Hi in this video we're gonna be talking about membrane proteins. So in this first section we're going to talk about different types of membrane proteins. So this one is going to be a lot of just vocab words and their definitions. So not super interesting, but you still have to know the vocab words. So I need to go through them now. There are many different types of membrane proteins. And the first one that I feel like people always think of when they think of proteins in the membrane is they think of trans membrane proteins. So these are proteins that extend through the entire lipid bi layer. So if I have a lipid bi layer here, imagine that these are these look more like DNA but imagine that this is a lipid bi layer then a trans membrane protein is going to extend through the entire membrane. And that's typically what people think of when they think of when they think of membrane proteins. Now, not all membrane proteins are like that, that is trans membrane protein. The second one you need to know about is integral membrane proteins. And these sometimes people get confused about because a lot of times they look very similar. A lot of times they do extend through the entire membrane but they don't have to they can actually sometimes just associate with one side or the other and not extend all the way through. So sometimes they look like this. Sometimes they look like this but how you classify an integral membrane protein is it's attached directly to the limits it's attached to the BI layer And so not all membrane proteins are attached to the bi layer. So this is actually a defining feature of integral membrane proteins. Now, if it's associated with just one side or the other, we call those monotone, pick integral membrane proteins because they are associated with only one side. The trans membrane always extends all the way through, integral can extend all the way through but doesn't have to. And so now let's go to the third one which is peripheral membrane proteins. And these are actually quite different because peripheral membrane proteins are bound to membranes but they aren't directly bound to membranes. Instead they're bound through direct interactions with other proteins. So what does that mean? It means that instead of binding to the membrane, like these integral membrane proteins do um peripheral proteins instead bind to proteins that are binding to the membrane. So here we have a PMP a peripheral membrane protein where it's attached to the bi layer but it's not directly binding the lipids itself, It's off to the periphery or off to the side because it's binding a protein that's binding the membrane. And so peripheral membrane proteins, they can be completely on the side of saul or they can be on the exercise regular service. Either one, they're not confined to just one side of the they're confined to one side of the membrane but they're not always on the side of all. We're always on the exercise of service. They can be mixed in together. And then the last one I want to talk about is a little bit harder to kind of conceptualize. And that's the lipid anchor protein. And pretty much what this is is it's bound to lipid. So it's not like a peripheral protein, it's bound to lipids, but instead of just kind of associating with them, which is what the integral membrane protein does, it's kind of just nearby and it has some interactions with it. A lipid anchor protein has a much straw stronger covalin bond. And remember those convey valent bonds are those really strong ones. So they really I mean they anchor themselves to that membrane and that is a strong interaction. And there's a ton of different membrane proteins that have this type of interaction with very strong interaction. And usually they're all named different things. So I wish I could just give you, oh, they're all called this but they're not. Um and so so ones that are mentioned in your book include the fatty acid anchor, which makes sense because they are anchored by a fatty acid. So they co violently attached to a fatty acid in the bi layer. Another one is an I suppressed elated protein. It's a type of lipid anchor protein and essentially what this is. You don't necessarily need to know it. But isolation is just a chemical reaction that can occur. And what happens is that chemical reaction allows it to be inserted into the membrane. But the one that you're going to need to know about. The one that we're probably gonna mention again. And the one that you're gonna see most often your book is the G. P. I. Anchored membrane protein. And what happens is that this protein is made in the er and then it becomes process so it becomes cleaved and some other things happen to it. But essentially whenever it's cleaved then this anchor gets added onto it and the anchor is called G. P. I. For short or like oh cell phosphate Illinois hospital. That's a mouthful there. So you can just say G. P. I. So the proteins made in the er it's cut it's cleaved off and this GPS anchors onto it when the anchor is attached onto this protein then that anchor binds or anchors it to the membrane. So these G. P. I anchored membrane proteins are really super important and super common lipid anchor protein type. It's a good example to recognize. So if we look here this is an example of a membrane you can see these red and orange things here are the lipids these fossil lipids in the membrane I've highlighted. There's a lot of things labeled here and we don't need to know all of them for membrane protein. So I've sort of just highlighted or boxed out in um read the ones I want to show you here. We have a trans membrane protein extending entirely through the membrane here. We have an integral protein which in this case is extending entirely through the membrane. And you can see that it's it's interacting a bit more with the lipids which is character the integral membrane protein. And we have the peripheral protein which is attached to one side and not both sides and it's not showing it attached to a protein here. But usually these are definitely attached to another type of protein. So those are three examples of course there's other things here, things like glycoprotein which are attached to sugar, glider, gold lipids, sugars onto lipids. Um and all sorts of other things here that are happening in the membrane by layer. It's not just the static place. It's obviously filled with lots of proteins and sugars and things. But those are just a few classifications that membrane proteins you're gonna want to know about. So with that let's move on.

