Actin Filaments

Hi in this video, we're gonna be talking about acting filaments. So we're going to go over a lot of things with active filaments. But first we're going to just start simply, we're going to talk about the structure. So active filaments, remember you can call them micro filaments, the same thing, no matter what their part of the side of skeleton 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, acting is going to, that's what acting is going to do. So acting is really important for moving that cell somewhere or changing the cell shape of the acting. And remember that's because the acting is right at the plasma membrane and the plasma membrane is really what's determining the cell shape. So, you want to change the cell shape, you've got to control the acting filaments now remember acting filaments that are composed of monomers. And those monomers are called G acting. So these individual g acting are those are those monomers. So they're kind of like best drawing them in circles later. But essentially each one of these is a g active. And together they make up a large acting filament and we call these active filaments, f acting filaments. So if you just have a single one that's gonna be a g acting. But if you have an entire filament that's gonna be f acting and in acting, it's not only just this one filament here, what happens is you actually get to 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 acted because we have our one that's round together and if we were to keep going, it would look like this. But if we have our second filament going like this, so they end up getting wound together like a rope. And that forms the F acting filaments. Now there are many different types of active. I'm talking about G acting and F acting because those are really the most important ones. But there's there's more than that and the different types of acting depend on where in the body it is so alpha active. That's going to be a muscle tissue, Beta acting is going to be a non muscle tissue and GAMMA is also going to be a non muscle. So there's a lot of 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 and which one, but just know that that's a that's a fact that you need to know. So the position of the acting sub unit. So the monomers here, anytime they act in subunits. Talking about monomers. What they do is those positions provide polarity to the f active filaments. So do you know what polarity is? So 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 has a positive side and a negative side is exactly the same way with polarity except instead of I mean I guess it's very similar depending on which term you use but acting, there's a minus end and a plus in similar to magnets. Right? But sometimes in some books and your professor may use these other terms which are pointed and barbed in. But essentially it's the same thing if you have a plus and 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. Kind of a directionality of sort to these active filaments. And knowing things about which side is the plus and which side is the minus is super. Super important. And we'll get to why in just one second. But I want to go over this, we have our G. Active, these are the monomers remember and we have our A. T. P. Which comes in. I haven't talked about this yet but I definitely will talk about a teepee in just a second. Um and a teepee comes in it binds to these g. Active monomers that allows for the formation of these filaments. And these are the f active elements because they contain two strings of protein filaments. So if we want to create active filaments and we want to remember their 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 active filaments prelim arise similarly similarly to that of micro tubules. So what does that mean? It means that on either side remember we have this proto filament here. This is going to be a plus side. This is going to be a minus. These G. Acting sub units can be added to this end or they can be added to this end. It doesn't matter. Right? As long as the pluses mining 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 gonna want to add to this end more than they're gonna want to add to this to the minus end. And so why is that like what what is the purpose of that? And this has to do with a. T. P. So um to add a monomer to a filament it has to hide relies or break down a teepee right? And so in order to um so whenever it gets added. So here we're gonna add this on, right and it has an A. T. P. Here then whenever it gets added that's going to release A. D. P. And that phosphate. And then that bond is going to form this G. Act in. Now how quickly that happens depends on which side of of the filament is going to be added. So if this happens slowly then the filament is going to grow. And I guess that's a little backwards. I usually think oh if it happens fast then it's gonna grow. But that is not the case here, it's kind of a little bit logically backwards. So if it hydra lies is slowly then it's going to grow. That hydra lies is quickly. So eight ep is broken down super fast. Then that actually destabilizes that bond. And then the loss of acting polymers happen. So if we have our filament here and we draw it out again right? We have our plus in we have our minus. 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 A T. P. Is hydrolyzed slowly. Right? And that allows everything to stabilize before um to stabilize and add a couple more on before hydrolyzed ation happens. But on this side it hydra lies is or a teepee 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 A. T. P. Is being hydrolyzed too fast with the slowest slower end. And that's causing a loss of these monomers. So when we talk about growth and loss there are two terms that is going to come up a lot and we've talked about these before but I do want to mention them again because people students really get these confused a lot. And that's the difference between dynamic instability and tread milling. 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 gonna be the fast end, remember. And what's happening here is that switching? So it's going from growth meaning that the A. T. P. Is hydra. Izing slowly and then it switches to shrinkage meaning that the A. T. P. Hydrolyzed too quickly. Which you can imagine that just happens right? Sometimes a teepee just hide relies is quickly and then that destabilizes that plus in and can shrink the molecule. So that is dynamic instability. When it's happening just at the plus in tread milling 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. And 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 tread milling. So let's go over this again. We've got over individual parts. We start off with our G. Active monomers, right? And A. T. P. Is added on to them, which activates them. So now they're activated when they have a teepee bound to them. Then we go through the process of creating that first preliminary ization which is called nuclear station. Right? And this is the hard part. But once nuclear station has actually occurred, then individual monomers can easily add on to this. Um This growing filament. And what we get is what we have are two filaments. We call this an acting filament. Remember these are round around each other. Kind of like twisting a rope would be right. And what happens is when A. T. P. Is um once these filaments are added that A. T. P. Is hydrolyzed and that creates that that creates ADP and phosphate and that releases the energy that can allow these things to grow. So with that let's move on.

