Membrane Proteins - Online Tutor, Practice Problems & Exam Prep

Membrane proteins are classified into four main types: transmembrane proteins, integral membrane proteins, peripheral membrane proteins, and lipid-anchored proteins. Transmembrane proteins typically form structures like alpha helices or beta barrels, allowing them to traverse the lipid bilayer. These proteins exhibit asymmetry in function and modifications across the membrane. Techniques such as detergent use, freeze fracturing, and FRAP (Fluorescence Recovery After Photo Bleaching) are employed to study membrane dynamics and fluidity, revealing the complex interactions and movements within cellular membranes.

Types of Membrane Proteins

Hi. In this video, we're going to be talking about membrane proteins. So in this first section, we're going to talk about different types of membrane proteins. This one is going to be a lot of just vocabulary words and their definitions. So not super interesting, but you still have to know the vocabulary 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 transmembrane proteins. These are proteins that extend through the entire lipid bilayer. So, if I have a lipid bilayer here, imagine that these look more like DNA, but imagine that this is a lipid bilayer. Then a transmembrane protein is going to extend through the entire membrane. And that's typically what people think of when they think of transmembrane or when they think of membrane proteins. Now, not all membrane proteins are like that. That is a transmembrane 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 lipids. It's attached to the bilayer. And so not all membrane proteins are attached to the bilayer. 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 monotopic integral membrane proteins because they are associated with only one side. So transmembrane 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. What does that mean? It means that instead of binding to the membrane, like these integral membrane proteins do, peripheral proteins instead bind to proteins that are binding to the membrane. Here we have a PNP, a peripheral membrane protein, where it's attached to the bilayer, 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 in the cytosol or they can be on the extracellular surface. Either one, they're not confined to just one side of the membrane, but they're not always in the cytosol or always on the extracellular surface. They can be mixed in together.

And then the last one I want to talk about is a little bit harder to conceptualize, and that's the lipid-anchored protein. And pretty much what this is, it's bound to lipids, 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-anchored protein has a much stronger covalent bond. Remember, the covalent bonds are those really strong ones. So they really anchor themselves to that membrane, and that is a strong interaction. There are a ton of different membrane proteins that have this type of interaction, this very strong interaction. And usually, they're called different things. So I wish I could just give you, oh, they're all called this, but they're not. And 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 covalently attach to a fatty acid in the bilayer. Another one is an isoprenylated protein. It's a type of lipid-anchored protein. And essentially what this is, you don't necessarily need to know it, but isoprenylation 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 going to mention again, and the one that you're going to see most often in your book is the GPI-anchored membrane protein. And what happens is that this protein is made in the ER, and then, it becomes processed or 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 GPI simply or glycosylphosphatidylinositol. That's a mouthful there. So you can just say GPI. So the protein's made in the ER. It's cut. It's cleaved off, and as GPI anchors onto it. When the anchor is attached to this protein, then that anchor binds or anchors it to the membrane. So these GPI-anchored membrane proteins are really, super important and super common lipid-anchored 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 phospholipids in the membrane. I've highlighted, there are a lot of things labeled here, and we don't need to know all of them for membrane proteins. So I've sort of just highlighted or boxed out in red, the ones I want to show you. Here we have a transmembrane 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 interacting a bit more with the lipids, which is characteristic of 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 are other things here, things like glycoproteins, which are attached to sugar, glycolipids, sugars onto lipids, and all sorts of other things here that are happening in the membrane bilayer. It's not just this static place, it's obviously filled with lots of proteins and sugars and things, but those are just a few classifications of membrane proteins you're going to want to know about. So with that, let's move on.

Alpha Helices and Beta Barrels

So, in this video, I'm going to be talking about two structures that transmembrane proteins have. Transmembrane proteins are, remember, the ones that traverse the membrane, so they go through the entire bilayer. And, to do that, there are two main structures that they form.

The first is an alpha helix. It is the most commonly formed structure by transmembrane proteins. 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 phospholipids, and the hydrophilic portion of the protein gets masked inside. This is important because the inner region of the membrane bilayer is hydrophobic. So, in order for it to extend through the membrane, those hydrophobic amino acids really need to be surrounding that alpha helix.

The second structure is a beta barrel, and this is typically formed by repeating sheets. Beta barrels are generally found in transmembrane proteins that form large channels, allowing large molecules to pass through the membrane. They are not as common, but they are really common in channels. Most transmembrane proteins that are just transmembrane proteins have the alpha helix.

