Microtubules: Study with Video Lessons, Practice Problems & Examples

Microtubules, composed of tubulin dimers (α-tubulin and β-tubulin), serve as cellular tracks for transporting vesicles and organelles. They exhibit polarity, with the plus end facilitating growth due to a GTP cap, while the minus end is prone to depolymerization. This dynamic instability allows for rapid assembly and disassembly, crucial during cell division. Centrosomes organize microtubule arrays, ensuring equal distribution of DNA and organelles. Understanding microtubule dynamics is essential for grasping cellular processes like mitosis and intracellular transport.

Microtubules

Hi. In this video, we're going to be talking about microtubules. So, microtubules are cytoskeleton elements, which we've already talked about, but, essentially, their function is they act as cellular tracks, so kind of like train tracks, that move vesicles and organelles throughout the cell. So microtubules, they are made up of tubulin, which is the small subunit that makes them up. And so, each tubulin is, actually, a dimer composed of an alpha subunit and a beta subunit. And the positions of the subunits are very, very carefully controlled because each position provides the tubulin with polarity. So the plus end is where the beta part is, and the minus end is where the alpha part is. And so, this is important because that means that one side is going to have a plus end and one side is going to have a minus end. So, it means that each side does different things and has, you know, different abilities. So, one of those abilities is that usually the plus end is where growth occurs, for the most part. It doesn't have to, but very fast the growth happens at the plus end. And so, not only are microtubules made up of just tubulin, like long strings of tubulin, instead, they have microtubule-associated proteins or MAPs, that bind them and stabilize them against disassembly, when they don't need to disassemble. So, here's the hydrotubulin. You can see that there's a dimer here. You have your alpha in red and your beta in yellow. And, these are very orderly positioned all the way down the microtubule subunit. And that gives them polarity where you have your beta subunit, which is going to be the plus end, and your alpha side, which is going to be what's called the minus end. Now, tubulin is attached to GTP. So, this we keep seeing this in cell bio. It's a big regulator. And so, this, GTP or this binding GTP affects microtubule polymerization. And, in case you don't remember what polymerization is, it just means growth So, I'm going to walk you through the steps of microtubule growth. So, the first thing is that protofilaments, which are going to be the small aggregates of tubulins, or these early late filaments of tubulin, form at microtubule organizing centers. So, this is where microtubules are nucleated, or undergo nucleation, which, if you remember, has absolutely nothing to do with the nucleus, but instead has to do with the small aggregation of tubulin subunits, which allow for the microtubule to grow. Nucleation is like the first step in microtubule growth. It happens at microtubule organizing centers in the cell. Then, from there, tubulin polymerization occurs, which is just growth. And, these tubulin dimers can actually be added to either side, they could be added to the plus end, or they can be added to the minus end. But, you are like, wait, you just told me they add faster at the plus end, which is true, and I'm going to tell you why. So, the reason that they do this has to do with why it's how it's bound to GTP. So, because tubulin dimers are bound to GTP, they can also hydrolyze them by GDP. So, the GTP, which is called the T form, or the GDP, which you may see as the D form, depends on kind of how fast the tubulin dimers add to each end. So, tubulin dimers have the ability to hydrolyze GTP to GDP, and they do it quickly upon addition. But, if they are hydrolyzed slowly, so, it means that they, when they bind to the microtubule, the growing microtubule, they are like wait just a little, not long, but just a little bit. They are just slow about it, about hydrolyzing from GTP to GDP, then what happens is the next dimer is added before hydrolysis can occur. And, so, when this happens, you actually get these stacks, tubulin dimers, that all have GTP and haven't yet gotten to the process of hydrolyzing. So, when this happens, it forms a GTP cap. And so, you have multiple tubulin GTPs that are bound to each other on this growing microtubule end, and they're not immediately hydrolyzed. So, they're going to be hydrolyzed, but not yet because they're slow about it. Alternatively, if it's hydrolyzed quickly, then GDP is just going to be super hydrolyzed so fast that before the next dimer can add, it's actually going to just be hydrolyzed. Now, the growth can happen on either end, but the growth is much easier at this end because it has this GTP cap, none of these are hydrolyzed yet, and it just the tubulin binds to this side much faster, much easier. So, this is going to be the quickest way, which if you remember what I said above, is also going to be the plus end. Whereas, here it's going to be the minus end, usually. And, if there's a bunch of GDP on the end, the tubulin is like, well, I don't have any energy to stay together. So, it's actually gonna start destabilizing it and breaking apart. So, what this looks like is this image here. So, you have your tubulin subunits. You have some that are bound to GTP and others that are bound to GDP. Now, these form together to form these small protofilaments, but eventually, they form microtubules and in the process of polymerization they grow. So you can see that there's this GTP cap with all these subunits that have GTP bound to them. You can see that in blue. So, the tubulin wants to add to this end, which is usually the plus end, and so it's polymerizing. It's growing at this end, these subunits are adding. Whereas, this end it's hydrolyzed so quickly when the subunits are added. So it is then you have a bunch of GDP-bound tubulin and subunits don't want to bind to that. They want to bind to the GTP cap. And so, when nothing is being added to it and has none of this energy coming in, it's going to depolymerize. These subunits are going to start breaking off, and this whole thing is going to be like unraveling kind of a zipper, from the from the minus end, but it's being created at the plus end. So, that's how tubulin polymerization and depolymerization works. Now, there are two terms that are very commonly used to describe this, and they sound so similar it's kind of hard to separate them. So, hopefully, I'll be able to explain this. So, the first is dynamic instability. So, this is when a microtubule end, so one of the ends, either the plus or minus, switches depolymerization. Now, treadmilling is different because this is when subunits are recruited to the plus end and shed or gotten rid of at the minus end. So, this deals with one end, and this deals with what's happening at both ends. So, for dynamic instability, this is what you see. So, you have GDP and GDP. We're focusing specifically on one end. So, when there is a bunch of GDP here, it's going to be growing. And then, what's called a 'catastrophe', which I think is a little dramatic, but it's just called a 'catastrophe' when all these things get hydrolyzed really quickly. Then, it starts destabilizing and shrinking. But then, occasionally, it can be rescued by adding a GTP onto it. And if it's rescued, then it's going to start growing again. And so, this is dynamic instability, what's happening at one end. Whereas, treadmilling is what I've showed you previously, which is what's going on at both ends. So, hopefully, that was clear. So, with that, let's now move on.

