Golgi Processing and Transport - Online Tutor, Practice Problems & Exam Prep

The Golgi apparatus processes proteins received from the endoplasmic reticulum (ER) through vesicular transport. Proteins are modified via glycosylation, including N-linked and O-linked types, crucial for stability and function. The Golgi consists of cisternae organized into cis, medial, and trans sections, facilitating protein maturation. Proteins can move through the Golgi via the vesicular transport model or the cisternal maturation model. Additionally, proteins are sorted for transport to their final destinations, utilizing anterograde and retrograde transport mechanisms, ensuring proper cellular function.

Leaving the ER

Hi. In this video, we're going to be talking about Golgi processing and transport. So, in order for proteins to get to the Golgi, they first have to leave the ER. This is what this video is going to be about. When proteins leave the ER, they automatically just end up at the Golgi, for the most part. Not everything, but most proteins that leave the ER end up in the Golgi. In order to leave the ER, they have to be transported out in vesicles. The vesicles that transport proteins are marked for exit from the ER and entrance into the Golgi by some type of sorting signal. These vesicles are called COP II vesicles, and that means they have this protein code called COP II, and we're going to go over different types of protein codes in different lessons. But just know right now that in order to leave the ER and travel to the Golgi, they have to transport out through vesicles. Only properly processed proteins can exit the ER for the Golgi. If the protein isn't folded right, if it doesn't look right, it's not going to leave the ER. The reason is that chaperone proteins can come in and control that folding, and then improperly folded proteins are going to be transported out of the ER, usually through some type of misfolded protein response or unfolded protein response, and left up to the proteasome to degrade them. If a protein wants to get to the Golgi, it has to be processed properly. Once it's packed into a vesicle and is properly folded, the protein wants to arrive at the Golgi, so that is going to depend entirely on vesicle fusion. The protein is in a vesicle, and if it wants to get into the Golgi, it's going to have to fuse. This type of fusion is given a special name, heterotypic fusion, which is membrane fusion from two different compartments. The two compartments here are going to be the ER and the Golgi, which have slightly different membrane components. Now, there is this interesting thing that can sometimes happen, and I have it here italicized because you don't necessarily need to know about it, but it is interesting, and you may read about it. It's called Vesicular Tubular Clusters. These are actually ER vesicles that fuse together to create a big compartment. These are tiny vesicles that add on each other, making this large ER vesicle compartment that will then fuse with the Golgi, delivering all of its content at once, instead of in little spurts throughout between all these different vesicles. What happens if a protein that wasn't supposed to leave the ER actually gets transported to the Golgi? Well, these proteins contain retrieval sequences that direct them back to the ER if they leave. An example of this is a sequence called KDEL, which is just like a tag that says, "I'm not supposed to be here, please get me back home," and that home is the ER. They get directed back to the ER through different types of vesicles. Here we have the ER and our Golgi. You can see that there's heterotypic fusion, where these little tiny vesicles all go, and they're all going to individually fuse. Then, you have these vesicular tubular clusters, which start out as little vesicles but eventually fuse together to create this large compartment that will fuse with the Golgi. That is how proteins leave the ER and arrive at the Golgi. So, let's now turn the page.

Golgi Structure

So, in this video, we're going to be talking about what the Golgi looks like. The Golgi complex, which you may also see referred to as the apparatus, has a distinct structure composed of flattened membrane-enclosed stacks. Each one of these stacks is called a cisterna, and they make up the Golgi. There are about 3 to 20 per Golgi, and that number depends on the cell type. Some have more, some have less.

The Golgi is organized into three sections: You have the cis-Golgi, which faces the ER; you have the trans-Golgi, which faces the plasma membrane; and then you have the medial Golgi, which sits between them both. So, if we look here, we have our cis face, which is going to face the ER—a vesicle that's coming in from the ER. Then, we have our trans face, which is going to face the plasma membrane, and here are the outgoing vesicles that are going somewhere out. And then, in between here, we have our medial, and each one of these sacs here is called the cisterna. So, this is the unique, blob-like looking Golgi. So with that, let's now turn the page.

