Question 1

Which of the following best describes the initial event in G-protein signaling after a ligand binds to its receptor?

Accepted Answer

The G-protein exchanges GDP for GTP, becoming activated.

Question 2

What is the role of phospholipase C in the G-protein signaling pathway?

Accepted Answer

It cleaves the phosphate head off a membrane lipid, generating DAG and IP3.

Question 3

Which molecule acts as a second messenger to open calcium channels in the endoplasmic reticulum?

Accepted Answer

IP3

Question 4

After IP3 binds to its receptor on the ER, what is the immediate cellular consequence?

Accepted Answer

Release of $$\mathrm{Ca$$^{2+}$$}$$ ions into the cytoplasm

Question 5

Which of the following best explains how increased cytoplasmic $$\mathrm{Ca$$^{2+}$$}$$ can lead to muscle contraction?

Accepted Answer

Calcium binds to calmodulin, activating a kinase cascade that ultimately leads to muscle contraction.

Question 6

Which of the following is the correct sequence of events in the G-protein signaling pathway described?

Accepted Answer

Ligand binds receptor → G-protein activation → Phospholipase C activation → IP3 and DAG production → $$\mathrm{Ca$$^{2+}$$}$$ release

Question 7

A mutation prevents IP3 from binding to its receptor on the ER. Which downstream effect would be most directly impaired?

Accepted Answer

Increase in cytoplasmic $$\mathrm{Ca$$^{2+}$$}$$

Question 8

Which molecule remains in the membrane after phospholipase C cleaves the phosphate head from the lipid?

Accepted Answer

DAG

Question 9

In a patient with a genetic defect in calmodulin, which cellular process would be most affected?

Accepted Answer

Activation of a parallel kinase cascade in response to $$\mathrm{Ca$$^{2+}$$}$$ signaling

Question 10

Which of the following best describes the role of DAG in the G-protein signaling pathway?

Accepted Answer

It activates kinases that propagate the signal.

Question 11

A researcher adds a drug that blocks the release of $$\mathrm{Ca$$^{2+}$$}$$ from the ER. Which of the following downstream effects would most likely be observed?

Accepted Answer

Reduced activation of calmodulin-dependent kinases

Question 12

Which of the following is a possible outcome of G-protein signaling through the pathway described?

Accepted Answer

Muscle contraction

Question 13

If a cell is exposed to a toxin that prevents GTP from binding to the G-protein, what is the most immediate effect on the signaling pathway?

Accepted Answer

Phospholipase C will not be activated.

Question 14

Which of the following best explains why IP3 is considered a 'second messenger' in this pathway?

Accepted Answer

It is produced inside the cell in response to an external signal and relays the signal by binding to ER receptors.

Question 15

A scientist observes that a cell has high levels of DAG but no increase in cytoplasmic $$\mathrm{Ca$$^{2+}$$}. $$Which step in the pathway is most likely defective?

Accepted Answer

IP3 binding to its receptor on the ER

Question 16

Which of the following best describes the relationship between DAG and IP3 in the G-protein signaling pathway?

Accepted Answer

Both are produced from the same lipid by the action of phospholipase C.

Question 17

Which of the following is NOT a direct effect of increased cytoplasmic $$\mathrm{Ca$$^{2+}$$} in $$this pathway?

Accepted Answer

Opening of $$\mathrm{Ca$$^{2+}$$}$$ channels in the ER

Question 18

A drug that mimics the action of IP3 would most likely cause which of the following?

Accepted Answer

Increased release of $$\mathrm{Ca$$^{2+}$$}$$ from the ER

Question 19

Which of the following best explains why G-protein signaling can lead to changes in gene transcription?

Accepted Answer

The signaling cascade activates kinases that can enter the nucleus and regulate transcription factors.

G-Protein Signaling Pathways in Cell Biology

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