AP Biology 4.2 - Introduction to Signal Transduction

This section of the AP Biology curriculum introduces students to the concepts and vocabulary behind signal transduction pathways and the many roles they serve within organisms. We’ll start by looking at an overview of signal transduction pathways. Then, we’ll take a look at each of the specific processes involved in signal transduction. First, we’ll see how receptor proteins kick things off by accepting a ligand and undergoing a conformational change. After this, we’ll look at the different aspects of signal transduction pathways, including some of the secondary messenger molecules that make them possible. Finally, we’ll see how these pathways lead to cellular responses and see some examples of cellular responses that are possible.

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Cells communicate by generating, transmitting, receiving, and responding to chemical signals.

Describe the components of a signal transduction pathway.

Describe the role of components of a signal transduction pathway in producing a cellular response.

Signal transduction pathways link signal reception with cellular responses.

Many signal transduction pathways include protein modification and phosphorylation cascades.

Signaling begins with the recognition of a chemical messenger – a ligand – by a receptor protein in a target cell —

  1. The ligand-binding domain of a receptor recognizes a specific chemical messenger, which can be a peptide, a small chemical, or protein, in a specific one-to-one relationship.
  2. G protein-coupled receptors are an example of a receptor protein in eukaryotes

Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals, resulting in the appropriate responses by the cell, which could include cell growth, secretion of molecules, or gene expression–

  1. After the ligand binds, the intracellular domain of a receptor protein changes shape, initiating transduction of the signal.
  2. Second messengers (such as cyclic AMP) are molecules that relay and amplify the intracellular signal.
  3. Binding of ligand-to-ligand-gated channels can cause the channel to open or close.

4.2 Introduction to Signal Transduction Overview

Believe it or not, cells have a lot in common with remote controlled toys. This type of toy has an antenna to receive incoming signals from the controller. Cells have proteins that sit on the cell surface and receive signals. The toy car has an electrical circuit that is activated by the signal; cells use a complex pathway of secondary messengers inside of the cell to pass the message along. In the car, after the signal has passed through the circuitry it is converted into movement, allowing the car to go forward or backward depending on the original signal. Likewise, cells have specific cellular responses to specific signals that allow them to create an appropriate response in different environments!

In this section, we’re going to see how cells can receive a signal,  pass the signal along using signal transduction, and ultimately create an appropriate cellular response. Plus, we’ll cover all the important vocabulary and concepts related to signal transduction. This information will definitely be on the AP Test. So, come along with us as we take an introductory look at signal transduction pathways.

Signal transduction is the process through which an external signal becomes an internal cellular response. A signal transduction pathway is part of a three-step process that turns signals into cellular responses

The process starts when a receptor protein receives a signal. The signal could be a signal molecule – also known as a ligand – or it could be a physical signal such as light or a change in electrical potential. Upon receiving the signal, the intermembrane domain of the receptor protein (the part of the protein on the inside of the cell) will catalyze a reaction. This begins the signal transduction pathway.

The various steps of cell signaling, including signal transduction
The various steps of cell signaling, including signal transduction

Signal transduction pathways are very diverse, and they are specific to each receptor protein and the signal it can receive. They often involve the activation of proteins, the phosphorylation and transfer of energy to specific substances, and the use of secondary messengers that relay and amplify the signal to different parts of the cell.

The ultimate conclusion of a signal transduction pathway is a cellular response. As we will see, cellular responses vary greatly depending on what the signal is, what type of cell receives the signal, and a wide variety of other external and internal factors.

Since it is important to keep the big picture in mind as we learn about the detailed inner-workings of cells, all of the signal transduction pathways we talk about will be related to how your body establishes and maintains a circadian rhythm. This complex process relies on light (or darkness) hitting your eyes and stimulating important regions of your brain. These regions relay the signal to your pineal gland, which releases the hormone melatonin. Melatonin then travels to other parts of your brain and body, putting your organ systems into a resting state and making you very tired.

Signal reception only happens at the target cell, since non-target cells do not have the requisite receptor proteins
Signal reception only happens at the target cell, since non-target cells do not have the requisite receptor proteins

The important thing to know about signal reception is that the process is highly targeted. The cell that is sending the signal produces a ligand (or chemical messenger) that can only bind to a very specific receptor protein on the target cell. Only the target cell expresses the genes necessary to create the receptor protein. A non-target cell does not express this protein, so it cannot receive the signal or produce a response. This is important because not all cell types must respond to every signal. However, keep in mind that cells can have many different types of receptor proteins that can respond to a wide variety of chemical signals. This all depends on a cell’s type and the role it plays in an organism.

