Exam #3 Review

Receptor Degradation and Inactivation

• Signaling Molecule as a Dimer:

  • The signaling molecule itself can be a dimer, pulling two receptor subunits together.

  • This induces a conformational change leading to enzyme activity.

• Receptor Binding and Interaction:

  • Each receptor binds to the signaling molecule, causing a conformational change that allows the receptors to interact.

• Result: Receptor degradation occurs.

Receptor Tyrosine Kinases (RTKs) and Autotransphosphorylation

• Enzymatic Activity:

  • RTKs possess inherent enzymatic activity.

• Autotransphosphorylation:

  • RTKs undergo autotransphosphorylation, activating and phosphorylating themselves.

  • $Autotransphosphorylation$

  • This leads to multiple phosphorylation points.

• Molecular Interactions:

  • Molecules associate with these receptors due to phosphorylation.

  • After enzyme activity, phosphorylation occurs, followed by the formation of a signaling complex through molecular interactions.

Receptor Inactivation Mechanisms

• Tyrosine Phosphatases:

  • Tyrosine phosphatases can be activated to remove phosphate groups from tyrosine residues.

  • Dephosphorylation inactivates the receptors, preventing interaction with other components.

• Ubiquitin Ligases:

  • Specific ubiquitin ligases can ubiquitinate the receptor, leading to downregulation.

  • Example: CBL (an E3 ubiquitin ligase) ubiquitinates the receptor.

• Clathrin-Coated Pits:

  • Ubiquitinated receptors are brought into early endosomes via clathrin-coated pits.

  • The ESCRT (endosomal sorting complex required for transport) process then sends them to the lysosome for degradation.

• Defects in ESCRT Proteins:

  • Defects in ESCRT proteins can lead to prolonged signaling.

• Defects in Ubiquitin Ligase System:

  • Defects in this system can also cause prolonged signaling, potentially leading to cancer.

Implications for Growth Pathways and Cancer

• Downstream Pathways of RTKs:

  • Many downstream pathways of RTKs are growth-related.

• Uncontrolled Cell Proliferation:

  • Issues in growth pathways can lead to uncontrolled cell proliferation and cancer.

Anchoring Proteins and Localization of Enzymes

• ACAPs (A-Kinase Anchoring Proteins):

  • ACAPs tether protein kinase A (PKA) to specific cellular locations.

  • This is an example of an anchoring mechanism.

  • Scaffolding proteins can also act as anchors, such as in the MAP kinase cascade.

• Localization of Other Enzymes:

  • Other enzymes in the cell have proteins that help localize them to specific cellular subdomains.

  • However, specific examples were not discussed in the same detail.

• Protein Kinase C (PKC):

  • PKC localization to the membrane is determined by the presence of diacylglycerol (DAG).

  • DAG is found near PLC activity, which is in proximity to the receptor.

  • No other localization mechanisms for PKC were covered.

• ACAPs and Autophagy:

  • ACAPs may have connections to autophagy, though this was not covered in the lecture.

PI3K-AKT-mTOR Pathway

• Activation of PI3K:

  • Activated RTKs can activate PI3 kinase.

• PI3K Function:

  • PI3K modifies the phosphorylation pattern of phosphatidylinositol phosphates (PIPs).

  • It converts PIP2 to PIP3.
    $PIP<em>2 \rightarrow PIP</em>3$

• PIP3 as a Docking Site:

  • PIP3 serves as a docking site for other proteins, including AKT.

• Activation of AKT:

  • AKT requires phosphorylation for complete activation.

  • mTOR and PDK1 are involved in this phosphorylation.
    $AKT + mTOR + PDK1 \rightarrow p-AKT$

  • AKT localizes to the membrane, positioning it for complete activation.

• AKT as a Kinase:

  • AKT, also known as protein kinase B, phosphorylates cellular targets.

  • AKT often phosphorylates targets to inhibit them.

Receptor Degradation and Inactivation

• Signaling Molecule as a Dimer:

  • The signaling molecule itself can be a dimer, effectively pulling two receptor subunits together upon binding. This dimerization induces a significant conformational change in the receptor complex, often leading to the activation of enzymatic activity within the receptor.

• Receptor Binding and Interaction:

  • Upon binding to the signaling molecule, each receptor undergoes a conformational change, facilitating their interaction. This interaction is crucial for initiating downstream signaling events.

• Result: Receptor degradation occurs, which is a mechanism to downregulate signaling. This degradation is often mediated by ubiquitination and subsequent trafficking to lysosomes.