So in this video, I'm going to be talking about two structures that trans membrane proteins have. So trans membrane proteins are, remember the ones that traverse first the membrane, so they go through the entire bi layer and to do that there are two main structures that they form. The first is an alpha helix most commonly commonly formed structure by trans membrane proteins. Um and so the alpha helix is important because it allows for the hydrophobic side chains of amino acids to be on the helix which interacts with the hydrophobic tails of the phosphor lipids and the hydra filic portion of the protein gets masked inside. And so this is important because the inner, the inner region of the membrane by layer is hydrophobic. So in order for it to extend to the membrane, those hydrophobic amino acids really need to be surrounding that alpha helix. Now second structure is a beta barrel and this is typically formed by repeating sheets. Now beta barrels are generally found in trans membrane proteins that form like large channels that allow for large molecules to pass through the membrane. So they're not as common, but they are really common in channels. But most trans membrane proteins that are just trans membrane proteins have the alpha helix. Now trans membrane proteins can also form as a single pass or a multi pass protein. So what a single pass mean is a single pass protein usually has one alpha helix that extends to the membrane, whereas multi pass proteins across the membrane multiple times. So we're looking at an example of this, we have our single pass and this is gonna be an alpha helix. Then we have our multi pass and this has multiple alpha helix sees. So there's 123 and it passes through the membrane multiple times. But we also have here our beta barrel. Mhm. And this is generally not described as single or multi. Um Clearly it's there's a lot of different sheets within the membrane but generally this form some kind of channel that passes molecules through to the other side of the membrane. So those are the common structures that membrane proteins can form in a membrane. So now let's move on.

So in this video we're going to be expanding on membrane proteins specifically talking about their organization and different functions in the membrane. So the first thing I want to talk about is the fact that like membrane lipids, trans membrane proteins are not equally distributed on each side of the bi layer. So out of all the proteins that I've talked about so far, only the trans membrane proteins actually are present and function on each side of the membrane. But even though they're present, they're usually each side of the trans membrane protein has a different some type of different function. So even though it's present, it's not acting the same and most of the proteins aren't even present on either side now, this is also true when it comes to protein modification. So things like uh like oscillation which is the addition of carbohydrates to in protein um can occur differently on each side of the bi layer. So um this is important because extra cellular league like oscillation was really important for protecting cells, preventing cell to cell contact that shouldn't occur and sort of blurring the barrier between the cell and the extra cellular matrix. So if we are to look again at this image um this just shows the symmetry of membrane proteins. So you can see that you have your trans membrane protein here. Um and there's it's definitely present on both sides but there you can see there are different modifications that happen on either side. You have some proteins that are only present on one side um and different. Here's a glycoprotein here with some type of oligarchs. Zachariah added onto it on this side of the membrane. And so this asymmetry allows for different sides of the membrane to have different functions based on its protein. Now. Also like membrane lipids, membrane proteins can move around and are also divided into domains so membrane proteins can move in the membrane. So the same terms that we use to describe, membrane lipids can also be used to membrane proteins, lateral diffusion, rotational diffusion and traverse diffusion have the same exact definition for membrane proteins as they do for membrane lipids and all describe a way that a membrane protein can move in a bi layer now like lipid rafts which if you remember where specific domains domains exist on cells with different protein compositions and different protein functions. So for instance epithelial cells or you can think of these as your gut cells. They contain the surface called the typical, the typical surface and the basal lateral surface. And they have different functions where the a pickle surface absorbs nutrients and the basal lateral transfers nutrients to blood. So if we're looking at what this looks like here, we have our cell and you have this a pickle surface here which is going to absorb the nutrients, the nutrients are going to pass through here and then you have this basal lateral surface with different proteins on it that responsible for taking those proteins and putting them into the blood. So these different domains are really important in membrane by layers and membrane function and membrane protein function. Now membrane proteins also have the ability to form these complex structures that help support the sound. So one of these structures is called the this the glycol