Select two signal mechanisms that activate transcription in response to an external signal. Any of these systems ultimately activates a transcription factor. For each of the systems you have selected, trace the route from signal binding to receptor to the final transcription factor activation. For each of the systems you have chosen, indicate one mutation that would result in failure to respond to the signal (here, we will assume that signal-receptor binding is intact because that possibility is just too easy) and one mutation that would result in constitutive activation of transcription.
Two signal mechanisms that activate transcription in response to an external signal are the G protein-coupled receptor (GPCR) signaling pathway and the receptor tyrosine kinase (RTK) signaling pathway.
1. GPCR Signaling Pathway:
- Signal Binding to Receptor: When an external signal binds to the GPCR on the cell surface, the receptor undergoes a conformational change.
- G Protein Activation: This conformational change leads to the activation of a G protein associated with the receptor.
- Activation of Effector Enzyme: The activated G protein activates an effector enzyme, such as adenylyl cyclase or phospholipase C, by binding to it.
- Production of Second Messengers: The effector enzyme produces second messengers, such as cAMP or inositol trisphosphate (IP3), respectively.
- Protein Kinase Activation: The second messengers activate protein kinases, such as protein kinase A (PKA) or protein kinase C (PKC), respectively.
- Transcription Factor Activation: The activated protein kinases then phosphorylate and activate specific transcription factors, which enter the nucleus and bind to DNA, initiating transcription.
Mutation that would result in failure to respond to the signal: A mutation in the GPCR that prevents G protein activation. Without G protein activation, the downstream signaling cascade will not be initiated, and the transcription factor will not be activated.
Mutation that would result in constitutive activation of transcription: A mutation in the protein kinase that phosphorylates and activates the transcription factor. This mutation could result in the protein kinase being constantly active, leading to continuous phosphorylation and activation of the transcription factor, even in the absence of the external signal.
2. RTK Signaling Pathway:
- Signal Binding to Receptor: The external signal binds to the receptor tyrosine kinase (RTK) on the cell surface, causing the receptor to form dimers or multimers.
- Autophosphorylation: The binding of the signal induces autophosphorylation of tyrosine residues within the RTK.
- Recruitment of Adaptor Proteins: The phosphorylated tyrosine residues serve as binding sites for adaptor proteins.
- Activation of Ras: The adaptor proteins facilitate the activation of Ras, a small GTPase protein.
- Phosphorylation Cascade: Activation of Ras leads to a phosphorylation cascade, involving protein kinases such as Raf, MEK, and ERK.
- Transcription Factor Activation: Ultimately, the activated ERK translocates into the nucleus and phosphorylates specific transcription factors, enabling their binding to DNA and transcriptional activation.
Mutation that would result in failure to respond to the signal: A mutation in the RTK that prevents autophosphorylation of tyrosine residues. This mutation would disrupt the recruitment of downstream signaling molecules and prevent the activation of the transcription factors.
Mutation that would result in constitutive activation of transcription: A mutation in Ras that renders it permanently active. In this case, the downstream signaling cascade would be continuously stimulated, leading to constant phosphorylation and activation of the transcription factors, even in the absence of the external signal.
Two signal mechanisms that activate transcription in response to an external signal are the cAMP-dependent pathway and the MAP kinase pathway. Let's explore the route from signal binding to the activation of transcription factor for each of these systems:
1. cAMP-dependent pathway:
- Signal Binding to Receptor: External signaling molecules, such as hormones or neurotransmitters, bind to their specific cell surface receptors.
- Activation of G Protein-Coupled Receptor: This binding activates the G protein-coupled receptor, which undergoes a conformational change.
- Activation of Adenylate Cyclase: The conformational change in the receptor activates the associated G protein, which then activates adenylate cyclase.
- Production of cAMP: Adenylate cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP).
- Activation of Protein Kinase A (PKA): cAMP binds to and allosterically activates protein kinase A.
- Translocation of PKA to the Nucleus: Activated PKA translocates to the nucleus.
- Phosphorylation of Transcription Factors: PKA phosphorylates specific transcription factors, leading to their activation.
- Transcriptional Activation: The phosphorylated transcription factors bind to specific DNA sequences and activate the transcription of target genes.
Mutation that results in failure to respond to the signal: A mutation in the G protein-coupled receptor that disrupts its conformational change upon signal binding would prevent downstream activation of adenylate cyclase and subsequent transcription factor activation.
Mutation that results in constitutive activation of transcription: A mutation in the adenylate cyclase gene that leads to its permanent activation, regardless of signal binding, would result in constitutive production of cAMP, prolonged activation of PKA, and subsequent constant activation of transcription factors.
2. MAP kinase pathway:
- Signal Binding to Receptor: External growth factors or mitogens bind to their specific cell surface receptors.
- Receptor Tyrosine Kinase Activation: The binding of the ligand induces dimerization and autophosphorylation of receptor tyrosine kinases.
- Recruitment and Activation of Adapter Proteins: Phosphorylated tyrosine residues on the activated receptor recruit adapter proteins.
- Activation of MAP Kinase Cascade: Adapter proteins initiate a series of phosphorylation events, forming a MAP kinase cascade.
- Translocation of MAP Kinase to the Nucleus: The final kinase in the cascade, MAP kinase, becomes phosphorylated and translocates to the nucleus.
- Phosphorylation of Transcription Factors: MAP kinase phosphorylates specific transcription factors to activate them.
- Transcriptional Activation: Phosphorylated transcription factors bind to specific DNA sequences, promoting transcription of target genes.
Mutation that results in failure to respond to the signal: A mutation in the receptor tyrosine kinase domain that prevents autophosphorylation would impair the activation of the entire MAP kinase cascade, leading to a failure in transcription factor activation.
Mutation that results in constitutive activation of transcription: A mutation in MAP kinase gene that results in its permanent activation, independent of upstream signaling events, would lead to constant phosphorylation of transcription factors, causing constitutive activation of transcription.