Question 1

How does activation of protein kinase B promote cell survival?

Accepted Answer

The answer of How does activation of protein kinase B...

Question 2

An SH2-containing protein contains a mutation that changes its binding pocket such that tyrosine and phosphotyrosine bind with equal affinity.As a result,MEK activity:

Accepted Answer

The mutation in the SH2-containing protein alters the binding pocket such that both tyrosine and phosphotyrosine bind with equal affinity. However, this does not necessarily mean that MEK activity is affected. MEK is downstream of the receptor activation and is activated by phosphorylation by Raf. The mutation in the SH2-containing protein only affects receptor dimerization and transautophosphorylation, which leads to downstream signaling. Therefore, MEK activity does not change as a result of this mutation. Option D correctly states this fact.

Question 3

Many kinases,including MAP kinase,protein kinase B,receptor tyrosine kinases,and JAKs,possess activation lips.What is an activation lip? What modification of the activation lip is required for activation of these kinases?

Accepted Answer

The answer of Many kinases,including MAP kinase,protein kinase B,receptor tyrosine...

Question 4

Using a luciferase reporter system in tissue culture cells,researchers from the University of California,San Francisco,found that ethanol stimulates transcription of genes in brain cells possibly involved in the adaptive responses to alcohol.This process was found to depend on PKA activation.By analogy to other PKA-dependent transcription activation pathways,describe a possible pathway for this transcription induction.What other proteins would be involved?

Accepted Answer

The answer of Using a luciferase reporter system in tissue...

Question 5

Which of the following are enzyme pairs that catalyze opposite reactions?

Accepted Answer

PI-3 kinase and PTEN phosphatase catalyze opposite reactions in the phosphoinositide 3-kinase (PI3K) signaling pathway. PI-3 kinase adds a phosphate group to phosphatidylinositol (4,5)-bisphosphate (PIP2) to generate phosphatidylinositol (3,4,5)-trisphosphate (PIP3), while PTEN removes the phosphate group from PIP3 to regenerate PIP2. The other enzyme pairs listed do not catalyze opposite reactions.

Question 6

Many PTB-containing proteins act as docking sites for multiple proteins.If these proteins are involved in the RTK signal transduction pathway,they most likely:

Accepted Answer

The answer of Many PTB-containing proteins act as docking sites...

Question 7

Which of the following signaling pathways can be activated by cytokines?

Accepted Answer

Cytokines can activate multiple signaling pathways, including JAK/STAT, PI-3 kinase, and Ras/MAP kinase, making all of the options correct.

Question 8

Which protein stabilizes an intermediate in the Ras GTP hydrolysis reactions?

Accepted Answer

Sos (Son of Sevenless) is a guanine nucleotide exchange factor that stabilizes the intermediate in the Ras GTP hydrolysis reaction by catalyzing the exchange of GDP for GTP on Ras. This leads to the activation of downstream signaling pathways. GRB2 (Growth Factor Receptor-bound protein 2), RTK (Receptor Tyrosine Kinase), and Raf (Rapidly Accelerated Fibrosarcoma) are also involved in the Ras signaling pathway but do not directly stabilize the intermediate in the GTP hydrolysis reaction.

Question 9

How can multiple MAP kinase pathways be segregated when they share a common component,like a downstream kinase?

Accepted Answer

Pathway-specific complexes assembled on molecular scaffolds can segregate multiple MAP kinase pathways even if they share a common downstream kinase because each complex would be specific to a particular upstream kinase, allowing for pathway specificity. This mechanism has been observed in the assembly of the ERK1/2/MAPK pathway. Options B, C, and D are unlikely to segregate MAP kinase pathways that share a downstream kinase because they do not provide a mechanism for specificity.

Question 10

Explain why,upon ligand binding,cell-surface receptors are often subjected to receptor-mediated endocytosis.

Accepted Answer

The answer of Explain why,upon ligand binding,cell-surface receptors are often...

