Chemistry 301 Introduction to Biochemistry

Study Guide :: UNIT 8

Signalling

Overview

In order to respond to their environment, cells use signal transduction. In essence, signal transduction is a multi‑step pathway resulting in amplification of a signal from the environment that elicits a change or response in cells. The signal binds a receptor on the cell surface and a series of reactions occurs, culminating in the binding of a transcription factor that induces the expression of a particular gene or group of genes encoding for proteins needed for that response. In immunology, signal transduction is important for the proliferation of T cells needed to defend against an invading pathogen. The cell receives the signal through binding a receptor, which induces the transcription of genes and then translation of proteins needed for T cell proliferation. This unit will cover the fundamentals of signal transduction, and present information on some of the types of signal transduction mechanisms in cells.

Unit 8 is divided into five lessons:

Learning Objectives

After completing this unit, you should be able to:

1. Define “signal transduction.”
2. Discuss the purpose and result of signal transduction in cells.
3. Outline the signal transduction pathways from ligand‑gated ion channel receptors, nuclear hormone receptors, G‑protein coupled receptors, and receptor tyrosine kinases.
4. Provide an example of each type of signal transduction pathway in cells.

Glossary

Lesson 1: Cell Signaling

Overview/Objectives

After completing this lesson, you should be able to:

1. Define the term “cell signaling.”
2. Outline the general principle behind signal transduction.
3. List the general types of signals.

Readings and Activities

1. Read “Cell Signaling” (pages 196–198 of the textbook).
2. You can also watch two video lectures on Signaling Mechanisms for all of the lessons in this unit:

   • #18 Biochemistry Signaling I Lecture for Kevin Ahern’s BB 450/550

     (A link to this is also provided on page 196 of the textbook.)

   • #19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550

     (Note that the current version of textbook does not provide this URL.)

Commentary

All organisms, from unicellular to complex multicellular ones, are able to respond to their environment and coordinate their many cellular activities through signal transduction mechanisms. Signal transduction follows a simple general principle:

Signal binds receptor → Pathway of intermediates → Target cell response

This general principle is illustrated on page 197 of the textbook. An overview diagram is below:

Adapted from: University of Tokyo, A Comprehensive Guide to Life Science, Fig. 14‑1B, https://csls-text3.c.u-tokyo.ac.jp/active/14_01.html

There are many types of receptors on the membrane surface of a cell, which transduce signals from the environment. The molecules that bind specifically to those receptors are called ligands or signal molecules. Signal molecules that mediate signal transduction between cells are called first messengers, and those that are produced and mobilized inside of cells in the pathway are referred to as second messengers.

Binding of a signal molecule from outside a cell to a receptor causes a receptor to transform and stimulate other molecules, referred to as activation. These activated receptors then activate other molecules in the cell. This pathway transmits information from the environment and amplifies the cell response, ensuring that the cell has an appropriate response to the environmental signal. This information may either be passed to the nucleus, where it will influence gene expression, or passed to intracellular proteins or organelles involved in cellular functions.

Now that you have an understanding of signal transduction, it is important to see how these general concepts fit into the larger, more detailed picture of this important cellular process. A detailed figure of the different signal transduction pathways is below:

File:Signal transduction pathways.svg. By cybertory, 2010. GFDL or CC BY-SA 3.0 via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Signal_transduction_pathways.svg.

This detailed figure shows the complexity of some signal transduction pathways in the cell. As this figure demonstrates, there are a number of pathways in cells, with many steps and second messengers culminating in a variety of cellular responses, which are dependent on the initial signal received. (You are responsible only for the material highlighted in the textbook and in the study guide commentary; you are not expected to know all the steps shown for every signal transduction pathway.)

There are many excellent figures online for signal transduction, which you may wish to consult as well. Even in its complexity, this figure does not outline all of the possible cell responses and second messengers. As you proceed through the other lessons in this unit, and learn about each type of receptor and resulting transduction pathway, you may wish to come back to this figure or others that you find to see how they all fit together.

Study Questions

1. On a cell surface, why are there so many different types of receptors with binding specificities to different molecules?
2. What are the three general steps in signal transduction?
3. List two general ways that signal transduction causes a change or response in a cell.

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at fst_success@athabascau.ca.

Lesson 2: Ligand‑gated Ion Channel Receptors

Overview/Objectives

After completing this lesson, you should be able to:

1. Define “ligand‑gated ion channel receptor.”
2. Provide an overview of the signal transduction pathway resulting from binding of a signal to a ligand‑gated ion channel receptor.
3. Identify the cell types that use signal transduction through ligand‑gated ion channel receptors.

