mRNA Export and Nuclear Structures - Online Tutor, Practice Problems & Exam Prep

The nucleus is a complex organelle containing various structures, including the nuclear envelope, nuclear pore complexes, nuclear lamina, and nucleolus. mRNA export involves the recognition of a nuclear export signal by the MRNP exporter, allowing only correctly processed mRNA to exit through nuclear pores into the cytosol for translation. Conversely, nuclear import requires a nuclear localization signal (NLS) on proteins, which is recognized by importin, facilitating their transport into the nucleus. Ran GTP plays a crucial role in releasing cargo proteins once inside, ensuring proper cellular function.

Nuclear Structures

Hi. In this video, we're going to be talking about mRNA export and the nucleus. So mRNA is produced in the nucleus, but right now, all we know about the nucleus is that it's an organelle and where the DNA is contained. The nucleus is actually a really diverse structure that has a lot of different compartments in it that all have different functions. This video is mainly going to be just a lot of vocabulary talking about different structures and functions of the nucleus. Like I just said, the nucleus consists of many subcompartments with different functions. One of these compartments is the nuclear envelope, and this envelope is formed of two lipid bilayers very similar to the plasma membrane except it has two lipid bilayers instead of just one. The outer membrane of the nucleus actually connects with the ER or the endoplasmic reticulum. The perinuclear space, which is the space between the two envelopes or between the two membranes, is actually at the same level as the ER lumen. It's kind of difficult to imagine, but the outer membrane of the nucleus is actually part of the membrane of the ER. Therefore, that parinuclear space is just sort of continuous with the ER lumen space. Embedded within the nuclear envelope are proteins called nuclear pore complexes, which are pores that are responsible for connecting the nucleoplasm, the inside of the nucleus, and the cytosol. These aren't just one protein; they're really made up of a bunch of different proteins called nucleoporin proteins. There are a ton of them, around 3,000 or 4,000 per pore per cell. They are responsible for blocking large molecules from getting into the nucleus. Small ones can pass, based on their structure, and we'll talk more about their structure later. The nuclear pores block really large things, around 3,000 Daltons.

Another structure of the nucleus is the nuclear lamina. This is a matrix of proteins that exist right under the nuclear envelope inside the nucleus and provide shape and structure. The proteins that make up the nuclear lamina are called lamins, and they line the inner surface of the nuclear membrane and really just help support it. They're kind of like a scaffold that provides the shape and structure to the nucleus. One of the last structures that I want to highlight is the nucleolus. This is where ribosomes are made. Within the nucleolus, there is a region called the nuclear organizing region, a stretch of DNA that contains the ribosomal RNA genes. This DNA sits within the nucleolus, ready to be transcribed, and everything responsible for this accumulates in the nucleolus and works to form the ribosome by transcribing these ribosomal RNA genes. These are just a few of the main nuclear structures that you should know in cell biology. But there are others, in case you come across them in your book or the professor mentions them in lecture. Some of these are called Cajal bodies, gems, and speckles. They all have different functions but are different structures of the nucleus. We're not going to talk about those now, as they're not as important as the other structures I mentioned.

One thing that is really important to know is that in biology classes, we've always talked about the nucleus as this region that holds DNA and chromosomes. But, do you think about whether these chromosomes are just thrown in the nucleus, or are there specific locations where they reside? Chromatin, which is the DNA and the protein, actually resides within specific regions of the nucleus. The nucleus controls where the DNA is going to be and ensures that it's in the correct location through heterochromatin. The heterochromatin on the chromosome will bind to specific regions in the nuclear envelope. This connects the chromosome to very specific regions of the nuclear envelope and allows the chromosomes to remain in their region and not float off to other regions they’re not supposed to be in.

So, this is a very simple drawing of the nucleus. But you can take a second if you would like and try to figure out where all of these things that I've just talked about are located. The first is the nuclear envelope, which has two bilayers, which is hard to see in this picture, but know that it's there. We have the nuclear pores, which block big molecules from entering the nucleus. We have the nucleolus down here, which produces ribosomes. And then, we also have some other things that are not on here, and that includes the nuclear lamina. If I were to draw this really fast, what it would look like is a meshwork that would go all the way around of proteins called lamin proteins. We will say nuclear lamina, but the lamina proteins or the lamin proteins help support the structure. And then, if we had some chromosomes in here, probably should make them green. We'll make them black. These heterochromatin regions, which are here, are going to bind to the nuclear envelope or the nuclear lamina, and secure the chromosomes to their correct location, so they don't float off into regions they're not supposed to be in. These are the main structures of the nucleus. Now, let's turn the page.

