Cell Signaling Quiz || Cell Biology Quiz

Advanced Cell Signaling Quiz

1. Which of the following scenarios would most likely lead to a sustained and amplified cellular response via a G-protein coupled receptor (GPCR) pathway, assuming the ligand concentration remains constant?

a) Increased activity of GPCR kinases (GRKs). b) A mutation in the α-subunit of the G-protein that impairs its intrinsic GTPase activity. c) Upregulation of phosphodiesterase activity. d) Increased expression of β-arrestin.

Correct Answer: b) Impaired GTPase activity means the $\alpha$-subunit remains in its active, GTP-bound state for longer, continuously activating effector enzymes and amplifying the downstream signal.

Hint: Consider the mechanisms by which G-proteins are inactivated and how their persistent activation would impact signal duration.

2. A cell line exhibits constitutive activation of the MAP kinase (MAPK) pathway, even in the absence of growth factors. Which of the following mutations would be most consistent with this observation?

a) A loss-of-function mutation in an upstream receptor tyrosine kinase (RTK) gene. b) A gain-of-function mutation in Ras that prevents GTP hydrolysis. c) Overexpression of a specific protein phosphatase that dephosphorylates MEK. d) A dominant-negative mutation in ERK (Extracellular signal-regulated kinase).

Correct Answer: b) Ras is a small G-protein that activates the MAPK pathway when bound to GTP. A mutation preventing GTP hydrolysis would lock Ras in its active state, leading to constitutive MAPK signaling.

Hint: Recall the sequential activation steps in the MAPK cascade and identify which component acts as a molecular switch.

3. Upon activation of a specific GPCR, intracellular calcium levels rise rapidly due to release from the endoplasmic reticulum (ER). This rise is primarily mediated by which second messenger?

a) Cyclic AMP (cAMP) b) Diacylglycerol (DAG) c) Inositol 1,4,5-trisphosphate (IP$_3$) d) Nitric Oxide (NO)

Correct Answer: c) IP$_3$ is produced by phospholipase C activation and binds to IP$_3$ receptors on the ER membrane, triggering the release of stored calcium.

Hint: Consider the products of phospholipase C activation and their direct targets for calcium mobilization.

4. Desensitization of GPCRs often involves phosphorylation by GPCR kinases (GRKs) and subsequent binding of arrestin. What is the primary functional consequence of arrestin binding to the phosphorylated receptor?

a) It enhances the receptor's affinity for its ligand. b) It promotes sustained activation of the associated G-protein. c) It facilitates the recruitment of clathrin and dynamin for receptor endocytosis. d) It directly activates adenylyl cyclase, increasing cAMP production.

Correct Answer: c) Arrestin acts as an adaptor protein that recruits components of the endocytic machinery, like clathrin and dynamin, leading to receptor internalization and removal from the cell surface.

Hint: Think about the fate of a GPCR once it has been phosphorylated by GRKs and bound by arrestin in the context of signal attenuation.

5. Steroid hormones, being lipid-soluble, typically exert their effects by binding to which type of receptor?

a) G-protein coupled receptors (GPCRs) on the cell surface. b) Receptor tyrosine kinases (RTKs) on the cell surface. c) Intracellular receptors in the cytoplasm or nucleus. d) Ligand-gated ion channels.

Correct Answer: c) Steroid hormones, due to their lipophilicity, can readily cross the plasma membrane and bind to receptors located in the cytoplasm or nucleus, acting as transcription factors.

Hint: Consider the chemical nature of steroid hormones and how it dictates their ability to cross the plasma membrane.

6. The PI3K/Akt signaling pathway is crucial for cell survival and growth. A common mechanism of its activation involves phosphorylation of PIP2 to PIP3 by PI3K. Which enzyme acts to reverse this step, thereby negatively regulating the pathway?

a) Phospholipase C (PLC) b) Protein Kinase B (Akt) c) PTEN (Phosphatase and Tensin homolog) d) Adenylyl Cyclase

Correct Answer: c) PTEN is a phosphatase that specifically dephosphorylates PIP3 back to PIP2, thus acting as a crucial tumor suppressor by negatively regulating the PI3K/Akt pathway.

Hint: Think about the role of phosphatases in reversing phosphorylation events and which specific phosphatase is known to counteract PI3K.

7. Which of the following best exemplifies signal amplification in a cellular signaling pathway?

a) A single ligand molecule binding to its receptor activates one effector molecule. b) Activated adenylyl cyclase producing multiple cAMP molecules from a single activated G-protein. c) Phosphorylation of a protein by a kinase, leading to its activation. d) Degradation of a second messenger by a phosphodiesterase.

Correct Answer: b) A single activated adenylyl cyclase enzyme can produce many molecules of cAMP, which in turn can activate many protein kinase A (PKA) molecules, leading to significant signal amplification.

Hint: Consider steps in a signaling pathway where one activated component can generate many molecules of the next component.

8. Nitric oxide (NO) is a crucial signaling molecule involved in vasodilation. Its mechanism of action typically involves:

a) Binding to a cell surface receptor and activating a G-protein cascade. b) Activating protein kinase C (PKC) by increasing diacylglycerol (DAG) levels. c) Directly activating soluble guanylyl cyclase, leading to increased cGMP production. d) Phosphorylating specific tyrosine residues on target proteins.

Correct Answer: c) NO diffuses into target cells and binds to the heme group of soluble guanylyl cyclase, activating it to produce cyclic GMP (cGMP), which then triggers downstream effects like vasodilation.

Hint: Think about the unique chemical nature of nitric oxide and its ability to freely cross cell membranes, as well as its specific intracellular target enzyme.

9. Which of the following scenarios represents a form of negative feedback in a signaling pathway?

a) A growth factor binding to its receptor leading to increased cell proliferation. b) Activation of a kinase that phosphorylates and inactivates an upstream receptor. c) Synthesis of a second messenger that activates multiple downstream effector proteins. d) Binding of a ligand to a receptor causing a conformational change that exposes an active site.

Correct Answer: b) This is a classic example of negative feedback, where a downstream component (kinase) acts back to inhibit an upstream component (receptor), thereby limiting the duration or intensity of the signal.

Hint: Consider mechanisms that lead to the attenuation or termination of a cellular response once the signal has been received and processed.

10. In the context of Wnt signaling, binding of Wnt ligand to its Frizzled receptor and LRP co-receptor leads to the stabilization and nuclear translocation of β-catenin. What is the primary role of β-catenin in the nucleus in this pathway?

a) It functions as a scaffolding protein to assemble a receptor complex. b) It directly dephosphorylates target proteins, altering their activity. c) It acts as a transcription factor or co-activator, regulating gene expression. d) It serves as an ion channel, facilitating ion flux across the nuclear membrane.

Correct Answer: c) Stabilized $\beta$-catenin translocates to the nucleus, where it binds to TCF/LEF transcription factors, converting them into transcriptional activators and thereby regulating the expression of Wnt target genes.

Hint: Consider the ultimate cellular response mediated by Wnt signaling, which often involves changes in cell fate and behavior.