Readings and Activities

1. Read “Ligand‑gated Ion Channel Receptors” (pages 199–200 of the textbook).
2. You can also watch two video lectures on Signaling Mechanisms for all of the lessons in this unit:

   • #18 Biochemistry Signaling I Lecture for Kevin Ahern’s BB 450/550

     (A link to this is also provided on page 196 of the textbook.)

   • #19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550

     (Note that the current version of textbook does not provide this URL.)

Commentary

As discussed in the text, ligand‑gated ion channel receptors are membrane receptors that are specifically bound by a signal that allows entry of a particular ion. Examples are Na⁺, K⁺, Ca²⁺, and Cl⁻ ions. Each receptor type facilitates the movement of only one ionic species. These gated channels create a pore, or channel, in the cell membrane. They are made up of multiple transmembrane proteins.

Signal transduction mediated by these types of receptors does not involve a complex pathway with second messengers and intermediates that ultimately affect transcription of genes. In this type of signal transduction, a signal is recognized by the ligand‑gated ion channel receptor and, in response, the channel opens allowing the passage of the ions from the extracellular environment into the cell. The presence of the ion in the intracellular environment can directly affect the cell and induce a response.

This type of signal transduction is rapid, and is seen in cells in neuromuscular junctions so that muscles can quickly respond to nerve impulses. Two examples of neurotransmitter signals are serotonin and acetylcholine. Acetylcholine can diffuse rapidly across the synaptic cleft and bind specific receptors on muscle cells. This allows the ligand‑gated ion channel to open releasing ions, such as Na⁺ and K⁺, into the cell. This change in electric potential in the cell leads to muscle contraction.

See the diagrams on page 199 of the text. For additional information, see the Wikipedia page Muscle contraction.

Study Questions

1. What is the structure of a ligand‑gated ion channel? How does signal transduction occur through these receptors?
2. Why is signal transduction through ligand‑gated ion channel receptors very rapid?
3. How does acetylcholine transmit signals to muscle cells?

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at fst_success@athabascau.ca.

Lesson 3: Nuclear Hormone Receptors

Overview/Objectives

After completing this lesson, you should be able to:

1. Outline signal transduction through nuclear hormone receptors.
2. Identify examples of signals that bind nuclear hormone receptors.

Readings and Activities

1. Read “Nuclear Hormone Receptors” (pages 200–201 of the textbook).
2. You can also watch two video lectures on Signaling Mechanisms for all of the lessons in this unit:

   • #18 Biochemistry Signaling I Lecture for Kevin Ahern’s BB 450/550

     (A link to this is also provided on page 196 of the textbook.)

   • #19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550

     (Note that the current version of textbook does not provide this URL.)

Commentary

Nuclear hormone receptors are those bound by steroid hormones; examples of steroid hormones are estrogen and testosterone. These steroid molecules are hydrophobic so, unlike other signals that cannot transverse the cell membrane and bind receptors on the outside of the membrane, these signals bind intracellular receptors. These receptors are referred to as nuclear receptors, and have the ability to regulate transcription of genes.

They are normally found in the cytoplasm of the cell bound to other proteins, such as HSP, and are inactive. However, when bound by a hormone, they dissociate from the protein they are bound to, and translocate into the nucleus. In the nucleus, these nuclear receptors become transcriptional regulators and influence the expression of the target genes by binding to their regulatory sequences (HRE, for hormone response elements).

This type of signal transduction is obviously slower than that mediated by ligand‑gated ion channels. The example presented in the text (see the figure on page 201) is an important mechanism for the regulation of genes by steroid hormones, which occurs over the lifetime of an individual. These steroid hormones and their modulation of gene expression are essential for development.

The estrogen nuclear hormone receptor is over‑expressed in around 70% of cases of breast cancer. This influences the cell response to the hormone in the cell. The presence of more of these receptors in cells likely causes tumorigenesis for a couple of reasons. First, binding of estrogen to more receptors stimulates proliferation of mammary cells with an increase in cell division and DNA replication, leading to mutations. Second, estrogen metabolism produces genotoxic waste. The overall result is disruption of the cell cycle, apoptosis, and DNA repair in cells which promotes tumour formation. (Adapted from the Wikipedia page Estrogen receptor.)

Study Questions

1. What signals do nuclear receptors recognize? How do these receptors transduce signals?
2. Why do steroid hormones such as testosterone and estrogen bind intracellular receptors?
3. Why are individuals who have an abundance of nuclear receptors specific for estrogen more susceptible to tumorigenesis?

