 By now, you might start to see how intimately related membrane proteins are to biological function, and that's why we're focusing on them. I'm going to show you one more really cool example. Remember that nerve impulse? I said that the voltage-gated ion channels is helping conducting the nerve signal insider cell. Well, at some point, we're going to get to the end of one nerve cell, and we need to make a jump to the next nerve cell. That's called the synaptic junction, and we know quite a lot about that from medicine. When you reach the end of a cell, you have a junction that might be a few tens of micrometers larger, so it's a tiny fraction of a millimeter, and what happens there is that you have small vesicles or small cells, well, sacs really inside the cell that contains neurotransmitters. They're just small chemical molecules. Many of them are amino acids, in fact. When this nerve signal reaches the end of that cell, these are released into the synaptic junction, and then the compounds diffuse tens of micrometers to this other cell. Do you see the small purple or yellow dots there? Those dots are membrane proteins. They're so-called ligand-gated ion channels. If a voltage-gated ion channels opens an ion channel in response to voltage change, what do you think a ligand-gated one does? It opens a channel when they bind a ligand, in particular those small neurotransmitter compounds. This is the acetylcholine receptor, which was the first structure was determined from torpedo fish by Nigel Anwen, relatively lower resolution, and was, in fact, one of the very first-cooled cryoen structures that was, at least, of a membrane protein. The acetylcholine compound, which is the neurotransmitter, will bind up here in the outer part of the channel. So this is a so-called extracellular domain, not sitting in the membrane, but outside the cell. When this molecule binds there, there is a gigantic earthquake in the entire protein, and it's causing the structure here to change, and it's pushing the loops between the extracellular and transmembrane domain, and as a result, the transmembrane domain will open up. This entire protein, if we look at it from the top instead, you might see there it goes into five subunits. It's a pentamer, and all these pentamers will then move away, just so slightly from each other, and make the central pore here large enough that it will be able to conduct ions. There is a wealth of different pentamerically congated ion channels. I won't have time to take you through all of them, but some of them lead positive ions, conduct positive ions, other conduct negative ions. Some of them are increased in the conductivity when compounds bind. Others basically end up being dampened by compounds, and this is the reason why we have a remarkable diversity in our cellular response. Here is an example of one that we've simulated. This is a bacterial channel called Glick, but do you see all the beta sheets up there? We have beta sheets in the extracellular domain, and then alpha helices in the central domain. The bacterial ones are much simpler than the human. In the bacterial case, all these five subunits are the same, and the human ones, they're typically five different ones, or at least two or three different ones. In fact, one of the most common channels that we're interested in, for instance, when it comes to anesthetics, there are 17 possible different subunits for the GABA channel. You can just calculate 17 to the power of five potential different combinations, which is likely why we can have so many different nerve cells, for instance, in our brains. It's a very large and active area of research, and we still understand how these channels open, partly due to research we've done in our lab, but connecting the actual channel, the molecular mechanism here to the genetic diversity, and how that results in cellular response, we have no idea about that yet. This is used by a number of animals. For instance, poison frogs. So poison frogs, they express a poison called epibatidine, and what epibatidine will do is going to bind inside these channels in the extracellular domain, and when epibatidine has bound, you can no longer bind the neurotransmitter, meaning that I can't conduct nerve signals. And of course, if I can't conduct nerve signals, my entire nervous system starts breaking down, which includes things like signaling my breathing or my heartbeats. So it's extremely poisonous. Cobras. Cobra toxin is also a toxin that attacks these channels. In fact, cobra toxin specifically attacks the acetylcholine receptor, and the cobra toxin itself is a small molecule that consists of a few beta strands, and we know that these beta strands will bind between the subunits up here in the extracellular domain, and when they have bound, these subunits can no longer move apart, which again will block the neurotransmission of signals. They bind extremely hard and rigid, which is good for the cobra catching its prey. But there is something else. What if you're a poison frog? Having epibatidine is great for killing your prey, but it's very bad if it's killing yourself. These are vertebrates. These animals too have nervous systems. So how does the frog, for instance, make sure that it doesn't die from its own poison? It turns out that these channels in the frog, the acetylcholine receptor in particular, it has a few mutations. It's just two or three amino acids that have been changed right next to this binding site so that in the frog itself, the frog has a binding site that is not sensitive to epibatidine. So the frog can express as much poison as it was. It's not going to hurt it while its prey does not have these mutations, and therefore the prey will die. This is not merely a theoretical reasoning important to understand basic biology, but exceptionally important in healthcare. If you ever need surgery, we're going to anesthetize you. And if a general anesthetic that is inhaled corresponds to small molecules ranging from, well, today I would say it's usually isofluorine or desfluorine. But these are small gas compounds that bind to the receptors I just showed you. This is a remarkable story in itself because for a very long time we used to believe that because all these compounds are gas soluble. So people thought that they were acting purely on the lifted bilayer. But the last 15 or 20 years now we have identified the binding sites in this ligand-gated ion channels where they bind, and today we know exactly how to design anesthetics. It's just that it's hard. This far we haven't been able to design any new anesthetics for molecular basis even though we know the binding sites where they act.