ER Processing and Transport: Study with Video Lessons, Practice Problems & Examples

Proteins enter the endoplasmic reticulum (ER) through two main processes: co-translational import and post-translational import. Co-translational import involves the signal recognition particle (SRP) binding to the ER signal sequence on mRNA, facilitating protein translocation via the translocon, powered by GTP hydrolysis. Post-translational import requires proteins to be unfolded and refolded by BiP after entering the ER. Additionally, proteins undergo modifications like glycosylation, which adds sugars for proper folding, and disulfide bond formation, crucial for structural integrity. The unfolded protein response (UPR) ensures only correctly folded proteins proceed to function.

ER Import

Hi. In this video, we're going to be talking about ER processing and transport. So this first video is going to focus specifically on 2 types of ER transport. And when we talk about ER transport, what we're talking about is how proteins are getting into the ER. So, the first type is co-translational import, and that's going to be the process of importing proteins into the ER as they're being translated, co-translational. So how this happens is that an mRNA is going to contain what's known as an ER signal sequence. Now, the mRNA that has that signal sequence is going to just be directed to the ER, and when it's there, it's going to recruit ribosomes and other things that it needs in order to be able to translate and also enter into the ER. So, how it does this is the ER signal sequence is recognized by what's known as the SRP, so the signal recognition particle, and that's going to bind to the signal sequence. Then, that particle recognizes its own receptor, the signal recognition particle receptor, that's located on the ER, and it binds to the SRP, which remember, is bound to the ER signal sequence. So, you have this complex of 3 things: the mRNA with the signal sequence, the particle that recognizes that signal sequence, and the receptor that recognizes that particle. Then, when all 3 of those are bound together and they're bound to all the right things, they come in contact with a pore in the ER called the translocon. And that is going to bind to all of these things and translocate that protein into the ER as it's being translated. To do that, it needs energy. Of course, everything needs energy. So, it uses that energy from GTP hydrolysis. So remember, that's going to be turning GTP into GDP. And then once that protein enters into the ER, it's been translated, it's now in the ER, it no longer needs that signal because it's already there. So this protein called a signal peptidase comes in and cleaves off that ER signal. So, what this looks like is if we have a protein here, it has a signal sequence or ER signal sequence. Oh, the little delay. Then, it recruits the signal recognition particle. You can see that binds here. This eventually binds the receptor and this results in translocation of the protein across to the ER lumen. So that's the first type. Now, let's go over the second, and that's going to be post translational import. This is going to be the process of importing proteins into the ER after or post translation. Now, in order for this to happen, the protein still has to be unraveled. You can't get this huge folded protein across these little pores. So, what happens is that there, the protein here is going to be unraveled into a single string, And then, it gets transported across, into the ER. And then once it's here, it needs to be refolded, so the protein responsible for that is called BIP, and that helps pull the protein across, and it interacts with it, and helps it refold once it's across. So, now that you have proteins in the ER, you want to keep them there because they have some kind of function. And so, the signal is, that does that is called an ER retention signal, which makes complete sense. Retains this protein in the ER. It's located on the C terminus, and it's what keeps proteins there. So, those are the 2 types of ways of getting protein into the ER. So with that, let's now turn the page.

Inserting Membrane Proteins

Okay. So in this video, we're going to talk about how proteins are inserted into the membrane. We know that there are a lot of transmembrane proteins. They exist throughout the plasma membrane and in other organelles. How do these proteins actually get into the membrane? They are not made in the membrane; they are made elsewhere but have to be inserted. We're going to discuss how the insertion process happens. The first method we'll talk about involves single-pass transmembrane proteins, which enter the membrane once, meaning they have one insertion.

You have this long string of a protein, and only at one point is it inserted into the membrane. It looks kind of like this one down here, which I'll go over in just a second. How do these proteins get into the membrane? There are specific sequences that allow them to insert. There are two sequences you need to know about: the start sequence and the stop sequence. Obviously, the start sequence begins the initiation process of getting into the membrane, while the stop sequence stops it. The sequence on the protein comes to the membrane it wants to insert into, and interacts with a translocon, a type of protein that allows insertion. When it encounters a stop sequence, it stops, and the protein is fixed in place wherever it is.

The start sequence, still embedded in the membrane, is then chopped off by another protein. These sequences can be located anywhere on the protein, not just on the N-terminus which is the initial part of the protein; they can be in the middle or at the end, wherever the protein is meant to insert into the membrane. Here is an example of a single-pass protein getting into the membrane. You see the protein line with a start sequence in red and a stop sequence. When the start sequence gets inserted into the membrane via the translocon, the protein is fed through little by little until it reaches the stop sequence where the process stops. Now you have a protein that has integrated into the membrane through both the start and stop sequences, but since this is a single-pass protein, a protein called a signal peptidase comes in, chops off the start sequence, and now we have a single-pass protein inserted into the membrane that dissociates from the translocon.