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at fst_success@athabascau.ca.

Lesson 4: G‑protein Coupled Receptors (GPCRs)

Overview/Objectives

After completing this lesson, you should be able to:

1. Outline signal transduction through G‑protein coupled receptors (GCPRs).
2. Describe the structure of a GCPR.
3. Outline the structure and function of G‑proteins.
4. Differentiate between the signal transduction pathways involving adenylate cyclase and phospholipase C.
5. Identify examples of signals that stimulate signal transduction through GCPRs.

Readings and Activities

1. Read “G‑protein Coupled Receptors (GCPRs)” (pages 201–208 of the textbook).
2. You can also watch two video lectures on Signaling Mechanisms for all of the lessons in this unit:

   • #18 Biochemistry Signaling I Lecture for Kevin Ahern’s BB 450/550

     (A link to this is also provided on page 196 of the textbook.)

   • #19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550

     (Note that the current version of textbook does not provide this URL.)

Commentary

G‑protein coupled receptors (GCPRs) are one of the two most commonly described types of signal transduction receptors, and you may be familiar with them from other courses. The pathways that result from activation through a GCPR can be more complex than the pathways described thus far in the unit. In contrast to ligand‑gated ion channel receptors, this response is slower as it involves more steps and regulation of gene expression. As in other lessons, this commentary provides an overview to assist in your understanding of the material in the textbook, and also provides additional information.

GCPRs are named for their association with a G‑protein (guanine nucleotide binding protein) on the cytosolic side of the membrane. GCPRs are located in the membrane and have 7 transmembrane loops. One end of the receptor has an external domain that binds the signal outside of the cell and the other end, in the cytoplasm, interacts with a G‑protein to initiate the signal transduction pathway. Once the signal binds the receptor on the outer side of the membrane, the receptor undergoes a conformational change, which allows it to interact with a G‑protein on the inside of the membrane. See page 202 of the text for two figures illustrating GCPRs and their association with G‑proteins.

It its inactive form, a G‑protein has three subunits, αβγ, and is bound to GDP. Binding of a signal to a GCPR and the subsequent conformational change allows the cytosolic end of the receptor to interact with a G‑protein. At this stage, the α subunit loses its GDP and binds to GTP and the α subunit dissociates from the βγ subunits. This is shown in the two figures on page 203 of the text. The α subunit and the βγ subunits can then interact upon targets in the cell.

Protein targets in the cell for signal transduction through a GCPR fall into two categories; the pathways for each are discussed below.

Ion Channels

Instead of a signal molecule interacting with an ion channel receptor directly, signal molecules can bind GCPRs, which leads to activation of a G‑protein that binds and opens an ion channel, resulting in changes in the membrane potential of the cell and a response by the cell.

Royce Shou, 2009. From Wikipedia File:GPGIC schematic.jpeg.   https://en.wikipedia.org/wiki/File:GPGIC_schematic.jpeg

Specific Enzymes

G‑proteins may regulate the activity of target enzymes by increasing or decreasing enzyme activity. Overviews of two enzyme targets can be found on pages 204 and 207 of the text. These enzymes are adenylate cyclase and phospholipase C, and are discussed briefly below.

• Adenylate cyclase. The activated α subunit of the G‑protein activates adenylate cyclase, causing an increase in cyclic AMP (cAMP). cAMP molecules bind and activate protein kinase A (PKA). Activated PKA phosphorylates other enzymes, making them active, or it phosphorylates proteins that can regulate transcription of genes. An example is the signal transduction and cell response following the binding of epinephrine to its receptor. Activation of a G‑protein and then PKA results in the phosphorylation and activation of the enzyme glycogen phosphorylase, which breaks down glycogen releasing glucose for use by the cell.

  

  Gene Expression in Response to files, Image 046. From G.R. Kantharaj, Molecular Biology for Masters. Bangalore, India, n.d., https://mol-biol4masters.masters.grkraj.org/html/Gene_Expression_II11-Gene_Expression_in_Response_To_files/image046.jpg

• Phospholipase C. The activated α subunit of the G‑protein activates phospholipase C, which cleaves the phospholipid phosphatidylinositol 4, 5 bisphosphate (PIP₂) into inositol triphosphate (IP₃) and diacylglyerol (DAG), two second messengers.