Receptor Tyrosine Kinases (RTKs) and Autotransphosphorylation

• Enzymatic Activity:

  • RTKs possess inherent enzymatic activity, specifically as tyrosine kinases, allowing them to phosphorylate tyrosine residues on target proteins.

• Autotransphosphorylation:

  • RTKs undergo autotransphosphorylation, a process where they phosphorylate themselves. This autophosphorylation is crucial for the activation of the kinase domain and creating docking sites for downstream signaling molecules.

  • $Autotransphosphorylation$

  • This leads to multiple phosphorylation points, each serving as a potential interaction site for different signaling proteins.

• Molecular Interactions:

  • Following autophosphorylation, the phosphorylated tyrosine residues serve as docking sites for various signaling molecules. These molecules associate with the receptors, leading to the formation of signaling complexes.

  • After enzyme activity and autophosphorylation, a signaling complex is formed through molecular interactions, initiating downstream signaling cascades.

Receptor Inactivation Mechanisms

• Tyrosine Phosphatases:

  • Tyrosine phosphatases are enzymes that remove phosphate groups from tyrosine residues. Activation of these phosphatases can lead to the dephosphorylation of RTKs, inactivating the receptors by preventing their interaction with downstream components.

• Ubiquitin Ligases:

  • Specific ubiquitin ligases, such as CBL (an E3 ubiquitin ligase), can ubiquitinate the receptor. Ubiquitination marks the receptor for downregulation.

• Clathrin-Coated Pits:

  • Ubiquitinated receptors are internalized into early endosomes via clathrin-coated pits. The ESCRT (endosomal sorting complex required for transport) process then directs these receptors to the lysosome for degradation.

• Defects in ESCRT Proteins:

  • Defects in ESCRT proteins can impair the proper sorting and degradation of receptors, leading to prolonged signaling and potential cellular dysfunction.

• Defects in Ubiquitin Ligase System:

  • Defects in the ubiquitin ligase system can result in reduced receptor ubiquitination and degradation, causing prolonged signaling and potentially contributing to the development of cancer.

Implications for Growth Pathways and Cancer

• Downstream Pathways of RTKs:

  • Many downstream pathways of RTKs are intimately involved in the regulation of cell growth, proliferation, and survival.

• Uncontrolled Cell Proliferation:

  • Issues or dysregulation in these growth pathways can lead to uncontrolled cell proliferation and contribute to the development and progression of cancer.

Anchoring Proteins and Localization of Enzymes

• ACAPs (A-Kinase Anchoring Proteins):

  • ACAPs tether protein kinase A (PKA) to specific locations within the cell, ensuring that PKA can efficiently phosphorylate its substrates at the appropriate sites. This is an example of an anchoring mechanism that enhances the specificity and efficiency of signaling.

  • Scaffolding proteins can also act as anchors, bringing together multiple components of a signaling pathway, such as in the MAP kinase cascade.

• Localization of Other Enzymes:

  • Other enzymes in the cell also rely on proteins to help localize them to specific cellular subdomains, ensuring their activity is spatially regulated. However, specific examples were not discussed in the same detail.

• Protein Kinase C (PKC):

  • PKC localization to the membrane is determined by the presence of diacylglycerol (DAG). DAG is produced near PLC activity, which is in proximity to the receptor, ensuring that PKC activation occurs in close proximity to the upstream signaling events.

  • No other localization mechanisms for PKC were covered.

• ACAPs and Autophagy:

  • ACAPs may have connections to autophagy, a cellular process involved in the degradation and recycling of cellular components, though this was not covered in the lecture.

PI3K-AKT-mTOR Pathway

• Activation of PI3K:

  • Activated RTKs can directly activate PI3 kinase, initiating the PI3K-AKT-mTOR signaling cascade.

• PI3K Function:

  • PI3K modifies the phosphorylation pattern of phosphatidylinositol phosphates (PIPs), converting PIP2 to PIP3.

  • $PIP2 \rightarrow PIP3$

• PIP3 as a Docking Site:

  • PIP3 serves as a critical docking site for other proteins, including AKT, facilitating their recruitment to the plasma membrane.

• Activation of AKT:

  • AKT requires phosphorylation at multiple sites for complete activation. mTOR and PDK1 are key kinases involved in this phosphorylation.

  • $AKT + mTOR + PDK1 \rightarrow p-AKT$

  • AKT localizes to the membrane, positioning it for complete activation by PDK1 and mTORC2.

• AKT as a Kinase:

  • AKT, also known as protein kinase B, is a serine/threonine kinase that phosphorylates a wide array of cellular targets, thereby regulating diverse cellular processes.

  • AKT often phosphorylates targets to inhibit them, providing a mechanism for feedback regulation and control of cellular signaling.