Okay so now we're gonna talk about some proteins that acting is associated with and how all of these things are organized. So there are many proteins associated with acting and and these assists and things like nuclear station which if you remember what nuclear nation is that's going to be the sort of you know beginning the start performing some type of element. But there can also be important for the actions function. So the thing that is we're really responsible for Nuclear Nation or helping new creation occur or that our P. R. RP too complex informants. And these assist in Nuclear Nation. Then you have some proteins called A. D. F. And Coughlin. And these um bind to act in and help enhance the disassociation of ADP acting. So that means that helps the disassociation of A. D. P. So that means that you know whenever it loses a DP is ready to gain a teepee or it can easily go to A. T. P. Now there's the opposite is pro filling reverses this action. Um and actually can stimulate the addition of active monomers into filaments. So those are really important proteins that you're gonna have to know. Um And just just as a reminder here remember these because we're working with A D. P. And A. T. P. If the A. T. P. Is gonna be hydrolyzed quickly that is going to destabilize it because A D. P. Acted is kind of just is not very stable. Um And usually this occurs on the minus and so that kind of just use that information if you get a little confused about what these things are doing. So enhances this association of A. D. P. So it's trying to make it more stable and fulfilling is going to reverse this um and stimulate the addition of active monomers into the filaments. So then um you have proteins that regulate acting in many different ways. They can do things like regulate clintonization. They can cap them from you know and prevent them from growing or destabilizing. They can cross link them to other things, sever them, bundle them attached to them and proteins have a lot of functions. So um here's an example of the RP two complex which if you remember is really responsible for nuclear station and you can see it here right here actually nuclear waiting. So you can see these small acting sub units are coming in and they're gonna eventually grow another acted branch off of this other one here and there's all these other different proteins, you don't need to know those but there's multiple proteins that do this process. So the cell organizers acting in a few distinct ways. Not all I'm not mentioning all of them but these are some important ones that your book is gonna mention. These are active bundles. Um and these are when acting filaments are cross linked into very closely packed parallel race. You can kind of think of like a bundle of sticks, they're all facing the same way and you know you can't have you know some being vertical and them being horizontal and call it a bundle. Um So this is very similar to an active bundle. Then you have active networks and these are actually cross linked into orthogonal race and so these are much more like all over the place. So kind of like this or you know it doesn't have to everything be like in this one direction like in a bundle but networks go all over the place and this is this actually forms this three D mesh. Works. You kind of think of it as mesh and when acting actually forms like this it can conform this form the structure that has similar characteristics to a gel. Like a semi solid gel. It's kind of like spongy and gross and stuff like that. So then you have the cell cortex and this is actually like an organ l or not quite organelles but it's a structure in the cell that acting will form and this is composed of active filaments and associated proteins. It lies beneath the plasma membrane and then you have micro villi and these are kind of like finger like extensions of the plasma membrane. So the plasma membrane will actually go out and it'll form these like finger like things that are typically involved in absorption. The micro villi are really like made of acting and so if a cell has a lot of micro villi, it's said to have a brush border. Can kind of think of like the micro villi as like like the little pokey is on a hairbrush. Um So it's called a brush border. And that's a layer of micro villi on the cell surface. So here's an example. So here would be a brush border because there's a ton of my micro ville I right here on top of the cell and you can see that these are all made of acting. But then you also have, so we'll say that this is the plasma membrane or some type of membrane. And you can see that there's we'll just say these green things are acting, he's acting networks and these acting networks for these um can form the cell cortex underneath the membrane. Um So that's active. So with that let's not 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