Let’s take a closer look at the receptor proteins that allow for the process of reception. This is a G-protein-coupled receptor. It is just one of many, many different types of receptors. However, these GPCRs are very common in all eukaryotes – from single-celled organisms to plants and animals. Plus, these receptors have many of the same basic structural motifs shared by all receptors. First, there is the ligand-binding domain. This is the part of the protein that can actually bind with a ligand, much like a substrate molecule binds to the active site of an enzyme. As the ligand binds, a conformational change takes place in the receptor protein.

The conformational change travels all the way through the hydrophobic domain that holds the protein within the lipid bilayer and into the intracellular domain of the receptor. The conformational change in the intracellular domain is what starts the series of reactions known as signal transduction. In the case of this G-protein-coupled receptor, the binding of a ligand causes a conformational change in the receptor protein. This change adds a GTP molecule to a G-protein and sends the G-protein into the cytoplasm to start a whole signal transduction pathway.

A G-protein-coupled receptor transfers a signal through the cell membrane
A G-protein-coupled receptor transfers a signal through the cell membrane

If we think about signal reception in terms of our model system, the circadian rhythm, we can see that there are tons of signal reception events in this one system. The eyes start the process by receiving photons from light that serve as a signal. The photons hit rhodopsin receptor proteins in the rod cells, which start a signal transduction pathway. However, there are many more signal reception events in this system. Each nerve cell in the brain receives a neurotransmitter signal, and melatonin – the hormone ultimately released by the pineal gland – is the ligand responsible for transferring the signal throughout the body.

The actual process of signal transduction is what links signal molecules and receptor proteins with the cellular responses they illicit. That being said, signal transduction pathways are incredibly diverse and rely on a huge variety of mechanisms to take place. In fact, a huge portion of an organism’s DNA is made for the sole purpose of producing signal transduction pathways that allow them to react to constantly changing environmental conditions.

So, instead of trying to cover all the different types of signal transduction pathways here, let’s just take a look at a general pathway that shows many of the important processes utilized in signal transduction pathways. As we saw in previous slides, the activation of the receptor protein causes a conformational change that can spark a variety of reactions. Most often, this conformational change activates an enzyme, which catalyzes a reaction within the cytosol.

The second messenger cAMP is often involved in phosphorylation cascades
The second messenger cAMP is often involved in phosphorylation cascades

While there are many enzymes that catalyze many different reactions within signal transduction pathways, a common theme here is the phosphorylation cascade. A phosphorylation cascade typically starts with the creation of a second messenger. The secondary messenger is a molecule that spreads throughout the cell and activates another enzyme, leading to a cellular response. In the case of a phosphorylation cascade, many secondary messengers are created that can activate thousands of proteins throughout the cell by relaying and amplifying the original signal.

If we consider the step of signal transduction in our larger model of the circadian rhythm, there are many different signal transduction pathways used in different cells based on the different signals within the entire process. The nerve cells in your eyes start a nervous impulse, which activates signal transduction pathways in your nerves that releases neurotransmitters to pass the nervous impulse into your brain. When this same signal reaches the pineal gland, it starts a signal transduction pathway that inhibits the production of melatonin. That is why sunlight is inhibitory and darkness is stimulatory.

When melatonin is actually released during the night, it activates hundreds of signal transduction pathways in different parts of your body.

Think of literally any cellular process. Now, consider that cells in most organisms have signal transduction pathways with this process as a cellular response. Phagocytosis, gene expression, exocytosis, the production of proteins, the modification of carbohydrates. All of these processes can be the ultimate cellular response created by cell signaling and signal transduction pathways.

Let’s look at a few of the cellular responses that take place in our model of the circadian rhythm. When it is daytime, the light striking your retina sends an electrical impulse through the cells of your nervous system. The cellular response in each cell is the same; the nervous impulse creates a signal transduction pathway that leads to the release of neurotransmitters. These neurotransmitters activate another pathway in the next nerve cell that sends this signal all the way to your pineal gland.

The overall process of creating your circadian rhythm involves many signal transduction pathways
The overall process of creating your circadian rhythm involves many signal transduction pathways

When this light signal hits your pineal gland, it creates a signal transduction pathway that actually inhibits the production of melatonin. As darkness falls this signal is stopped because the eyes are no longer receiving enough light. Thus, the cells in the pineal gland are no longer inhibited and can produce and export melatonin. Melatonin then becomes a signal to cells that night time has come and affects several melatonin receptors throughout the body.

Most importantly, melatonin binds to melatonin receptors in the brain. These receptors start signal transduction pathways that slow the speed and reactivity of neurons throughout the brain. Not only does this make you sleepy, but it leads to a slower heart rate, breathing rate, and other things that help you sleep and rest your brain. However, there are also melatonin receptors in places you wouldn’t expect. For example, there are melatonin receptors on the cells that create bone tissue that seem to activate these cells. This may be why bed rest and increased melatonin levels can help broken bones heal faster!