Vesicular Budding, Transport, and Coat Proteins - Online Tutor, Practice Problems & Exam Prep

Vesicle transport involves two main pathways: the secretory pathway, which moves materials from the endoplasmic reticulum (ER) through the Golgi to the cell surface, and the endocytic pathway, which brings substances into the cell. Vesicles are coated with proteins, primarily clathrin, COPI, and COPII, which aid in their formation and targeting. SNARE proteins facilitate vesicle fusion by ensuring specificity between vesicles and their target membranes, forming a tightly bound four-helix bundle. Energy from ATP is required to dissociate SNARE complexes post-fusion, allowing for repeated vesicle transport processes.

Overview

Hi. In this video, we're going to be talking about vesicle budding transport and coats. So this is going to be really just a short video overview of what I'm even talking about when I talk about vesicle transport and budding. So transport vesicles are little tiny membrane-enclosed organelles that move molecules between organelles and the plasma membrane. There are two main pathways here that we're going to talk about, and each one has a different mechanism and different proteins that work in them.

The first that I want to talk about is the secretory pathway. You can think of this as secretion, so things getting out of the cell. It begins internally in the cell, starting in the ER, moving through the Golgi, but ends at the cell surface. This is the secretory pathway, secreting something. Then you have the endocytic pathway, which is kind of the entrance pathway. This pathway begins at the plasma membrane at the surface of the cell, and it's going to bring all those molecules in. Those molecules get sorted and can go anywhere in the cell, such as the endosome, lysosome, Golgi, ER, or nucleus. But they have to enter through the endocytic pathway.

Short video, but these are the two pathways, and then we will get more into specifics later. Here we have some vesicles; some are being transported out. So through here, the secretory vesicles are leaving; they're being secreted. But then you have the alternate pathway of something potentially coming in and traveling through the Golgi, maybe then to the ER, and then maybe even to the nucleus if it decides. And that's going to be the endocytic pathway of entrance. So with that, let's get more into the specifics and move on.

