The CAMP-PKA-CREB pathway regulates many important biological processes, from hormone synthesis to inducing long-term memory in the brain. The following statements describe the effects of mutations in the components of the pathway on gene transcription by CREB. A. Loss of function mutation in a cAMP binding site of the PKA regulatory subunit leads to the inactivation of gene expression. B. Activating mutation in the GTP-binding domain of the a subunit of Gs leads to the activation of gene expression. C. Inactivating mutation that prevents the regulatory subunit of PKA to bind the catalytic subunit leads to the activation of gene expression. D. Inactivating mutation in the PKA phosphorylation site of CREB leads to the activation of gene expression. Which one of the following statements is INCORRECT?
Only D
Understanding the CAMP-PKA-CREB Pathway
The CAMP-PKA-CREB pathway is a crucial signaling route involved in various cellular processes, including gene regulation. Let's break down how this pathway normally works and then analyze the effects of mutations in its components.
Normally, the pathway proceeds as follows:
- Binding of an external signal (like a hormone) to a G protein-coupled receptor activates the Gs protein.
- The activated Gs alpha subunit binds to and activates Adenylyl Cyclase.
- Adenylyl Cyclase converts ATP into cyclic AMP (cAMP).
- cAMP binds to the regulatory subunits of Protein Kinase A (PKA).
- Binding of cAMP causes the release of the catalytic subunits of PKA, activating PKA.
- Active PKA catalytic subunits phosphorylate target proteins, including the transcription factor CREB (cAMP Response Element-Binding protein).
- Phosphorylated CREB binds to specific DNA sequences called CREs (cAMP Response Elements) in the promoter regions of genes.
- Binding of phosphorylated CREB recruits co-activators, leading to the activation of gene transcription.
Now, let's examine the given statements about mutations in this pathway and their effect on gene transcription by CREB.
Analyzing Statement A: PKA Regulatory Subunit Mutation
Statement A describes a loss of function mutation in a cAMP binding site of the PKA regulatory subunit. The regulatory subunit normally binds cAMP to release the catalytic subunit. If the cAMP binding site is mutated and cannot bind cAMP, the regulatory subunit will remain bound to the catalytic subunit.
- Effect on PKA: PKA remains in its inactive state because the catalytic subunits are held by the regulatory subunits and cannot be released and activated by cAMP.
- Effect on CREB: Since PKA is inactive, it cannot phosphorylate CREB.
- Effect on Gene Expression: Unphosphorylated CREB does not efficiently bind co-activators and activate gene transcription. Therefore, this mutation would lead to the inactivation of gene expression by CREB.
Based on this analysis, Statement A seems to be a correct description of the outcome of such a mutation.
Analyzing Statement B: Gs Alpha Subunit Mutation
Statement B describes an activating mutation in the GTP-binding domain of the alpha subunit of Gs. The Gs alpha subunit is normally active when bound to GTP and inactive when GTP is hydrolyzed to GDP. An activating mutation in the GTP-binding domain means the Gs alpha subunit stays bound to GTP and remains constitutively active.
- Effect upstream: Active Gs alpha continuously activates Adenylyl Cyclase.
- Effect on cAMP: This leads to constantly elevated levels of cAMP.
- Effect on PKA: High cAMP levels continuously activate PKA by binding to the regulatory subunits and releasing the catalytic subunits.
- Effect on CREB: Active PKA continuously phosphorylates CREB.
- Effect on Gene Expression: Phosphorylated CREB activates gene transcription. Therefore, this mutation would lead to the activation of gene expression by CREB.
Based on this analysis, Statement B seems to be a correct description of the outcome of such a mutation.
Analyzing Statement C: PKA Catalytic Subunit Binding Mutation
Statement C describes an inactivating mutation that prevents the regulatory subunit of PKA from binding the catalytic subunit. The regulatory subunit normally keeps the catalytic subunit inactive by binding to it.
- Effect on PKA: If the regulatory subunit cannot bind the catalytic subunit, the catalytic subunit is effectively always free and active, regardless of cAMP levels.
- Effect on CREB: Active PKA continuously phosphorylates CREB.
- Effect on Gene Expression: Phosphorylated CREB activates gene transcription. Therefore, this mutation would lead to the activation of gene expression by CREB.
Based on this analysis, Statement C seems to be a correct description of the outcome of such a mutation.
Analyzing Statement D: CREB Phosphorylation Site Mutation
Statement D describes an inactivating mutation in the PKA phosphorylation site of CREB. CREB needs to be phosphorylated by PKA at a specific site (Serine 133 in mammals) to become transcriptionally active and recruit co-activators.
- Effect on CREB: Even if PKA is active (due to high cAMP), it cannot phosphorylate CREB if the phosphorylation site is mutated and non-functional.
- Effect on Gene Expression: Unphosphorylated CREB does not efficiently activate gene transcription, even if it can bind to the CRE element. Therefore, this mutation would lead to the inactivation (or significantly reduced activation) of gene expression by CREB.
Statement D says this mutation leads to the activation of gene expression, which contradicts our analysis. This statement is INCORRECT.
Identifying the Incorrect Statement
Comparing our analysis with the statements:
- Statement A: Leads to inactivation (Statement says inactivation) - Correct.
- Statement B: Leads to activation (Statement says activation) - Correct.
- Statement C: Leads to activation (Statement says activation) - Correct.
- Statement D: Leads to inactivation (Statement says activation) - Incorrect.
The statement that is INCORRECT is Statement D.
Important Questions from Cellular communication
In a neuron, proteins and membranes are primarily synthesized in the cell body. These materials must be transported down the axon to the synaptic region using microtubules in an anterograde fashion. Such axonal transport is directed by
Acetylcholine is a potent neurotransmitter, which is released from the neurons. After release they diffuse across the synaptic cleft and combine with nicotinic acetylcholine receptor molecules in the membrane of the postsynaptic cell. The interaction of acetylcholine with the nicotinic acetylcholine receptor produces large transient increase in the permeability of the membrane to specialized ions resulting in signal transduction for nerve impulse. Acetylcholine receptor is a
Column X represents the type of junctions and column Y represents the proteins associated with the junctions.
Column X
Column Y
A.
Anchoring junction
i.
Claudins
B.
Occluding junction
ii.
Delta-Notch
C.
Channel-forming junction
iii.
Desmoglein
D.
Signal-relaying junction
iv.
Connexin
Which one of the following options is a correct match between terms of Columns X and Y?
Bacteria employ various mechanisms to invade or enter host cells, which are either phagocytic or non-phagocytic in nature. Given below are some of the mechanisms which are generally used for carrying out this process.
A. Some bacteria express a protein called invasin that is recognized by host-cell β1 integrins.
B. Actin polymerization along with assembly of clathrin coat results in the internalization of bacteria by zipper mechanism.
C. Some bacteria, including Salmonella enterica, use trigger mechanism to inject a set of effector molecules in the cytosol through type III secretion system.
D. Some bacteria attach to host cell surface receptors inducing local elevation of Ca2+ in cell cytosol, leading to the fusion of lysosomes with bacteria containing plasma membrane vesicles.
Which one of the following represents the combination of correct mechanisms for invading non-phagocytic cells?
In C. elegans, PAR proteins segregate at the cell cortex in the zygote to establish cell polarity. This is dependent on the regulation of the cortical actin cytoskeleton by RhoA. An investigator sought to directly inhibit actin polymerization to analyze the impact of this inhibition on PAR protein localization. Which one of the following chemicals would be the most suitable?