  IP₃ and DAG work together to activate a protein kinase. IP₃ diffuses to the endoplasmic reticulum, and causes Ca²⁺ release from calcium channels. This action, along with DAG, activates protein kinase C (PKC). The release of Ca²⁺ also directly results in intracellular changes and activity. PKC phosphorylates a number of proteins in the cell. This amplifies the cell response to the signal.

  PKC can lead to the activation of different transcriptional regulators, which move into the nucleus and initiate the transcription of a number of genes that encode proteins needed for the cell response. There are many different types of PKC, and the signal transduction cascade and targets are dependent on the cell type. Some of the cell responses to signal transduction cascades, from PLC to PKC and down, involve membrane structure events, immune responses, cell growth, and learning and memory through phosphorylation of a variety of different proteins.

  The figure on page 207 of the textbook shows a portion of phospholipase C signaling (up to activation of PKC). Some cell responses resulting from activation of PKC can be found under Function on the Wikipedia page “Protein kinase C.”

Study Questions

1. Describe the structure of a G‑protein coupled receptor (GCPR).
2. In the absence of a signal, a G‑protein in the cell has three subunits, αβγ. What happens to these G‑protein subunits in the presence of a signal?
3. How can ion‑gated channels be activated through a GCPR?
4. Outline the signal transduction pathway and cell response to the signal epinephrine.
5. How does phospholipase C (PLC) initiate a signal transduction cascade resulting in increased transcription of target genes?

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at fst_success@athabascau.ca.

Lesson 5: Receptor Tyrosine Kinases (RTKs)

Overview/Objectives

After completing this lesson, you should be able to:

1. Outline signal transduction through receptor tyrosine kinases (RTKs).
2. Describe the function of RAS.
3. Discuss the functions of MAP kinases.
4. Give an example of a signal transduction cascade resulting from RTKs.

Readings and Activities

1. Read “Receptor Tyrosine Kinases” (pages 208–211 of the textbook).
2. You can also watch two video lectures on Signaling Mechanisms for all of the lessons in this unit:

   • #18 Biochemistry Signaling I Lecture for Kevin Ahern’s BB 450/550

     (A link to this is also provided on page 196 of the textbook.)

   • #19 Biochemistry Signaling II Lecture for Kevin Ahern’s BB 450/550

     (Note that the current version of textbook does not provide this URL.)

Commentary

Signaling from receptor tyrosine kinases (RTKs) is similar in ways to that of G‑protein coupled receptors, in the way the cascade is activated and in the fact that the response is slower because many proteins are phosphorylated in complex cascades and some of the end targets affect the transcription of genes.

Like GCPRs, RTKs have an extracellular domain that binds the signal and an intracellular domain involved in activation of signal transduction. These regions are connected by a transmembrane α‑helix. However, unlike GCPRs, RTKs dimerize to be active (joining of two receptors) and the two parts autophosphorylate several tyrosine residues on the cytoplasmic tails of each other. Three figures on page 209 of the textbook illustrate these concepts.

Ras is a signaling protein that is activated by autophosphorylation of an RTK. Ras is very similar to a G‑protein. It is a monomeric guanine nucleotide binding protein on the cytosolic face of the plasma membrane that binds GDP in the absence of a signal (inactive), and GTP in the presence of a signal (active); and, like G‑proteins, Ras can hydrolyze GTP. One figure on page 209 and one on page 210 of the textbook show the activation of Ras.

Binding of a signal to an RTK causes two receptor molecules to associate (dimerize), which causes the tyrosine kinase activity of each cytoplasmic tail to be turned on. The autophosphorylation (phosphorylation of each tail by the other tail) on tyrosine residues allows these tails to serve as binding sites for signaling proteins. One of these proteins in the complex interacts with Ras and activates it by stimulating the exchange of GDP for GTP. Once activated, Ras triggers a phosphorylation cascade of three protein kinases called MAP kinases (mitogen activated protein kinases). This is shown in a figure on page 210 of the textbook. The final MAP kinase phosphorylates a variety of target proteins to initiate a cell response, including enzymes and transcriptional activators. The combined effects of phosphorylated enzymes and induction of genes alter the physiological state of the cell; this response may be quite complex, depending on the signals that are present.