mRNA Export

Okay. So now we're going to talk about mRNA export from the nucleus. After transcription and processing of the mRNA, it must be transported out of the nucleus into the cytosol because translation occurs in the cytosol, and we need the mRNA to become a protein. So, how does the nucleus decide which RNAs get transported into the cytosol? Well, there is a process using a protein called an MRNP exporter that recognizes a signal on the mRNA called a nuclear export signal. The MRNP exporter can recognize that nuclear export signal and allow for the mRNA to export out of the nucleus. How does it get out? It exits through the nuclear pore complexes, which connect the nucleoplasm to the cytosol. It's important to note that only correctly processed RNA can be exported from the nucleus. Anything that is incorrectly processed is taken by the exosome, which degrades any improperly processed RNA and introns left in the nucleus. Also, whenever the mRNA is transported, it goes in the 5' prime direction. If we look at just a cell here, you can see that there is a nucleus. Inside is the DNA. Transcription happens, then processing occurs, resulting in this mature RNA. Let's say there's a nuclear export signal here that the MRNP can recognize and allow for export. This goes into the cytosol for eventual translation. So that's mRNA export. Let's now move on.

Nuclear Import

Okay. So this video is going to talk about nuclear import, which is the process of getting proteins that need to be in the nucleus inside of the nucleus. So all the proteins that need to get inside the nucleus have the same signal, and that signal is called a nuclear localization signal. And this NLS is required for proteins that need to get into the nucleus. Now it's extremely important. You don't need to know exactly what the sequence is. I'm just going to tell you it. It does consist of lysine and arginine or a lot of it. But this sequence is really important, not only because it allows proteins to get into the nucleus, but also because it's really easily regulated. So, of course, not every protein that needs to be in the nucleus needs to always be there. So there is this regulation process of making sure that proteins that need to get into the nucleus are only coming in whenever they're necessary. So how this happens is by blocking the NLS sequence whenever it's not needed. So if other proteins or molecules can come and sort of hide the NLS sequence, then that will keep that protein in the cytosol. But when those molecules or proteins release, then the NLS is released. And, that can be recognized and allow for nuclear import. And, regulation of this process is, of course, extremely important because we don't need all these proteins in the nucleus all the time.

So, once the NLS sequence is released, how does it actually get in? Well, it gets in because there is this protein called the importin protein, and that recognizes the NLS sequence in the cytosol. So once importin recognizes the NLS, they are bound together, this sort of cargo protein and the importin. And then, that can actually transport via the nuclear pore into the cytosol. But, now, we have a problem because we have the cargo protein inside the nucleus, but it's bound to this other protein that we don't need or really want. So, we have to make sure that the cargo, like, dissociates from the importin. So, how it does this is through a third protein. You know, it sort of getting complicated, but this third protein is called Ran GTP. And, when Ran GTP binds the importin complex and the cargo, that releases the cargo, which is great because now we have the cargo inside the nucleus, which is exactly what we wanted. But, now we also have this other complex that we don't really need inside the nucleus. We have this importin and that's bound to Ran GTP. So, when this happens, when importin is bound to Ran GTP, we're lucky because this complex actually can go back through the nuclear pore and end up in the cytosol. Great. But now there's one final problem. Once this complex gets into the cytosol, what happens is Ran GTP becomes Ran GDP, which means it loses a phosphate. And, that allows for the release of importin, which can then act again. So, that's kind of the process of what happened.

Let's actually figure it out by an image. So, the first step here is we have the cargo protein with the NLS sequence, and that binds to importin. Now when those 2 are bound, this can get in past the nuclear pore, which is the nuclear pore, and get inside the nucleus. But we have a problem because once it's inside, we don't want it bound to importin. So what happens is this third step with Ran GTP binds, and that releases the cargo. And, the cargo is free to go do whatever it needs to do. But, we have a problem because we have this complex now inside the nucleus. But, luckily, what happens is this complex is transported out of the nuclear pore into the cytosol. And so now we have the GTP and the importin inside the cytosol. But what we need to happen is we need these to separate. So, what happens is GTP becomes GDP and that allows for the separation of importin. So this is going to be step 5. And when those 2 are separated, this can go back and sort of start all over with step 1. So that is the process of nuclear import. And I get it's kind of confusing, but we run through these processes a lot with GTP and GDP transitions sort of controlling these kinds of feedback loops of different processes in cell biology. So this is one example of one of these with nuclear import, but we're going to see a lot of these GTP, GDP controls in future topics. So now, let's move on.