This example demonstrates single-passes, but not all membrane proteins are single-pass proteins. Some are multi-pass proteins, which means they pass through the membrane multiple times and thus have more than one insertion. In these proteins, the start and stop transfer sequences do not look different; they have the same sequence, and it's only the order that determines whether they act as a start or stop. This pattern continues until it runs out of these transfer sequences. In this example, if we assume multiple additional sequences, it could result in a protein with multiple passes through the membrane, potentially up to seven times, which is a common occurrence with membrane proteins.

That is how proteins, either single-pass or multi-pass, are inserted into the membrane. With that, let's move on.

ER Protein Modifications

Okay. In this video, I'm going to be talking about different ER modifications that happen for proteins. So, proteins, they have to go out into the cell, do a lot of things, and so before they want to head out in the cell, they need to make sure that they look right. So, they need to have all the different modifications, their hair and their makeup done, before they can head out into the cell to do things. So, the ER is a major source of this modification. Kind of think of it as like a hair salon or something. Fixing up these proteins so that they can get ready to go out and do their functions. So, the first type of modification that happens is going to be glycosylation, and this is going to be the addition of a sugar, in case you don't remember what glycosylation is. And glycosylation occurs in the ER first. So, how this happens is there's this precursor molecule, dolichol, and, that gets added onto any protein that's going to be glycosylated. So it's the same molecule and anything that needs it is going to be, it's going to be added on first. So for all the women out there used to, or know anything about makeup, or men too, I'm not going to be choosy. Dolichol is kind of like the foundation, so that gets added on first before anything else happens. And then, once that precursor's added on, it can be modified. You know, you can add things to it, you can take things away from it, but that's going to be the foundation that goes on any protein that needs glycosylation. And so, this is super important for proteins because these sugars that are added on or the foundation that is added on are tags, they mark this protein for proper folding, and it's been determined actually that the chaperone proteins that are responsible for protein folding actually bind the protein through binding different sugars that have been added on through glycosylation. So, the chaperones bind the sugars, and that helps them fold everything into the correct place. So, glycosylation is super important. Then, you have this GPI anchor. I'm not even going to attempt to say that, I suggest you don't either. You'll always see it as GPI. And so this is an anchor that's added to proteins that are destined for the plasma membrane. So, why is this anchor added? Well, this anchor is added because if you add a if you add an anchor to a protein, that anchors it to the plasma membrane. But then, at any other point, you want to just kind of cut it off, just cut it off and release it into the night. And so, GPI anchors are added to proteins that are generally going to be released into the cell or released into the extracellular environment. So, GPI anchors is another thing that happens in the ER and super, super important. So, this is an example of glycosylation, so this is what I said about, you know, your foundation. So you have all these different oligosaccharides that can be added on to proteins, but it all starts with this molecule, and this molecule is added on and then it's further modified to whatever it wants to be in different components, or different cellular organelles. But it starts out all the same way as this molecule. Now, another thing that happens is this protein called protein disulfide isomerase helps with the formation of disulfide bonds. So, disulfide bonds are, you know, these really strong bonds that help keep the protein together and just sort of help keep it shaped and bond in the way that it should be. So protein disulfide isomerase helps to do that. And then, you have the unfolded protein response. So, you can kind of imagine this as like your friend when you go out at night because, it's going to detect the misfolded protein. So, it's going to tell those proteins and say, hey, you really don't look good enough to go out tonight. So, what we're going to do is we're just actually going to redo this whole thing. So, the unfolded protein response has these proteins called ERAD proteins, and this is kind of like your friend. And so, the ERAD proteins, they come in, they say, oh dear goodness, you do not look right. So, these proteins that are misfolded, they say, no, you can't go out like that. So, what they do is they transport them to the cytosol and eventually, degrade them. But let's hope your friends don't degrade you when you're not looking that great and want to go out. So this is what happens with the unfolded protein response. You have this protein transported in the ER, and then normally, it looks good, it looks like it's folded well and it's ready to go out into the world. But sometimes, something happens and it aggregates or misfolds, or it doesn't look very good, say, you got a new haircut and it's really not good for you. And so the unfolded protein response and these ERAD proteins come in, and they say, nope. You're not going to go out. So, what they do is they transport these things into the cytosol and degrade them. And so, you can kind of think of that as like your friend pushing you into your closet and telling you to find a new outfit. So, that's the unfolded protein response. So, with that, let's now move on.