Vesicular Coats

Okay. So now we're going to talk about vesicle coats. And so what I mean with the vesicle coat is exactly what it sounds like. So when we go outside, we put on a coat because we're cold. Well, vesicles, they put on their coats, but their coats are actually proteins. And so, this happens most of the time, not all the time, but the majority of the time, vesicles, when they bud off of organelles or the plasma membrane or wherever they're going, they need to be protected by that, by a protein coat. And, that helps direct them to the proper place, and it helps them form, and it helps them do everything they need to do to transport materials. So, there are 3 types of vesicle coats. These are all 3 proteins. The first is going to be clathrin-coated vesicles, and these travel between the Golgi and the plasma membrane. Then you have the COP 1 and COP 2, which each transport through different plates. So, COP 1 does Golgi towards ER and COP 2 is buds from the ER. So you'll also notice if you kind of think about the positioning of the cells that if it buds from the Golgi towards the ER, that's actually going to be inwards movement. Whereas buds from the ER, that's going to be outward movement. And so these are the 3 main protein coats that transport things throughout the cell. So what it might look like is you have all these different things that need to get inside these stars. They bind to receptors on the plasma membrane, and then you have protein coats here that cause help form that vesicle, and eventually, this coat protein is surrounding the entire vesicle. These little red things here are the protein, and that helps the vesicle form and transport to the proper location. Now, one of the main ones that we're going to talk about is the clathrin coat and that's because clathrin coats are extremely important in driving vesicle formation. So these are kind of the, instigators especially at the plasma membrane. So how this happens is there's actually these proteins called adapter proteins, and these bind both the clathrin and the transmembrane proteins that are being transported. So the important thing here is that clathrin isn't actually binding to anything that it's transporting. Clathrin is just acting as the coat. It's not, you know, interacting with particular proteins. It's not interacting with anything that it needs to get in. It's just acting as a coat, so it's just coming on. But how it connects to those things that need to get inside the cell are through adapter proteins. So clathrin binds the adapter proteins, and the adapter proteins are the ones that are specific for the the things that need to get in. And so one of the things that cargo or that adapter proteins bind are things like cargo receptors. And so, sometimes, the vesicle needs to transport soluble molecules. So these are things that are just kinda floating outside of the cell or in a certain vesicle or wherever it is. We'll just say outside of the cell. So, there's a molecule that needs to get inside. It needs to be transported, but it's just kind of floating out there. Well, it's got to somehow interact with the membrane, so it can get into the vesicle. So, it does that through binding cargo receptors. And then those cargo receptors bind adapter proteins, which then go and bind clathrin. So once all of these interactions have taken place and the vesicles ready to bud, what you get is this other protein, dynamin. And this, you can kind of think of as, I guess, a ring. So it comes in and it simples a ring around the neck, and it uses the energy from GTP and just pinches it off. So what this looks like here. So you have some type of, soluble cargo, and it needs to bind to its cargo receptor. Once it's here, you have an adapter. Protein come in, so you have your receptor, your cargo, and your adapter. And the clathrin is going to come in, and it's going to bind the adapter. Then, once you have enough of these formed, then the vesicle starts budding. So you have your clathrin coat here, you have your adapter here, you have your receptor here, and you have your soluble, thing here. So if you want to think of it, it's kind of like CARS. Right? So you have your clathrin. Let me actually write this down here. RS. You have your clathrin. You have have your adapter. I can't spell for anything. Oh, my goodness. A d a v t o r. You have your receptor and you have your soluble cargo CARS. So then, this eventually forms into a vesicle, and, the dynamine will come in right here, and it'll pinch that off, so that it actually forms this vesicle that can be transported anywhere in the cell. So remember CARS, and that's with clathrin. So, obviously, this needs to be regulated. Everything in the cell needs to be regulated. So how it's regulated are through GTPases, which regulate the recruitment of the coats. So, obviously, vesicles won't form without the coats, so if the coats never get there, then they won't form. Those things won't be transported. So, GTPases sort of regulate this process by controlling whether or not the coats get to the membrane. So, when a coat protein comes to the membrane and it binds to an adapter, this triggers this transition, so, GDP goes to GTP. Then, once GTP is activated, that assembles more, coat proteins to the to the area. Once those coat proteins are here, RAB proteins are the GTPases, that are responsible for controlling the specificity. So the coat proteins that arrive, they they are all gonna be the same type of coat protein because these GTPases, control which coat proteins can get there. And so, yeah. So each vesicle has a different combination of RAB proteins, and each one of those controls which coat gets there. And then, once the coat gets there, it changes to GTP, and that GTP, sort of, recruits all these all the same coat. So, we see all these different RAB proteins. So, here, early endosome has RAB5, here is a RAB7. Now, you don't need to remember these RAB numbers. But, know that all of these that I've circled, these are different RAB proteins. And so, each one is a different GTPase. RABs are GTPases. And they recruit different, coat here, and the one that's here, and this one as well. And so, the the specificity of the coat protein that's recruited is entirely controlled by these, RAB GTPases. So these are kind of like the bouncers. You know, they say, you are allowed to come and you're not. So those are the RAB proteins. So with that, let's now move on.

SNARE Proteins

Great. In this video, we are going to focus on SNARE proteins, and these are super important proteins for vesicle fusion. SNARE proteins are the proteins that allow the vesicle to fuse because you can kind of imagine that it's not exactly easy for a membrane to fuse. First, they have to get to the location, then they have to actually get close enough to fuse. SNARE proteins help in both getting close enough for membrane fusion to occur and also providing specificity. You don't just want the first thing the vesicle runs into have it fused with that; you want it to fuse where it needs to go. So, SNARE proteins have these two functions to allow them to catalyze that membrane fusion and, also, to provide specificity. There are two types of SNAREs which reside within the membrane.

You have T-SNAREs. These are two to three target SNAREs, and they reside on the target organelle. So, if a vesicle needs to get to the Golgi, there are T-SNAREs that are specific for the Golgi. If the vesicle needs to get to the ER, then there are T-SNAREs that are specific for the ER. So, this is where the specificity comes in. Then you have a V-SNARE, and this is the protein that resides on the vesicle itself. And so, this V-SNARE is like searching out the matching T-SNAREs so that it can find which one it's supposed to bind to and help that fusion. Once those two SNAREs meet, so once the proper V-SNARE meets the proper T-SNARE, then they're like, okay, awesome. We're together. You know? We're going to stay together.