Golgi Protein Modifications

Okay, so now we're going to talk about glycosylation and other protein modifications that happen in the Golgi. So, for a protein that wants to get out and do something, it's got to look pretty. So, that first starts in the ER, say with its makeup, and then it goes to the Golgi to do its hair. And so, the Golgi complex is a major location where proteins are modified. So, the first modification I want to talk about is the first one that we talked about in the ER, and that's going to be glycosylation. So, glycosylation, remember, is the addition of carbohydrates, Golgi. So the first is going to be N-linked glycosylation, and that is when a sugar is linked to a nitrogen atom, so that's going to be an N. And then you have O-linked glycosylation, which is going to be added to a hydroxyl group, which if you remember what that is, is an OH, O-linked. And notice also as well that these happen on different amino acids, although you don't need to know which amino acids they happen on, not for cell biology anyways. And so protein glycosylation is important for protein folding and stability. So here's your two forms, so here's your N-linked, which you can see is added here. And here's your O-linked, which you can see is added here. So you have your glycosylation added onto these different amino acids. So, there is a term called terminal glycosylation, and that is going to be the final modification that happens to glycosylation. So, remember, glycosylation all starts out the same way. It all starts out with its foundation, but all these different modifications can happen to it so that it creates all these different sugars on these proteins. But there is a final one that happens, and it's called terminal glycosylation, and this final one occurs in the Golgi. So, once it leaves the Golgi, it's not going to have any more modifications done to it. So, it needs to make sure it finishes it here. So, for instance, one terminal glycosylation that happens is for N-terminal glycosylation. That's going to occur in the ER, and it removes glucose and mannose sugars in the Golgi. Now, you don't need to necessarily know this, just know this, this happens here. And so, there are some enzymes responsible for this, I've italicized them in case you read about them and are just wondering what these do, but these are the enzymes responsible for this modification. Now, proteins undergo a lot of different modifications in the Golgi, and each modification occurs in a different cisterna location or region or whatever you want to put here within the Golgi. And so, each one of the Golgi regions, there are functional differences. So, the cis does different things than the trans, which does different things in the medial. So here, we have protein modifications. So remember, this protein is going to start out here in the ER, but eventually, it gets transferred to the Golgi. So this whole thing here is the Golgi. And you can see that modification, so it starts out with all these different colors and ends up with these. So these modifications are happening in the Golgi before they're transported to the plasma membrane. So the Golgi is a major place for protein modification. With that, let's now turn the page.

Golgi Protein Transport

Hi. So in this video, we're going to talk about Golgi maturation and protein transport. So far, we've been focusing specifically on getting protein to the Golgi or what happens to proteins when they're in them, but we need to really spend some time talking about how proteins actually travel through the Golgi because it has a unique way of traveling. There are two ways that proteins can move throughout the Golgi. The first is called the vesicular transport model, which means that those Golgi cisternae we talked about don't move. They're just completely stationary, they're always there, And instead, what happens is that a protein that needs to travel through the Golgi will do so in little vesicles. Now, there's another model that says cisternae maturation model. And this means that, the cisternae themselves actually move up, So, they travel up, and eventually, it gets to the point where it becomes the last one. And then, that last one will butt off into a bunch of different vesicles. And it is eventually replaced by the one below it. And so, scientists aren't actually sure, you know, which one of these is more used, but there's been this really hot debate over which one of these happened. And so, but current evidence suggests that the proteins actually move using a combination of the 2 pathways.

So, let's look at both these pathways. Here, you have the cisternae maturation model. So, what you have here is that the cisternae themselves are moving. These are these green arrows here. These are being transported this way, the entire cisternae, until it reaches the end and these butt off into all these different vesicles, and it's replaced by the one below it. Then, you have a vesicular, model, where the cisternae actually don't move, so there's no movement here. But instead, what you get is you get vesicle movement in between the cisternae that transport proteins throughout the Golgi. And so, it's actually currently thought that both of these methods are moved differently by different proteins. So, protein transport between different organelles in the cell and the Golgi occurs in 2 different ways. So this is going to be between organelles. So the first one is called anterograde transport and that moves from the ER to the Golgi and towards the plasma membrane. So, this moves outwards. Then you have retrograde transport, which moves the opposite way. So this is going to move inwards. This starts at the plasma membrane, goes to the Golgi, and then goes to the ER.

