Actin Filaments: Study with Video Lessons, Practice Problems & Examples

Actin filaments, or microfilaments, are crucial components of the cytoskeleton, primarily facilitating cell movement and shape changes. Comprised of G-actin monomers that polymerize into F-actin filaments, they exhibit polarity with a plus and minus end, influencing growth dynamics. Proteins like the Arp2/3 complex and formins assist in nucleation, while profilin and cofilin regulate actin stability. Actin can form bundles, networks, and structures like microvilli, enhancing cellular functions such as absorption. Understanding these dynamics is essential for grasping cellular motility and structural integrity.

Structure

Hi, in this video we're going to be talking about actin filaments. So, we're going to go over a lot of things with actin filaments, but first, we're going to just start simply. We're going to talk about the structure. So, actin filaments, remember you can call them microfilaments, it's the same thing no matter what. They're part of the cytoskeleton and their main function is going to be cell movement. So if the cell needs to get from one place to another, or if it just needs to change its shape for whatever reason, that's what actin is going to do. Actin is really important for moving the cell somewhere or changing the cell shape. And remember, that's because the actin's right at the plasma membrane, and the plasma membrane is really what's determining the cell shape. So, if you want to change the cell shape, you've got to control the actin filaments. Remember, actin filaments are composed of monomers, and those monomers are called G actin. So, these individual G actin monomers are what make up a large actin filament, and we call these actin filaments F actin filaments. So, if you just have a single one, that's going to be a G actin, but if you have an entire filament, that's going to be F actin. And, in actin, it's not only just this one filament here, what happens is you actually get two that end up being kind of wrapped around each other like a rope. So, this is a really poor drawing because I am not an artist, but I'll show you below in this image. You can see here that we have our different filaments, but this together here is going to be the F actin, because we have one that's round together, and if we were to keep going, it would look like this, but we have our second filament going like this. So, they end up getting wound together like a rope, and that forms the F actin filaments. Now, there are many different types of actins. I'm talking about G actin and F actin because those are the most important ones, but there are more than that, and the different types of actins depend on where in the body it is. So, alpha actin is going to be in muscle tissue. Beta actin is going to be in non-muscle tissue, and gamma is also going to be a non-muscle. So, there are many different types. These aren't necessarily important, just know that there are many different types and these different types are in different tissues. You don't have to memorize which types are in which one, but just know that that's a fact that you need to know. So, the position of the actin subunits, the monomers here, anytime I say actin subunits, I'm talking about monomers, what they do is those positions provide polarity to the F actin filament. So, do you know what polarity is? Polarity, what that means is that this end here is different from this end. So, that they have two different ends. And kind of like a magnet, you know, it has a positive side and a negative side. It's exactly the same way with polarity, except, instead of, I mean, I guess it is very similar depending on which term you use, but in actin, there is a minus end and a plus end, similar to magnets. Right? But sometimes in some books, and your professor may use these other terms, which are pointed and barbed end. But essentially, it's the same thing. If you have a plus end over here and a minus end over here, then when these come together, it's going to be minus-plus. Right? And that's going to make anything on this side the plus end of the filament and anything on this side the minus end. And so that is polarity, a kind of directionality of sort to these actin filaments. And knowing things about which side is the plus and which side is the minus is super important and we will get to why in just one second. But, I want to go over this. We have our G actin, these are the monomers remember? And, we have our, ATP, which comes in. I haven't talked about this yet, but I definitely will talk about ATP in just a second. And, ATP comes in, it binds to these G actin monomers that allows for the formation of these filaments and these are the F actin filaments because they contain two strings of protofilaments. So, if we want to create active filaments and we want to remember they're dynamic, so they move a lot. There's a lot always being formed, and some are being taken away or degraded. How do we actually do that? And so, actin filaments polymerized similarly to that of microtubules. So what does that mean? It means that on either side, remember we have this protofilament here, this is going to be a plus side, this is going to be a minus. These G actin subunits can be added to either end. It doesn't matter. Right? As long as the plus is lining up with the minus, it can be added on either end. However, one end is favored. So the plus end here goes faster than the minus end. So, active filaments are going to want to add to this end more than they're going to want to add to this, to the minus end. And so, why is that? What is the purpose of that? And this has to do with ATP. So, to add a monomer to a filament, it has to hydrolyze or break down ATP. Right? And so, whenever it gets added, so here we're going to add this on. Right? And it has an ATP here. Then whenever it gets added, that's going to release ADP and that phosphate, and then that bond is going to form this G actin. Now, how quickly that happens depends on which side of the filament is going to be added. So, if this happens slowly, then the filament is going to grow, and that's a little backwards. I usually think, oh, if it happens fast, then it's going to grow, but that is not the case here. It's kind of a little bit logically backwards. So, if it hydrolyzes slowly, then it's going to grow. If it hydrolyzes quickly, so ATP is broken down super fast, then that actually destabilizes that bond, and then the loss of actin polymers happens. So, if we have our filament here, let me draw it out again. Right? We have our plus end, we have our and this is occurring faster over here and it's occurring slower over here. And the reason that it's doing that is because here, ATP is hydrolyzed slowly. Right? And that allows everything to stabilize before, it allows it to stabilize and add a couple more on before hydrolysis happens. But, on this side, it hydrolyzes, or ATP breaks down quickly, and then what happens is it's just too fast. And, so, when it happens too fast, it can't stabilize, and if it doesn't stabilize, then these start falling off. Okay, so then you lose it at the minus end. So, the plus end is faster, and the minus end is slower because the ATP is being hydrolyzed too fast at the slower end, and that's causing a loss of these monomers. So, when we talk about growth and loss, there are two terms that are going to come up a lot. And we've talked about these before, but I do want to mention them again because students really get these confused a lot. And that's the difference between dynamic instability and treadmilling. And the best way to remember this is that dynamic instability is happening at one end. It's happening at the plus end here. So, that's going to be the fast end, remember. And what's happening here is it's switching. So, it's going from growth, meaning that the ATP is hydrolyzing slowly, and then it switches to shrinkage, meaning that the ATP hydrolyzed too quickly, which you can imagine that just happens. Right? Sometimes ATP just hydrolyzes quickly, and then that destabilizes that plus end and can shrink the molecule. So that is dynamic instability when it's happening just at the plus end. Treadmilling is happening at both the plus and the minus, And what happens is we're growing at the plus end, and we're shrinking at the minus end. So if it's only talking about one end, you're going to say that's dynamic instability. If it's talking about something happening at both, that's treadmilling. So let's go over this again. We've gone over our individual parts. We start off with our G actin monomers. Right? And, ATP is added to them, which activates them. So, now they're activated when they have ATP bound to them. Then we go through the process of creating that first polymerization, which is called nucleation. Right? And this is the hard part, but once nucleation has actually occurred, then individual monomers can easily add on to this, this growing filament. And what we get is when we have our two filaments, we call this an actin filament. And remember, these are wound around each other, kind of like twisting a rope would be. Right? And, what happens is when, ATP is, once these filaments are added, that ATP is, hydrolyzed and that creates ADP and the phosphate, and that releases the energy that can allow these things to grow. So with that, let's move on.

