mRNA Export and Nuclear Structures

Hi in this video we're going to be talking about M. RNA export and the nucleus. So M. RNA is produced in the nucleus but right now all we know about the nucleus is that it's an organ I'll and where the D. N. A. Is contained. But the nucleus is actually this really diverse structure that has a lot of different compartments in it that all have different functions. So this video is mainly going to be just kind of a lot of vocabulary talking about different structures and functions of the nucleus. So like I just said the nucleus consists of many sub compartments with different functions. So one of these compartments is the nuclear envelope and this envelope is formed of two lipid bi layer is very similar to the plasma membrane except for it has two lipid bi layers instead of just one. So the outer membrane of the nucleus actually connects with the er or the endo plasma particular. Um and then the para nuclear space which is the space between the two envelopes between the two membranes is actually at the exact same as the er lumen. So it's kind of difficult to imagine. But actually the outer membrane of the nucleus is actually part of the membrane of the er therefore that para nuclear space is just sort of continuous with the er lumen space. Now inside the are embedded within the nuclear envelope. Are these proteins called nuclear pore complexes And these are pores that really are responsible for connecting the nuclear plasm to the inside of the nucleus and the side is all. So these are just one, these aren't one protein. They're really made up of a bunch of different proteins called nuclear porn proteins. Um there's a ton of them around 3000 or 4000 pores per cell. So it's a lot of different pores. And they are really responsible for blocking large molecules from getting into the nucleus. So small ones can pass um that's just based on their structure and we'll talk more about their structure later. But the nuclear pores block really large things around 3000 daltons. You don't need to know that number. Just sort of know this this is a large molecule that the nuclear pore is responsible for blocking. Now another structure of the nucleus is the nuclear lamin A. To and this is just a sort of this matrix of proteins um that really exists right under the nuclear envelope inside the nucleus. That provides shape and structure. Um So the proteins that make up the nuclear lamin A. Are called Lammens. Um and those are proteins that line the inner surface of the nuclear membrane and really just sort of help support it. They're kind of like a scaffold um that provide the shape and the structure to the nucleus. And then I'm one of the last structures that I really want to hit highlight there more than these but the final one is the nuclear list and that is the location where ribosomes are made. And so um within the nuclear Alice there is this region called the nuclear organizing region and this is a stretch of D. N. A. That contains the ribosomes RNA genes. And so that D. N. A. Sits within the nuclear lists are sort of ready to be transcribed and everything that's responsible for making those RNA all those proteins transcription factors or synthesis facilitators all sort of accumulate in this nuclear list. And that's they all work to form the river is owned by transcribing these ribosomes RNA genes. Now those are just a few of the main um nuclear structures that you really should know um in cell biology. But there are others in case you run across these in your book or the professor mentioned them in lecture. Um Some of these are called cage. Nobody's jim speckles and they all have different functions really but they just are different structures of the nucleus. But we're not gonna talk about those just because they're not as important as these other structures that I mentioned. Um Now one thing that is really important to know is before we've always been talking biology classes. We've always talked about the nucleus as this region that holds D. N. A. And chromosomes but you don't really think about you know are these chromosomes just sort of thrown in the nucleus. Are they specific location where they kind of reside. And so chroma tin which is remember the D. N. A. And the protein actually reside within specific regions of the nucleus. So it's not just sort of they're all in there floating around in the nucleus. They actually contain very specific regions where the D. N. A. Sits. And so how the nucleus controls where the D. N. A. Is going to be and how it sort of make sure that it's in the correct location is actually through hetero chroma tin. So the hetero chroma genes on the chromosome will bind to specific regions in the nuclear envelope. And so that sort of connects the chromosome to very specific regions of the nuclear envelope. And that is what allows the chromosomes to remain in their region and not sort of float off to other regions that 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 sort of try to figure out you know where all of these things that I just talked about. So the first one is the nuclear envelope. Remember this is has to buy layers. Just kind of hard to see in this picture but know that that's there. We have the nuclear pores and those block big molecules from entering into the nucleus. We have the nuclear list down here which remember produces ribosomes. And then we also have some other things that we that are on here and that includes the nuclear lamin A. So if I were to just draw this really fast, what it would look like is kind of this a mesh work that would go all the way around of proteins called lamin proteins. So we'll say nuclear laminar with the laminar proteins or the layman proteins that helps support the structure. And then if we had some chromosomes in here, probably should make them green, we'll make them black. These hetero chromosome regions which are here are going to bind to the nuclear envelope for the nuclear lamin A. And sort of secure the chromosomes to their correct location so they don't float off into regions they're not supposed to be in. So these, so these are the main structures of the nucleus. So now let's turn the page.

