphates from tyrosines on the activated receptor. RTKs are usually activated by signal-induced dimerization, which allows the receptors to phosphorylate themselves and activate intracellular signaling proteins that are stimulated by the phosphorylated receptor. After it is activated, the receptor is dephosphorylated, and thereby inactivated, by a protein tyrosine phosphatase. Therefore, a mutation in the gene that encodes the protein tyrosine phosphatase will inappropriately increase the activity of the receptor and promote uncontrolled cell proliferation. Mutations that prevent dimerization of the receptor (including mutations that prevent ligand binding) or autophosphorylation (which requires the kinase activity of the receptor) will inactivate the receptor.