#Question id: 1255
#Unit 4. Cell Communication and Cell Signaling
Following statements are regarding to the mechanism of action of cholera toxin.
A. cholera toxin is a homodimeric protein, target G proteins, interfering with normal signaling in host cells.
B. cholera toxin associates with a small G protein, known as ARF6.
C. association with ARF6 activates cholera toxin, which catalyzes the transfer of ADP-ribose from NAD+ to the critical Arg residue in the P loop of the alpha subunit of Gs.
D. ADP-ribosylation blocks the GEF activity of Gs and thereby renders Gs permanently inactive.
E. This results in continuous activation of the adenylyl cyclase of intestinal epithelial cells, chronically high cAMP, and active PKA dephosphorylates the CFTR Cl- channel and a sodium-proton exchanger in the intestinal epithelial cells.
Which of the following combination is correct?
#Question id: 1256
#Unit 4. Cell Communication and Cell Signaling
Scaffolds for MAP kinase pathways are well documented in yeast, fly, and worm cells, but their presence in mammalian cells has been difficult to demonstrate. Perhaps the best documented scaffold protein in metazoans is Ksr (kinase suppressor of Ras).
A. Ksr functions as a molecular scaffold by binding several signaling components of the MAP kinase cascade, including both MEK and MAP kinase; and thus can enhance MAP kinase activation by regulating the efficiency of these interactions.
B. Ksr function as a protein kinase which constitutive activate RTK signalling pathway.
C. Ksr function as a receptor proteins which inhibits RTK signalling pathway.
D. Ksr function as a protein phosphatase which inhibits RTK signalling pathway.
Which of the following statement is correct about Ksr?
#Question id: 1257
#Unit 4. Cell Communication and Cell Signaling
Following statements are regarding to protein kinase A which phosphorylates multiple intracellular target proteins expressed in different cell types.
A. Inactive PKA is a tetramer consisting of two regulatory (R) subunits and two catalytic (C) subunits.
B. Each R subunit contains a pseudo-substrate domain whose sequence resembles that of a peptide substrate and binds to the active site in the catalytic domain but is not phosphorylated; thus the pseudo-substrate domain inhibits the activity of the catalytic subunits.
C. active PKA is turned off by binding of cAMP Each R subunit has two distinct cAMP-binding sites, called CNB-A and CNB-B.
D. Binding of cAMP by an R subunit of PKA occurs in a cooperative fashion; that is, binding of the first cAMP molecule to CNB-B increases the Kd for binding of the second cAMP to CNB-A. Thus small changes in the level of cytosolic cAMP can cause proportionately large changes in the number of dissociated C subunits and, hence, in cellular kinase activity.
Which of the following statements are incorrect?
#Question id: 1258
#Unit 4. Cell Communication and Cell Signaling
Following statements are regarding to the desensitization mechanism of GPCR.
A. Several residues in the cytosolic domain of the β-adrenergic receptor are phosphorylated by the enzyme rhodopsin kinase.
B. β-Arrestin binds to specific phosphorylated tyrosine residues in the C-terminal segment of G protein–coupled receptors (GPCRs).
C. Clathrin and AP2, two other proteins bound by β-arrestin, promote endocytosis of the receptor.
D. β-Arrestin also functions in transducing signals from activated receptors by binding to and activating several cytosolic protein kinases like Src activates the MAP kinase pathway, leading to phosphorylation of key transcription factor.
Which of the following combination is incorrect?
#Question id: 1259
#Unit 4. Cell Communication and Cell Signaling
Following statements are regarding to the phosphoinositide pathway.
A. many activated RTKs and cytokine receptors initiate phosphoinositide pathway by recruiting the enzyme phosphatidylinositol-3 kinase (PI-3 kinase) to the membrane.
B. PI-3 kinase adds a phosphate to the 3′ carbon in one of two separate phosphatidylinositol substrates, leading to formation of two phosphatidylinositol 3-phosphates: PI(3,4)P2 or I(3,4,5)P3
C. PI-3 kinase pathway can trigger cell division and prevent apoptosis, thus ensuring cell survival by the activation of PKB.
D. Activated PKB phosphorylates FOXO3a, which induces the expression of several antiapoptotic genes.
Which of the following combination is correct?
#Question id: 1260
#Unit 4. Cell Communication and Cell Signaling
Which of the following statement regarding RTK/Ras pathway is incorrect?