Okay so now we're gonna talk about M. RNA export from the nucleus. So um after transcription and processing of the M. R. N. A. It has to be transported out of the nucleus for the nucleus into the side of hell. Because translation occurs in the side of saul. And we need the M. RNA to become a protein. So how does the nucleus decide which are N. A. S. Um get transported into the side of saul? Well um there is a process using a approaching called an M. R. And P exporter that recognizes a signal on the M. RNA called a nuclear export signal. And so the M. R. M. R. N. P exporter can recognize that nuclear export signal and allow for the M. RNA to export out of the nucleus. And so how does it get out? It gets out through the nuclear pore complexes which connect the nuclear plasm to the side of saul. And so it's really importantly it's important to know that only correctly processed RNA can be exported from the nucleus. Anything that is incorrectly processed is actually taken by the X. Zone and um sort of degrades two grades any improperly processed R. N. A. And N. Tron is left in the nucleus. And also whenever the embargo is transported it goes in the five prime direction. So over to look at just a cell here you can see that there's a nucleus here. Um And inside is the D. N. A transcription happens then processing happens to to um result in this mature RNA from here let's say there's a nuclear export signal here that the M. R. And P. Can recognize and allow for export. And this goes into the side of saw um for eventual translation. So that's M. RNA export. Let's not move on.

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 N. L. S. Is required um 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 gonna tell you it does consist of licensing and Argentina are 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 of 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 N. L. S. Sequence whenever it's not needed. So if other proteins or molecules can come and sort of hide the N. L. S. Sequence then that will keep that protein in the side of salt. But when those molecules or proteins release then the N. L. S. Is released and that can be recognized and allowed 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 N. L. S. Sequence is released um how does it actually get in? Well it gets in because there is this protein called the important important protein and that recognizes the N. L. S. Sequence in the side, it's all so once important recognizes the N. L. S. They're bound together this sort of cargo protein and the important. And then that can actually transport the of the nuclear pore into side is all. 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 dis associates from the important. So how it does this is through a third protein. You know, it's sort of getting complicated. But this third protein is called RAN GTP. And when RAN GTP binds the important complex in 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 important and that's bound to random GTP. So when this happens when important is bound to RAN GTP, we're lucky because this complex actually can go back through the nuclear pore and end up in the side of Saul great. But now there's one final problem, anyone can figure it out right. We need the important to be able to act again. So once this complex gets into the side of saul what happens is RAN GTP becomes ran G. D. P. Which means it loses a phosphate. And that allows for the release of important which can then act again. So that's kind of the words to what happened. Let's actually figure this out by an image. So the first step here is we have the cargo protein with the N. L. S. Sequence. And that binds to important. Now when those two are bound this can get in pass through the nuclear pore, which is this nuclear pore and get inside the nucleus. But we have a problem because once it's inside we don't want it bound to important. So what happens is this third step with rand GTP binds and that releases the cargo and the car goes 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 side. Us all. And so now we have the G. T. P. And the important inside the side is all. But what we need to happen is we need these two separate. So what happens is G. T. P becomes G. D. P. And that allows for the separation of important. So this is gonna be step five. And when those two are separated, this can go back and sort of start all over with step one. 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 kind 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.