Transmembrane proteins can also form as a single pass or a multi-pass protein. A single pass protein usually has one alpha helix that extends through the membrane, whereas multi-pass proteins cross the membrane multiple times. So if we're looking at an example of this, we have our single pass, which is an alpha helix. Then, the membrane multiple times. But, we also have here our beta barrel. This is generally not described as single or multi. Clearly, there are many different sheets within the membrane, but generally, this forms 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.

Organization of Membrane Proteins

So in this video, we're going to be expanding on membrane proteins, specifically talking about their organization and different functions in the membrane. The first thing I want to talk about is the fact that like membrane lipids, transmembrane proteins are not equally distributed on each side of the bilayer. Out of all the proteins that I talked about so far, only the transmembrane proteins are present and function on each side of the membrane. But, even though they're present there, usually, each side of the transmembrane protein has a 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. This is also true when it comes to protein modifications. So, things like glycosylation, which is the addition of carbohydrates to a protein, can occur differently on each side of the bilayer. This is important because extracellularly glycosylation is 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 extracellular matrix. If we were to look again at this image, this just shows the asymmetry of membrane proteins. You can see that you have your transmembrane protein here, and 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. Here's a glycoprotein here with some type of oligosaccharide added onto it on this side of the membrane. 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. The same terms we use to describe membrane lipids can also be used to describe 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 bilayer. Now, like lipid rafts, which if you remember were specific domains, domains exist on cells with different protein compositions and different protein functions. For instance, epithelial cells, or you can think of these as your gut cells, they contain the surface called apical, the apical surface, and the basolateral surface. They have different functions where the apical surface absorbs nutrients and the basolateral transfers nutrients to the blood. We are looking at what this looks like. Here, we have our cell, and you have this apical surface here, which is going to absorb the nutrients. The nutrients are going to pass through here. Then you have this basolateral surface with different proteins on it that are responsible for taking those proteins and putting them into the blood. These different domains are really important in membrane bilayers and membrane function and membrane protein function. Membrane proteins also have the ability to form these complex structures that help support the sound. One of these structures is called the glycol colloids, and it's formed by glycoproteins and glycolipids, which coat the outside of the plasma membrane. It contains proteoglycans, which are proteins linked to polysaccharides, and glycoproteins, which are proteins linked to oligosaccharides. Another type is the cell cortex, and this actually sits on the inner surface of the plasma membrane. It interacts with the cell cytoskeleton, and it's this combination of membrane proteins and cytoskeleton that anchor the membrane and help support the cell, and it also limits membrane protein movement. Membrane proteins can also support membrane bending and membrane protein insertion. The insertion of hydrophobic protein domains controls the intensity of bending. You can imagine, the cell is circular, so at some point, it does have to curve and the membrane does have to bend. You can think of this as membrane bending, or curvature. It's probably a better word. How it does this is it inserts these hydrophobic proteins, at specific domains, which allow for the membrane to actually curve into the circular structure that we are used to thinking about. Looking at this example, let me back up. Here we have this collection of proteins on the cortex. These membrane proteins are anchored here to all of these other types of proteins. In this case, it would be the ECM and different cytoskeleton components that are really responsible for supporting the cell structure and function. Now with an understanding of basic membrane protein functions, let's move on.

Laboratory Techniques

So in this video, I'm going to 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 think you use to wash your clothes or your dishes. Hopefully, you do both, and you use a detergent to do it. And what a detergent is, you have probably never even thought about it. But it is really just small molecules that have a hydrophobic tail and they form micelles in water, which are just kind of very small lipid layers. And, when mixed in membranes, these detergents can actually interact with the hydrophobic portions of the membrane and disrupt the bilayer detergents to mess up the bilayer 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, 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 kind of disrupt the protein's 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 is actually used to reveal the inner surface of the cell and the cell's cortex. So how this happens is that you have membranes or cells, and you freeze them really quickly, like super, super quickly. Then once they're completely frozen, flash frozen, they are pierced with a diamond knife, 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 bilayer.