colonics. Um And it's formed by glycoprotein and like oh lipids which coat the outside of the plasma membrane. Um It contains protein like cans which are proteins linked to policy aka rides um And glycoprotein which are proteins linked to ali go sacha rides another type is the cell cortex and this actually sits on the inner surface of the plasma membrane interacts with the sell side O skeleton. And it's this combination of membrane proteins inside a skeleton that anchor the membrane and helps support the cell. Um And it also limits membrane protein movement. Now, membranes proteins can also support membrane uh bending and membrane protein insertion and so insertion of hydrophobic protein domains control the intensity of bending. So you can imagine, you know, the cell is circular so at some point it does have to curve and it has the membrane actually does have to bend. Um So you can think of this as membrane bending or curvature. It's probably a better word. Um And how it does this is it inserts these hydrophobic proteins um at specific domains which allow for the membrane to actually curve into the circular structure that we're used to thinking about. So looking at this example, let me back up here, we have this collection of proteins on the extra cellular surface, as well as the intracellular surface and that makes up the cell cortex. Um and these membrane proteins are anchored here to all of these other types of proteins. So in this case it would be the E C. M. And different side of skeleton um components that are really responsible for supporting the cell structure and function. So now, with an understanding of basic membrane protein functions, let's move on.

So in this video I'm gonna be going over just a few techniques that are used by laboratories to study membranes in cells. So the first is detergent which is exactly what you know you think you use to wash your clothes or your dishes, hopefully you do both um and you use a detergent to do it. Um And what a detergent is um you've probably never even thought about it but it's really just small molecules that have a hydrophobic tail and they form my cells and water which are just kind of sort of very small lipid uh layers. And when mixed in membranes, these detergents actually can interact with the hydrophobic portions of the membrane and disrupt the membranes in order to extract proteins out. So if you need to get membrane proteins out of the lipid bi layer you use detergents to mess up the bi layer and or any type of lipid and isolate the protein. It's how your clothes get clean. Second thing is X ray crystallography. This is a technique used to study the structure of proteins um But it's really not effective with membrane proteins and this is because in order to isolate membrane proteins you have to use detergents but detergents are really strong and therefore can disrupt the proteins normal function or structure and therefore making X ray crystallography really difficult to use to look at membrane proteins but they still sometimes attempt it even if it's not that great. Another technique is called freeze fracturing and this actually is used to reveal the inner surface of the cell and the cells cortex. And so how this happens is that you have membranes or cells and you freeze them really quickly, like super super, super quickly. Then once they're completely frozen, flash frozen, they're pierced with a diamond knife. Um and this diamond knife has the ability to actually split hydrophobic areas. And so when those hydrophobic areas are split open, what you're left with are membrane proteins that are actually still embedded in either one side of the bi layer. So that is a technique that's often used. And then finally there's this technique called frappe which stands for fluorescent recovery after photo bleaching. And this is generally the technique that's used to study membrane fluidity. So how this happens is you label lipids or proteins with some kind of molecule that floor esus. And then you can bleach a fluorescent molecule by exposing it to a certain wavelength of light. And so by what I mean by bleaches, I mean that it loses its fluorescence. But if you only so if the wholesale for instance is has this fluorescent molecule on the lipid bi layer. But you only bleach this really small surface eventually if the lipids are moving, that's going to sort of slowly creep back in from other lipids nearby. So what they do is they bleach it and then they watch for recovery of fluorescence which will only occur if the lipids or proteins are moving into the bleached area. So what this would look like is if you have labeled your lipid bi layer with all these red fluorescent proteins or LED fluorescent molecules. So all of them have it. You can see there's like red all over the place. And this is what you would look like. If you're using a fluorescent microscope, then you bleach it somehow through some wavelength of light. So you lose that fluorescent in a really small proportion. So you get this black helps if I don't use red on red, but you get this black circle here where it's been bleached and over time these eventually diffuse out, so it starts getting blurry, but eventually the entire bleached area disappears because these bleached phosphor lipids have moved out. Um And so we use this method to know more about membrane fluidity and you can use it for lipids or proteins, whichever one. So now let's move on.