Examples of signals for RTK transduction pathways are insulin and epidermal growth factor. Binding of insulin to its RTK receptor increases the uptake of glucose into fat and muscle cells and reduces the synthesis of glucose in the liver to maintain glucose homeostasis. The insulin transduction pathway is influenced by feeding versus fasting states, hormones, and stress levels. When carbohydrates are consumed, the rise in blood glucose causes the pancreas to release insulin. When insulin binds its RTK (insulin receptor), it leads to the cascade described above through activation of MAP kinases, which affects cell differentiation and mitogenesis, and the enzyme PI‑3K (phosphoinositide 3‑kinase), the pathway of which results in either the storage of usage of glucose through GLUT‑4 vesicle. GLUT‑4, which is responsible for passive diffusion of glucose, binds to PI‑3K after bringing glucose into the cell. The PI‑3K isolates the glucose which is sent to the mitochondria to make ATP. The excess glucose is stored in the cell as glycogen.  (Some information is taken from the Wikipedia page Insulin signal transduction pathway. Please note that, while some information on the page is disputed, the material explained in this lesson is not affected.)

The following figure shows a simplified general representation of the insulin signal transduction pathway.

File:BIOE Article Pic.svg, James Forman, 2011, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=31625497

Although Ras is not indicated in this figure, it would be in the complex of proteins near the cytosolic tails of the insulin receptor; the “phosphorylation of enzyme” would apply to Ras, which goes on to activate the MAP kinases and PI‑3K. Please also note that the pathways are more complex than is explained in the textbook or shown in this figure. It is important to have a general understanding of these pathways, but you are not required to know all of their details.

Now that we have covered all of the general types of signal transduction pathways, it is important to reflect on some important concepts. Signal transduction plays a critical role in biochemistry for two reasons:

• Signal transduction shows how the molecules we have studied in earlier units interact for a coordinated response, and how organisms are constantly responding to their environment.
• Signal transduction highlights how metabolic reactions in the cell occur as a result of signals to the cell and precise regulation of these pathways.

At any given time, a cell may receive multiple signals that set off a number of different signal transduction pathways and cell responses. In addition, a cell response to a particular signal may be influenced by other signals the cell is receiving. All together, the combination of a large number of different receptors and signals and complex pathways enables cells to respond to an endless diversity of environmental conditions.

Study Questions

1. Describe the structure of a receptor tyrosine kinase (RTK).
2. What is the function of Ras? How is this protein similar to a G‑protein?
3. The binding of insulin to its receptor triggers two pathways. Outline each one and indicate the cell response for each.

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at fst_success@athabascau.ca.

Answers to Unit 8 Study Questions

Lesson 1: Cell Signaling

1. There are many different receptors, because there are many different signals that a cell can receive. Many receptors with a binding specificity for many types of signals ensures that the cell can respond to a variety of environmental conditions or requirements in a coordinated fashion.
2. The three general steps in signal transduction are: a) signal binds receptor, b) pathway of intermediates and reactions, c) target cell response.
3. Two ways that signal transduction can cause a cell response are: a) influence gene expression, b) influence intracellular proteins involved in metabolism or cellular function.

Lesson 2: Ligand‑gated Ion Channel Receptors

1. Ligand‑gated ion channel receptors are membrane receptors that are made up of multiple transmembrane proteins to create a pore or a channel in the membrane. These are specific for a particular ion, and allow the movement of only one ionic species. Examples of ions are Na⁺, K⁺, Ca²⁺, and Cl⁻. A signal is recognized by the ligand‑gated ion channel receptor, and the channel opens to allow the passage of an ion. The presence of the ion in the intracellular environment induces a response. An example is in cells in neuromuscular junctions. This type of signal transduction is rapid.
2. Signal transduction through ligand‑gated ion channel receptors is very rapid, because there are no pathways involved or intermediates and second messengers. The binding of the signal to the receptor induces an influx of a particular ion and changes the cell environment quickly, causing a response.
3. Acetylcholine binds the receptor on muscle cells, opening the channel to release Na⁺ and K⁺. This change in electric potential leads to muscle contraction.

Lesson 3: Nuclear Hormone Receptors

1. Nuclear receptors recognize hormones. These receptors are found in the cytoplasm of the cell, where they are bound to proteins such as HSP (heat shock protein). In this form, they are inactive. When they are bound by a hormone, they dissociate from the protein they were bound to and translocate into the nucleus, where they act as transcriptional regulators and influence the expression of target genes. They do this by binding to the regulatory sequences of the target genes called hormone response elements (HRE). This type of signal transduction is slower.
2. Steroid hormones such as testosterone and estrogen bind to intracellular receptors because they are hydrophobic and are unable to cross the membrane.
3. Individuals that have an abundance of nuclear receptors specific for estrogen are more susceptible to tumorigenesis because: a) binding of estrogen to more receptors stimulates proliferation of mammary cells with an increase in cell division and DNA replications, leading to mutations; and b) estrogen metabolism produces genotoxic waste, which results in disruption of the cell cycle, apoptosis, and DNA repair in cells, and promotes tumour formation.

