Hi. In this video, I'm going to be doing an overview of cell surface receptors. There are three types of cell surface receptors, which makes it easy to remember.
The first is going to be an ion channel coupled receptor. This is a receptor that responds to different types of electrical gradients, stemming from ions, which makes complete sense. Ion channels also have a unique ability to convert chemical signals from ion gradients into electrical signals or vice versa. This is most commonly found in nerve signaling that we've talked about before. Ion channels are super important for that. Now, we're going to spend more time talking about the other two because we've talked about ion channels in the past.
The next one that we are going to talk about is G protein coupled receptors. They work pretty much by, once they're activated, going on to activate G proteins that are found in the cytosol. These G proteins then act on whatever they want to within the cell. They can act on enzymes, ion channels, and different proteins. Generally, this results in some kind of signaling cascade where the activation of a G protein leads to the activation of an enzyme, which leads to the activation of another protein which goes on and on, creating this huge cascade of events. We will talk a lot about G protein coupled receptors in future videos.
The final type is an enzyme coupled receptor, which you may see as a protein kinase receptor. These receptors act as enzymes, usually in some type of complex. There is actually a portion on this protein, usually in the C-terminal domain, that acts as an enzyme. When these are activated, they turn into enzyme complexes that can then go on to activate other things.
Here are the three types of cell surface receptor:
- You have your ion channel receptor, which will pass ions through and respond to different electrical or chemical gradients.
- Your G protein coupled receptor, which activates G proteins which go on to do other things in the cell.
- Your enzyme coupled receptor, which usually has some type of domain, usually in the cytosol, that acts as an enzyme once activated and usually works in a receptor complex. Here, I've shown it as dimers, which is going to be the most common form.
Even though there are three different classes, there are commonalities that exist between the three main receptor types and their signaling pathways. Usually, these receptors are activated by binding to some type of ligand, normally an extracellular ligand, something outside the cell that binds to the receptor. Once bound, it can activate a variety of different things depending on the receptor type, whether it’s an ion channel, G protein, or enzyme/protein kinase. Sometimes this ligand will remain for a really long time, which you can imagine will just continually activate the signaling pathway, which the cell may want, but generally does not. So, if the ligand stays for a really long time, the cell usually ends up down-regulating the receptor. This means that the cell says it doesn't need this much ligand, but since it's out there, it needs to respond less to it. The only way it can do that is by removing the receptor from the plasma membrane so that it can't interact with it anymore.
Another key component found downstream of every single one of these receptors are protein kinases. Remember, protein kinases add phosphates, and phosphatases remove phosphates. The addition or removal of a phosphate results in the activation or inhibition of a protein. Lastly, we need to know about a second messenger, which are molecules that act to signal downstream. The receptor and ligand bind, that activates the receptor, then the receptor goes on to activate something else, and that something else is usually some type of second messenger, which then goes on to signal downstream.
These second messengers make up what are known as signal transduction pathways, which are collections of stepwise events. The receptor activates protein A, protein A activates protein B, protein B activates protein C, protein C activates protein D, and so on. These are long and complicated pathways, but they are called signal transduction pathways. Usually, it gets to the end where protein Z has now been activated. So, what is protein Z? What does that protein do once it's activated? Usually, this protein acts as a transcription factor to activate or inhibit different genes. Generally, these signaling pathways lead to some difference in gene expression by the end.
This is just an example of a signaling transduction pathway. We do not at all need to understand what all these abbreviations mean, just sort of understand that it's complicated. If we have a ligand that binds to a receptor, this receptor activates a protein, it then activates these three things, one of which goes on to activate a series of proteins, which is going to be a signal transduction pathway. But eventually, what happens is that one of them activates some type of transcription factor, which travels to the nucleus to support gene expression and support whatever it wants to support, whether that's survival, apoptosis, proliferation, or differentiation. Whatever the signal is supposed to do, it will do by activating a transcription factor in the end. So, that's the overview of cell surface receptors and signal transduction pathways. So, with that, let's now move on.