Match the following nuclear locations to its function 

i. Nuclear pore complexes _________
ii. Nuclear lamina _________
iii. Nucleolus _________
iv. Nuclear Envelope _________ 

a. Matrix of proteins that provide nuclear shape and structure 

b. Processing of ribosomes 

c. Separate the nucleus from the cytoplasm 

d. Allow for transport between the nucleus and cytoplasm

Okay. So this practice problem is a matching practice problem, and it says match the following nuclear locations to its function. So, let's just go through these together and see if we can figure it out as we go. So the first one says nuclear pore complexes. So what are nuclear pore complexes? Remember, those are the groups of proteins that allow, and that connect the nucleoplasm and the cytoplasm. So, do any of these match that description? Right. So, looking at these, it would be d and allows for transport between the nucleus and the cytoplasm. Now the second one is the nuclear lamina. So do you remember what is the nuclear lamina? Right. So that is the meshwork of proteins that provide shape and structure to the nucleus and then kind of reside right under the nuclear envelope. And, so, we're looking at these answers. The first one that comes shows up is a, which is the matrix of proteins that provide nuclear shape and structure. So 3 says, nucleolus. So do we remember what the nucleolus does? Right. That's the place of ribosome formation. So, out of the two choices left, it's very clearly b. And then, finally, the last one has to be c. It's the only answer left, but it is also the nuclear envelope, which is responsible for separating the nucleus from the cytoplasm, which matches here. So, D, A, B, C. So, those are the nuclear locations and their functions. So, let's move on.

Ribosomes are synthesized in which of the following locations?

• A. Nuclear Pore Complex
• B. Nuclear Lamina
• C. Nucleolus
• D. Cytoplasm

Import of molecules into the nucleus requires the use of importin and RAN-GTP. When RAN-GTP binds to importin, what happens to the molecule?

• A. The molecule is released into the cytoplasm
• B. The molecule is released into the nucleus
• C. The molecule is bound in the cytoplasm
• D. The molecule is bound in the nucleus

Unprocessed mRNA is exported from the nucleus to be processed before translation.

• A. True
• B. False

Here’s what students ask on this topic:

What is the role of the nuclear pore complex in mRNA export?

The nuclear pore complex (NPC) plays a crucial role in mRNA export by serving as the gateway between the nucleoplasm and the cytosol. After mRNA is transcribed and processed in the nucleus, it must be transported to the cytosol for translation. The NPC allows this transport by recognizing the nuclear export signal on the mRNA, which is facilitated by the MRNP exporter. Only correctly processed mRNA can pass through the NPC, ensuring that defective mRNA is degraded by the exosome. This selective transport is vital for maintaining cellular function and protein synthesis.

How does the nuclear envelope differ from the plasma membrane?

The nuclear envelope differs from the plasma membrane in several key ways. Firstly, the nuclear envelope consists of two lipid bilayers, whereas the plasma membrane has only one. The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (ER), making the perinuclear space contiguous with the ER lumen. Additionally, the nuclear envelope contains nuclear pore complexes that regulate the transport of molecules between the nucleus and the cytosol. These structural differences are essential for the specialized functions of the nuclear envelope, including protecting genetic material and regulating gene expression.

What is the function of the nucleolus in the nucleus?

The nucleolus is a prominent substructure within the nucleus responsible for ribosome biogenesis. It contains the nuclear organizing region, a stretch of DNA with ribosomal RNA (rRNA) genes. These genes are transcribed within the nucleolus, and the resulting rRNA is combined with ribosomal proteins to form ribosomal subunits. These subunits are then transported to the cytosol, where they assemble into functional ribosomes. The nucleolus thus plays a critical role in protein synthesis by producing the components necessary for ribosome assembly.

What is the significance of the nuclear localization signal (NLS) in nuclear import?

The nuclear localization signal (NLS) is essential for the import of proteins into the nucleus. Proteins that need to function within the nucleus contain an NLS, which is recognized by the importin protein in the cytosol. The importin-NLS complex is then transported through the nuclear pore complex into the nucleus. Once inside, Ran GTP binds to the complex, releasing the cargo protein. This regulated process ensures that only necessary proteins enter the nucleus, maintaining cellular function and gene regulation. The NLS is crucial for the spatial and temporal control of protein localization.

How does the nuclear lamina contribute to the structure of the nucleus?

The nuclear lamina is a dense fibrillar network located just beneath the inner nuclear membrane. It is composed of lamin proteins that provide structural support and shape to the nucleus. The nuclear lamina acts as a scaffold, maintaining the integrity of the nuclear envelope and organizing chromatin. It also plays a role in DNA replication, cell division, and gene expression by anchoring chromatin to specific regions of the nuclear envelope. This structural framework is essential for the proper functioning and stability of the nucleus.