Question 11

All genes regulated by PKA contain a cis-acting DNA sequence that binds to the phosphorylated form of a transcription factor called:

Accepted Answer

Genes regulated by PKA contain a cis-acting DNA sequence called the cAMP response element (CRE), which binds to the phosphorylated form of the transcription factor CREB (cAMP response element binding protein). Therefore, the correct choice is B, CREB. The other options (A) β-catenin, (C) c-Jun, and (D) TCF, are transcription factors that are not specifically associated with PKA-mediated gene regulation.

Question 12

How can multiple MAP kinase pathways be segregated when they share a common component?

Accepted Answer

The answer of How can multiple MAP kinase pathways be...

Question 13

Predict the consequences of a temperature-sensitive mutation in which phosphorylation of the β-catenin protein is blocked.Above the permissive temperature:

Accepted Answer

Phosphorylation of β-catenin targets it for degradation. If phosphorylation is blocked, β-catenin will not be degraded, leading to increased levels and constitutive activity, as it can then enter the nucleus and activate transcription of target genes.

Question 14

In the NF-B signaling pathway,which of the following molecules is downstream of I-B kinase?

Accepted Answer

The I-B kinase (IKK) phosphorylates I-B proteins, which leads to I-B degradation and releases NF-B from inhibition, allowing it to translocate into the nucleus and activate target genes. Therefore, NF-B is downstream of I-B kinase in the signaling pathway. TAK1 and TNF-α are upstream regulators of the pathway.

Question 15

Transcriptional activation downstream of the MAPK pathway involves:

Accepted Answer

The answer of Transcriptional activation downstream of the MAPK pathway...

Question 16

Which of the following is NOT true about the role of adapter proteins in the activation of Ras by receptor tyrosine kinases?

Accepted Answer

Sos does not act as a GAP (GTPase-activating protein) to convert Ras·GDP to active Ras·GTP. Rather, Sos acts as a guanine nucleotide exchange factor (GEF) to facilitate the exchange of GDP for GTP on Ras, leading to its activation.

Question 17

Before ligand binding,receptor tyrosine kinases:

Accepted Answer

Receptor tyrosine kinases (RTKs) contain activation lip tyrosines within the kinase active site that need to be phosphorylated to activate their kinase activity. Ligand binding induces dimerization of RTKs, which brings the two kinase domains in close proximity, allowing them to trans-phosphorylate each other's activation lip tyrosines and activate their kinase activity. Before ligand binding, RTKs are in an inactive state. They may contain binding sites for SH2-domain containing proteins and can bind to GRB2 within the cytosolic domains, but these are not unique features of inactive RTKs.

Question 18

By what mechanism does PI-3 phosphate promote activation of protein kinase B (PKB)?

Accepted Answer

PI-3 phosphate promotes the activation of protein kinase B (PKB) by recruiting PKB to the plasma membrane, recruiting the activating kinase PDK1 to the plasma membrane, and releasing inhibition of the catalytic site by the PH domain. All these mechanisms work together to facilitate the activation of PKB.

Question 19

Which of the following explains why Ras is activated quickly by RTKs?

Accepted Answer

Ras is maintained at the plasma membrane through a lipid-mediated attachment, which allows it to quickly interact with RTKs (Receptor Tyrosine Kinases) upon their activation. This proximity to RTKs facilitates its rapid activation. Ras itself is not directly phosphorylated by RTKs, nor does it bind phosphotyrosine residues on RTKs. Its activation involves the exchange of GDP for GTP, which is catalyzed by guanine nucleotide exchange factors (GEFs) activated by RTK signaling.

Question 20

The order of events in a signaling pathway can be determined by the analysis of mutants.Cells that express a mutant defective Raf protein cannot be stimulated to proliferate uncontrollably by constitutively active Rasᴰ (dominant active).This indicates:

Accepted Answer

Ras being upstream of Raf in the signaling pathway is indicated by the fact that cells with a defective Raf cannot be stimulated by an active Ras to proliferate uncontrollably. This suggests that Ras's activity is dependent on Raf functioning properly downstream in the pathway for the signal to continue, leading to cell proliferation.

Quiz 15: Signaling Pathways That Control Gene Activity