Match the following term with its definition 

I. Co-translational import     ______________________ 

II. Post-translational import ______________________ 

III. ER retention signal         ______________________ 

IV. Translocon                      ______________________ 

A. Pore in the ER membrane that binds SRP and SRPR to translocate the protein into ER

B. Signal sequence located on the C-terminus and keeps proteins within the ER 

C. Process of importing proteins into the ER as they're being translated 

D. importing proteins into the ER after they've been translated

Okay, so this is a matching question. It says match the following term with its definition. So, one says, cotranslational. So what is that? That's going to be transporting proteins as they're being translated, cotranslational. So if we look at these answers here, the answer that it says, as they're being translated, is going to be c. Then, post-translational import, what do you think that is? Right, that's going to be protein import after they've been translated, which is the one right below it, d. Then, we have the ER retention signal, which if you remember what that is, that's going to be a signal that retains proteins within the ER. So that's going to be b, and then we're left with only one answer, a, but that's going to be translocon, and so that is a pore in the ER membrane that translocates proteins into the ER. So, the answer is c, d, b, a. So, with that, let's now move on.

Which of the following is responsible for recognizing the ER signal sequence?

• A. Signal Recognition Particle
• B. Signal Recognition Particle Receptor
• C. Translocon
• D. Stop Transfer Sequence

A protein contains 5 start/stop transfer sequences. How many times will this protein cross the membrane?

• A. 2
• B. 5
• C. 10
• D. 3

Glycosylation of proteins in the ER is associated with which of the following molecules or responses?

• A. Unfolded protein response
• B. Protein disulfide isomerase
• C. Glycosylphosphatidyl-inositol (GPI) anchor
• D. Dolichol

Here’s what students ask on this topic:

What is the difference between co-translational and post-translational import into the ER?

Co-translational import involves the simultaneous translation and translocation of proteins into the ER. An mRNA with an ER signal sequence is recognized by the signal recognition particle (SRP), which directs the ribosome to the ER membrane. The translocon then facilitates the entry of the nascent protein into the ER, powered by GTP hydrolysis. In contrast, post-translational import occurs after the protein has been fully synthesized in the cytosol. The protein must be unfolded to pass through the translocon and is subsequently refolded by BiP chaperone proteins once inside the ER. Both processes ensure proteins are correctly localized to the ER for further processing.

How do single-pass and multi-pass transmembrane proteins get inserted into the membrane?

Single-pass transmembrane proteins have one insertion into the membrane. They contain a start-transfer sequence that initiates insertion into the translocon and a stop-transfer sequence that halts the process. The start sequence is cleaved off, leaving the protein embedded in the membrane. Multi-pass transmembrane proteins have multiple start and stop sequences, allowing them to pass through the membrane several times. The translocon encounters these sequences in an alternating manner, inserting and stopping the protein multiple times, resulting in a protein that spans the membrane several times. The number of passes depends on the number of start and stop sequences.

What role does glycosylation play in ER protein processing?

Glycosylation is the addition of sugar molecules to proteins, which occurs in the ER. This process starts with the addition of a precursor molecule, dolichol, to the protein. Glycosylation is crucial for proper protein folding, as the added sugars serve as tags that chaperone proteins recognize and bind to, facilitating correct folding. Properly folded proteins are essential for their function and stability. Glycosylation also plays a role in protein sorting and trafficking, ensuring that proteins reach their correct destinations within the cell.

What is the unfolded protein response (UPR) and why is it important?

The unfolded protein response (UPR) is a quality control mechanism in the ER that ensures only correctly folded proteins proceed to function. When misfolded proteins are detected, UPR activates ER-associated degradation (ERAD) proteins, which transport these faulty proteins to the cytosol for degradation. This process prevents the accumulation of misfolded proteins, which can be toxic to the cell. UPR is crucial for maintaining cellular homeostasis and preventing diseases related to protein misfolding, such as neurodegenerative disorders.

What is the function of the signal recognition particle (SRP) in ER protein import?

The signal recognition particle (SRP) plays a critical role in co-translational import of proteins into the ER. It recognizes and binds to the ER signal sequence on the nascent polypeptide emerging from the ribosome. This binding pauses translation and directs the ribosome-nascent chain complex to the ER membrane by interacting with the SRP receptor. Once docked, the complex is transferred to the translocon, where translation resumes, and the growing polypeptide is translocated into the ER lumen. SRP ensures that proteins destined for the ER are accurately targeted and efficiently imported.