Organization

Okay. So now we're going to talk about some proteins that actin is associated with and how all of these things are organized. So, there are many proteins associated with actin, and these assist in processes like nucleation, which, if you remember, is the start of forming a type of filament. But this can also be important for actin's function. The proteins that are really responsible for nucleation or helping nucleation occur are the ARP or ARP2/3 complex and formins. These assist in nucleation. Then, you have some proteins called ADF and cofilin, which bind to actin and help enhance the dissociation of ADP from actin. This means that when actin loses ADP, it is ready to gain ATP, or it can easily transition to ATP. On the opposite end, profilin reverses this action and can stimulate the addition of actin monomers into filaments.

These are really important proteins that you're going to have to know. Just as a reminder, because we're working with ADP and ATP, if the ATP is going to be hydrolyzed quickly, that is going to destabilize it because ADP-actin is not very stable, and usually, this occurs on the minus end. This information might help if you get a little confused about what these proteins are doing. Profilin reverses the action of cofilin, enhancing the dissociation of ADPs, trying to make it more stable, and stimulates the addition of active monomers into the filaments.

Then, you have proteins that regulate actin in many different ways. They can do things like regulate polymerization, cap the filaments to prevent them from growing or destabilizing, cross-link them to other structures, sever, bundle, and attach to them. Proteins have a lot of functions. Here's an example: the ARP2 complex subunits come in and eventually grow another actin branch off of another one. There are multiple proteins that do this process.

