BIOL 271 MODULE 5 EXAM: ADVANCED
SIGNAL TRANSDUCTION AND
CELLULAR COMMUNICATION
1. Which of the following describes the mechanism by which a G protein-coupled receptor
(GPCR) activates its downstream effector?
A. The receptor undergoes autophosphorylation on tyrosine residues.
B. The activated receptor acts as a guanine nucleotide exchange factor (GEF) for the alpha
subunit.
C. The ligand enters the cell and binds to the alpha subunit directly.
D. The G protein alpha subunit binds to the ligand in the extracellular space.
Answer: B
Conceptual Explanation: GPCRs function by inducing a conformational change that allows
the receptor to act as a GEF, promoting the release of GDP and the binding of GTP on the G-
alpha subunit.
2. In the phospholipase C (PLC) signaling pathway, which second messenger is responsible for
releasing calcium ions from the endoplasmic reticulum?
A. Diacylglycerol (DAG)
,B. Cyclic AMP (cAMP)
C. Inositol 1,4,5-trisphosphate (IP3)
D. Phosphatidylinositol 4,5-bisphosphate (PIP2)
Answer: C
Conceptual Explanation: PLC cleaves PIP2 into DAG and IP3. IP3 then binds to ligand-
gated calcium channels on the ER to release calcium into the cytosol.
3. What is the specific role of the SH2 domain in signaling proteins?
A. To bind to proline-rich sequences in adapter proteins.
B. To catalyze the hydrolysis of GTP to GDP.
C. To bind to phosphorylated tyrosine residues on other proteins.
D. To anchor the protein to the plasma membrane via lipid modification.
Answer: C
Conceptual Explanation: SH2 (Src Homology 2) domains specifically recognize and bind
to phosphorylated tyrosine residues, facilitating the assembly of signaling complexes.
4. Cholera toxin causes massive diarrhea by affecting which signaling component?
A. Inhibiting the G-alpha-q subunit.
B. Blocking the binding of epinephrine to its receptor.
C. Degrading the secondary messenger cAMP.
, D. Chemically modifying the G-alpha-s subunit to lock it in the active state.
Answer: D
Conceptual Explanation: Cholera toxin ADP-ribosylates the G-alpha-s subunit, preventing
GTP hydrolysis. This keeps adenylyl cyclase active, leading to high cAMP levels and fluid
loss.
5. Which of the following is a characteristic feature of Receptor Tyrosine Kinases (RTKs)?
A. They contain seven transmembrane alpha-helices.
B. They function as monomers even after ligand binding.
C. They directly activate protein kinase A (PKA).
D. Ligand binding induces dimerization and trans-autophosphorylation.
Answer: D
Conceptual Explanation: RTKs require ligand-induced dimerization to bring the kinase
domains together for trans-autophosphorylation of their cytoplasmic tails.
6. Ras is a small GTPase that acts downstream of RTKs. What protein is required to deactivate
Ras?
A. Ras-GEF (Sos)
B. Ras-GAP
C. Grb2
D. Raf kinase
SIGNAL TRANSDUCTION AND
CELLULAR COMMUNICATION
1. Which of the following describes the mechanism by which a G protein-coupled receptor
(GPCR) activates its downstream effector?
A. The receptor undergoes autophosphorylation on tyrosine residues.
B. The activated receptor acts as a guanine nucleotide exchange factor (GEF) for the alpha
subunit.
C. The ligand enters the cell and binds to the alpha subunit directly.
D. The G protein alpha subunit binds to the ligand in the extracellular space.
Answer: B
Conceptual Explanation: GPCRs function by inducing a conformational change that allows
the receptor to act as a GEF, promoting the release of GDP and the binding of GTP on the G-
alpha subunit.
2. In the phospholipase C (PLC) signaling pathway, which second messenger is responsible for
releasing calcium ions from the endoplasmic reticulum?
A. Diacylglycerol (DAG)
,B. Cyclic AMP (cAMP)
C. Inositol 1,4,5-trisphosphate (IP3)
D. Phosphatidylinositol 4,5-bisphosphate (PIP2)
Answer: C
Conceptual Explanation: PLC cleaves PIP2 into DAG and IP3. IP3 then binds to ligand-
gated calcium channels on the ER to release calcium into the cytosol.
3. What is the specific role of the SH2 domain in signaling proteins?
A. To bind to proline-rich sequences in adapter proteins.
B. To catalyze the hydrolysis of GTP to GDP.
C. To bind to phosphorylated tyrosine residues on other proteins.
D. To anchor the protein to the plasma membrane via lipid modification.
Answer: C
Conceptual Explanation: SH2 (Src Homology 2) domains specifically recognize and bind
to phosphorylated tyrosine residues, facilitating the assembly of signaling complexes.
4. Cholera toxin causes massive diarrhea by affecting which signaling component?
A. Inhibiting the G-alpha-q subunit.
B. Blocking the binding of epinephrine to its receptor.
C. Degrading the secondary messenger cAMP.
, D. Chemically modifying the G-alpha-s subunit to lock it in the active state.
Answer: D
Conceptual Explanation: Cholera toxin ADP-ribosylates the G-alpha-s subunit, preventing
GTP hydrolysis. This keeps adenylyl cyclase active, leading to high cAMP levels and fluid
loss.
5. Which of the following is a characteristic feature of Receptor Tyrosine Kinases (RTKs)?
A. They contain seven transmembrane alpha-helices.
B. They function as monomers even after ligand binding.
C. They directly activate protein kinase A (PKA).
D. Ligand binding induces dimerization and trans-autophosphorylation.
Answer: D
Conceptual Explanation: RTKs require ligand-induced dimerization to bring the kinase
domains together for trans-autophosphorylation of their cytoplasmic tails.
6. Ras is a small GTPase that acts downstream of RTKs. What protein is required to deactivate
Ras?
A. Ras-GEF (Sos)
B. Ras-GAP
C. Grb2
D. Raf kinase