And then, finally, there's this technique called FRAP, 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 fluoresces, and then you can bleach a fluorescent molecule by exposing it to a certain wavelength of light. And, by what I mean by bleach is, I mean that it loses its fluorescence. But, if you only, so, if the whole cell, for instance, has this fluorescent molecule on the lipid bilayer, but you only bleach this really small surface, eventually, if the lipids are moving, that is going to 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 bilayer with all these red fluorescent proteins, or lead 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 fluorescence in a really small proportion, so 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 phospholipids have moved out, 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.

Which of the following is not a type of membrane protein?

• A. Integral membrane proteins
• B. Peripheral membrane proteins
• C. Proteogylcan membrane proteins
• D. Lipid anchored membrane proteins

Which of the following membrane proteins does NOT attach to the membrane by binding to lipids?

• A. Integral membrane proteins
• B. Peripheral membrane proteins
• C. GPI-anchored membraned proteins
• D. Lipid-anchored membrane proteins

Which of the following secondary structures is most commonly found in membrane proteins?

• A. Coiled coil
• B. Beta sheet
• C. Alpha helix

Like lipids, membrane proteins have the same ability to act like a fluid and move around in the membrane.

• A. True
• B. False

What is the name of the collection of membrane bound and transmembrane proteins that are interconnected on the cytoplasmic surface of the plasma membrane?

• A. Proteoglycans
• B. Glycocalyx
• C. Cell Cortex
• D. Actin groupings

Which of the following techniques is best used when studying membrane fluidity?

• A. X-ray crystallography
• B. Detergents
• C. Freeze fracturing
• D. FRAP

Here’s what students ask on this topic:

What are the different types of membrane proteins and their functions?

Membrane proteins are classified into four main types: transmembrane proteins, integral membrane proteins, peripheral membrane proteins, and lipid-anchored proteins. Transmembrane proteins span the entire lipid bilayer and often form structures like alpha helices or beta barrels, facilitating the passage of molecules. Integral membrane proteins are embedded in the lipid bilayer and can either span the membrane or associate with one side. Peripheral membrane proteins are bound to the membrane indirectly through interactions with other proteins. Lipid-anchored proteins are covalently attached to lipids within the bilayer, providing strong anchorage. Each type of protein plays a crucial role in various cellular functions, including transport, signaling, and maintaining cell structure.

How do transmembrane proteins traverse the lipid bilayer?

Transmembrane proteins traverse the lipid bilayer by forming specific structures, primarily alpha helices and beta barrels. Alpha helices are the most common structure, where hydrophobic side chains of amino acids interact with the hydrophobic tails of phospholipids, allowing the protein to span the membrane. The hydrophilic portions are masked inside the helix. Beta barrels, formed by repeating beta sheets, create large channels that allow molecules to pass through the membrane. These structures enable transmembrane proteins to maintain stability and function within the hydrophobic environment of the lipid bilayer.

What techniques are used to study membrane proteins?

Several techniques are used to study membrane proteins, including the use of detergents, x-ray crystallography, freeze fracturing, and FRAP (Fluorescence Recovery After Photo Bleaching). Detergents disrupt the lipid bilayer to isolate proteins. X-ray crystallography helps determine protein structures but is less effective for membrane proteins due to detergent interference. Freeze fracturing involves freezing cells and splitting them with a diamond knife to reveal membrane proteins. FRAP involves labeling proteins with fluorescent molecules, bleaching a small area, and observing the recovery of fluorescence to study membrane fluidity and protein movement.

What is the role of glycosylation in membrane proteins?

Glycosylation, the addition of carbohydrates to proteins, plays a crucial role in membrane proteins. It occurs differently on each side of the bilayer, contributing to the asymmetry of membrane proteins. Extracellular glycosylation is important for protecting cells, preventing unwanted cell-to-cell contact, and blurring the barrier between the cell and the extracellular matrix. Glycosylated proteins, such as glycoproteins and proteoglycans, form structures like the glycocalyx, which coats the outside of the plasma membrane and supports cell structure and function.

How do membrane proteins contribute to cell structure and function?

Membrane proteins contribute to cell structure and function in several ways. They form complex structures like the glycocalyx, composed of glycoproteins and glycolipids, which coat the plasma membrane and provide protection. The cell cortex, a network of proteins on the inner surface of the membrane, interacts with the cytoskeleton to anchor the membrane and support the cell. Membrane proteins also facilitate membrane bending and curvature by inserting hydrophobic protein domains, allowing the membrane to maintain its shape. Additionally, they play roles in transport, signaling, and maintaining the asymmetry and fluidity of the membrane.