The cell organizes actin in a few distinct ways, not all of which I'm mentioning, but these are some important ones that your book is going to mention. These are actin bundles when actin filaments are cross-linked into very closely packed parallel arrays. You can think of it like a bundle of sticks, all facing the same way. Then, you have actin networks, which are cross-linked into orthogonal arrays, and these are much more dispersed. This forms a 3D meshwork, and you can think of it as a mesh. When actin forms like this, it can create a structure with similar characteristics to a semi-solid gel, which is spongy.

Then you have the cell cortex, which is a structure in the cell formed by actin filaments and associated proteins, lying beneath the plasma membrane. Then you have microvilli, which are finger-like extensions of the plasma membrane, typically involved in absorption. The microvilli, made primarily of actin, create a brush border, which is a layer of microvilli on the cell surface. Here's an example: imagine a brush border with a ton of microvilli on top of the cell. These are all made of actin. Also, consider the green structures as actin networks forming the cell cortex beneath the membrane.

So that's actin. With that, let's now move on.

Which of the following proteins are associated with actin nucleation?

• A. Arp 2/3
• B. ADF
• C. Profilin
• D. Integrin

Actin monomers are added to both the minus end and the plus end of a growing actin filament?

• A. True
• B. False

Which of the following terms describes the addition of monomers at the plus end and the loss of monomers at the minus end?

• A. Dynamic instability
• B. Treadmilling

If ATP at the minus end is hydrolyzed quickly, what happens to an actin filament?

• A. The filament grows at the minus end
• B. The filament is destabilized at the minus end
• C. The filament grows at both the minus and plus ends
• D. The filament is destabilized at both the minus and plus ends

Here’s what students ask on this topic:

What is the structure of actin filaments?

Actin filaments, also known as microfilaments, are composed of G-actin monomers that polymerize to form F-actin filaments. These filaments are double-stranded helices, resembling two intertwined ropes. Each G-actin monomer binds ATP, which is hydrolyzed to ADP upon incorporation into the filament. The filaments exhibit polarity with a plus (barbed) end and a minus (pointed) end, influencing their growth dynamics. The plus end typically grows faster due to slower ATP hydrolysis, while the minus end grows slower and is more prone to depolymerization.

What are the functions of actin filaments in cells?

Actin filaments play crucial roles in various cellular functions. They are essential for cell movement, enabling processes like cell migration and cytokinesis. Actin filaments also contribute to maintaining and changing cell shape by forming structures like lamellipodia and filopodia. Additionally, they are involved in intracellular transport, anchoring organelles, and forming specialized structures such as microvilli, which enhance absorption in epithelial cells. Actin's dynamic nature allows it to rapidly assemble and disassemble, facilitating these diverse cellular activities.

How do actin filaments exhibit polarity?

Actin filaments exhibit polarity due to the orientation of their G-actin monomers. Each filament has a plus (barbed) end and a minus (pointed) end. The plus end is where ATP-bound G-actin monomers are preferentially added, leading to faster growth. In contrast, the minus end typically undergoes slower growth and is more prone to depolymerization. This polarity is crucial for the directional movement of cells and the dynamic behavior of actin filaments, as it influences the rates of polymerization and depolymerization at each end.

What proteins are involved in the regulation of actin filaments?

Several proteins regulate actin filaments. The Arp2/3 complex and formins assist in nucleation, initiating filament formation. Profilin promotes the addition of actin monomers, while cofilin enhances the disassociation of ADP-actin, facilitating turnover. Other proteins, such as tropomyosin, stabilize filaments, and capping proteins prevent further polymerization at filament ends. Cross-linking proteins like filamin organize actin into networks, and severing proteins like gelsolin break filaments into shorter fragments. These regulatory proteins ensure the dynamic and functional versatility of actin filaments in cells.

What is the difference between dynamic instability and treadmilling in actin filaments?

Dynamic instability and treadmilling are two behaviors of actin filaments. Dynamic instability occurs at the plus end, where the filament alternates between phases of growth (slow ATP hydrolysis) and shrinkage (rapid ATP hydrolysis). Treadmilling involves simultaneous growth at the plus end and shrinkage at the minus end, maintaining a constant filament length. In treadmilling, actin monomers are added to the plus end while being lost from the minus end, creating a steady-state flow of subunits through the filament.