Lesson 4: G‑protein Coupled Receptors (GPCRs)

1. G‑protein coupled receptors (GCPRs) are associated with a G‑protein (guanine nucleotide binding protein) on the cytosolic side of the membrane. GCPRs are located in the membrane and have seven transmembrane loops. One end of the receptor has an external domain that binds the signal outside of the cell; the other end, in the cytoplasm, interacts with a G‑protein to initiate the signal transduction pathway. Once the signal binds the receptor on the outer side of the membrane, the receptor undergoes a conformational change, which allows it to interact with a G‑protein on the inside of the membrane.
2. In the presence of a signal, the GCPR undergoes a conformational change allowing the cytosolic end to interact with the G‑protein. The three subunits of a G‑protein separate because the α subunit loses its GDP and binds to GTP and then dissociates from the βγ subunits. The α subunit and the βγ subunits can then interact upon targets in the cell.
3. Instead of a signal molecule interacting with an ion‑gated channel receptor directly, signal molecules can bind GCPRs, which lead to activation of a G‑protein that binds and opens an ion channel, resulting in changes in the membrane potential of the cell and a response by the cell.
4. Binding of epinephrine to its GCPR leads to activation of a G‑protein. Protein kinase A (PKA) is activated and phosphorylates the enzyme glycogen phosphorylase. This activates the enzyme, which allows the cell to break down glycogen releasing glucose for use by the cell.
5. Phospholipase C (PLC) initiates a signal transduction cascade resulting in increased transcription of target genes in this manner:
   1. Target binds GCPR and the receptor undergoes conformational change which allows it to interact with G‑protein.
   2. G‑protein subunits are activated and dissociate (α subunit binds GTP instead of GDP).
   3. α subunit activates PLC, which cleaves phosphatidylinositol 4, 5 bisphosphate (PIP₂) into inositol triphosphate (IP₃) and diacylglycerol (DAG).
   4. IP₃ and DAG work together to activates protein kinase C. IP₃ diffuses to endoplasmic reticulum and causes Ca²⁺ release and this action along with DAG activates PKC. The Ca²⁺ release also causes direct cellular changes.
   5. PKC phosphorylates a number of proteins which leads to the activation of different transcriptional regulators which move to the nucleus and initiate the transcription of particular genes.

Lesson 5: Receptor Tyrosine Kinases (RTKs)

1. Receptor tyrosine kinases (RTKs) have an extracellular domain that binds the signal, and an intracellular domain involved in activation of signal transduction. These regions are connected by a transmembrane α‑helix. When a signal binds an RTK, it associates with another RTK and the two intracellular domains autophosphorylate tyrosine residues on the cytoplasmic tails of each other.
2. Ras is a signaling protein that is similar to a G‑protein because it is a monomeric guanine nucleotide binding protein on the cytosolic face of the plasma membrane. It binds GDP in the absence of a signal (inactive) and GTP in the presence of a signal (active). It is also like a G‑protein because it can hydrolyze GTP.
3. The insulin transduction pathway is influenced by feeding versus fasting states, hormones, and stress levels. When carbohydrates are consumed, the rise in blood glucose causes the pancreas to release insulin. When insulin binds its RTK (insulin receptor), it leads to two cascades:
   • MAP kinase: Binding of insulin to RTK causes association of two RTKs and tyrosine kinase activity (autophosphorylation) of cytoplasmic tails. The tails serve as binding sites for signaling proteins. One of these interacts with Ras and activates it by stimulating the exchange of GDP for GTP. Ras triggers a phosphorylation cascade of three MAP kinases (mitogen activated protein kinases). The final MAP kinase phosphorylates target proteins such as enzymes and transcriptional activators which affects cell differentiation and mitogenesis.
   • PI‑3K (phosphoinositide 3‑kinase). Binding of insulin to RTK causes association and activation of Ras as described above. Ras activates PI‑3K, which results in either the storage of usage of glucose through GLUT‑4 vesicle. GLUT‑4, which is responsible for passive diffusion of glucose, binds to PI‑3K after bringing glucose into the cell. The PI‑3K isolates the glucose, which is sent to the mitochondria to make ATP. The excess glucose is stored in the cell as glycogen. Signaling and activation of PI‑3K can also trigger a pathway that influences the synthesis of lipids, proteins, and cell survival and proliferation.

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at fst_success@athabascau.ca.