So, in both of these processes, the Golgi acts as a sorting hub. It recognizes sorting sequences and it says, okay, you're going into this vesicle because this vesicle is going to the ER. Or, you're going into this vesicle because this vesicle is going to the plasma membrane. And so, there are a bunch of different protein receptors in the Golgi that bind these sorting signals and trigger the proper sorting into different vesicles. So, here we have the 2 different types of transport. So, like I said, this transport here is going out, out, and this retrograde transport here is coming in. So those are the two ways that proteins move between the Golgi and other organelles. So now, let's move on.

Which of the following is not a method of Golgi transport?

• A. Retrograde transport
• B. Vesicular transport model
• C. Cisternal transport model
• D. Medial golgi transport

Which side of the Golgi faces the endoplasmic reticulum?

• A. Cis-Golgi
• B. Trans-Golgi
• C. Medial-Golgi

True or False:Each Golgi cisternae matures by moving upwards through the Golgi.

• A. True
• B. False

Which of the following transport moves molecules from the plasma membrane to the Golgi?

• A. Cisternal transport
• B. Vesicular transport
• C. Retrograde transport
• D. Anterograde transport

Here’s what students ask on this topic:

What is the role of the Golgi apparatus in protein processing and transport?

The Golgi apparatus plays a crucial role in processing and transporting proteins received from the endoplasmic reticulum (ER). Proteins are transported to the Golgi in vesicles, where they undergo various modifications, such as glycosylation. Glycosylation involves the addition of carbohydrates, which can be N-linked (attached to a nitrogen atom) or O-linked (attached to a hydroxyl group). These modifications are essential for protein folding, stability, and function. The Golgi is organized into cisternae, which are flattened membrane-enclosed stacks, and is divided into cis, medial, and trans sections. Proteins move through the Golgi via vesicular transport or cisternal maturation models and are sorted for transport to their final destinations, ensuring proper cellular function.

How do proteins move through the Golgi apparatus?

Proteins can move through the Golgi apparatus via two main models: the vesicular transport model and the cisternal maturation model. In the vesicular transport model, the Golgi cisternae remain stationary, and proteins are transported between them in vesicles. In the cisternal maturation model, the cisternae themselves move, with the cisternae at the cis face maturing and moving towards the trans face, eventually budding off into vesicles. Current evidence suggests that both models are used, depending on the specific proteins being transported. This dual mechanism ensures efficient protein processing and sorting within the Golgi.

What are the different sections of the Golgi apparatus, and what are their functions?

The Golgi apparatus is divided into three main sections: the cis Golgi, the medial Golgi, and the trans Golgi. The cis Golgi is the entry face that receives proteins from the ER. The medial Golgi is the middle section where most protein modifications, such as glycosylation, occur. The trans Golgi is the exit face that sorts and packages proteins into vesicles for transport to their final destinations, such as the plasma membrane or lysosomes. Each section has distinct functions and contributes to the overall processing and sorting of proteins within the Golgi.

What is glycosylation, and why is it important for protein function?

Glycosylation is the process of adding carbohydrate groups to proteins, which occurs in the ER and Golgi apparatus. There are two main types: N-linked glycosylation, where sugars are attached to a nitrogen atom, and O-linked glycosylation, where sugars are attached to a hydroxyl group. Glycosylation is crucial for protein folding, stability, and function. It helps in proper protein folding by stabilizing the protein structure and protecting it from degradation. Additionally, glycosylation can influence protein interactions and cellular localization, making it essential for the proper functioning of proteins within the cell.

What are COPII vesicles, and what role do they play in protein transport?

COPII vesicles are transport vesicles that carry proteins from the endoplasmic reticulum (ER) to the Golgi apparatus. These vesicles are coated with COPII proteins, which help in the budding and formation of the vesicle from the ER membrane. Only properly folded and processed proteins are packaged into COPII vesicles, ensuring quality control. Once formed, these vesicles transport the proteins to the Golgi, where they undergo further modifications and sorting. COPII vesicles are essential for the efficient and accurate transport of proteins within the cell.