And so, they actually come together to form this 4-helix bundle. So remember, there are two to three here, one here. So that's going to equal four as long as this is three. But it forms this bundle that's really tightly bound to each other. And this tight boundness results in the vesicle fusing together. They just get so wrapped around each other and so tight that the vesicle really can't escape. So the only thing that it's going to do is it's going to fuse. And, after it fuses, you still have the SNAREs bound together. So what you have to do is you actually have to have energy come in and break them apart. So, that energy that comes in and breaks them apart is ATP. And there is this special factor called the NSF, here is the long word for it, but don't worry about that. NSF uses ATP hydrolysis to release the bound SNAREs. This is an important vocab word to know when it comes to SNARE proteins. So, what we see here is we have our vesicle, we have our one V-SNARE, and we have our target organelle here. So, you can see there are the different three T-SNAREs. And these form this 4-helix bundle, their specificity, and are targeted to specific organelles and what allows them to fuse. They are really important. So, with that, let's now move on.

Which of the following is not a protein coat?

• A. COPI
• B. COPII
• C. COPIII
• D. Clathrin

Which of the following coats is used on vesicles that transport molecules from the Golgi to the ER?

• A. COPI
• B. COPII
• C. COPIII
• D. Clathrin

Vesicle fusion requires all but which of the following?

• A. T SNARES
• B. V SNARES
• C. Trans SNARE complex
• D. Rab GTP proteins

Here’s what students ask on this topic:

What are the main pathways involved in vesicle transport?

Vesicle transport involves two primary pathways: the secretory pathway and the endocytic pathway. The secretory pathway moves materials from the endoplasmic reticulum (ER) through the Golgi apparatus to the cell surface, facilitating the secretion of substances out of the cell. In contrast, the endocytic pathway brings substances into the cell from the plasma membrane. These substances are then sorted and directed to various organelles such as endosomes, lysosomes, the Golgi, or the ER. Both pathways are essential for maintaining cellular function and homeostasis.

What are the different types of vesicle coat proteins and their functions?

There are three main types of vesicle coat proteins: clathrin, COPI, and COPII. Clathrin-coated vesicles are involved in transport between the Golgi apparatus and the plasma membrane. COPI-coated vesicles mediate retrograde transport from the Golgi to the ER, while COPII-coated vesicles facilitate anterograde transport from the ER to the Golgi. These coat proteins help in vesicle formation, provide structural support, and ensure that vesicles are directed to their correct destinations within the cell.

How do SNARE proteins facilitate vesicle fusion?

SNARE proteins are crucial for vesicle fusion with target membranes. There are two types of SNAREs: v-SNAREs on vesicles and t-SNAREs on target membranes. When a vesicle approaches its target, the v-SNARE and t-SNARE interact to form a tightly bound four-helix bundle. This interaction brings the vesicle and target membrane close enough to fuse. After fusion, ATP and the NSF protein are required to dissociate the SNARE complex, allowing the SNAREs to be reused in future transport processes.

What role do GTPases play in vesicle coat recruitment?

GTPases, such as RAB proteins, regulate the recruitment of vesicle coat proteins. When a coat protein binds to the membrane, it triggers the conversion of GDP to GTP. This activation recruits more coat proteins to the membrane, facilitating vesicle formation. RAB GTPases ensure specificity by controlling which coat proteins are recruited, acting like bouncers that allow only the correct coat proteins to assemble on the vesicle. This regulation is crucial for the proper targeting and transport of vesicles within the cell.

What is the function of clathrin in vesicle formation?

Clathrin plays a vital role in vesicle formation, particularly at the plasma membrane. It acts as a coat protein that helps shape the vesicle and provides structural support. Clathrin does not directly interact with the cargo being transported; instead, it binds to adapter proteins, which in turn bind to cargo receptors. This interaction facilitates the budding of the vesicle. Once the vesicle is formed, the protein dynamin pinches it off from the membrane, allowing it to be transported to its target destination.