Microtubules and Cell Division

Okay. So, now we're going to talk about microtubules and cell division. This is going to be really short because we're going to go over a lot of this when we actually get to cell division, but I just want to introduce it here. So, the first thing I want to introduce is the centrosome. These are part of locations in the cell, organelles in the cell that actually are responsible for organizing microtubule arrays during cell division. These arrays are really important because microtubules are what move everything around. They move replicated organelles, they move replicated DNA, and they make sure that everything is sorted into cells when they divide. That's super important because you don't want one cell with like 75% of the DNA and one with only 25%. And, you don't want one with all the microtubules or all the mitochondria and one with none; I think it's got to be equally distributed, and so, centrosomes and microtubules do that.

So, centrosomes contain central pairs, which I know you've seen in your bio 101 class. Central pairs are super important because they act as the nucleation site for microtubule growth. So remember, nucleation has nothing to do with the nucleus. Instead, it has to do with where these cytoskeleton elements form. Centrioles have nucleation sites for microtubule growth during cell division. These tubulin dimers come into these centrioles, and they're added with their minus end towards the centrioles and plus end out towards the cytoplasm where they grow and attach to DNA or organelles or whatever they're going to attach to during cell division. This is what they look like. They have this very unique structure with this triplet. So, you see the 1, 2, 3. 1, 2, 3, which is arranged around. So, they have these triplets. Now, there's a pair. So, you can see there's 1 here and 2 here. And this is where nucleation of microtubules occurs during cell division. So, like I said, super short, but also super important. So now let's move on.

Under which condition is a GTP cap formed during microtubule formation?

• A. If the microtubule end is hydrolyzed slowly
• B. If the microtubule end is hydrolyzed quickly

Treadmilling is when a microtubule end switches from polarization to depolarization.

• A. True
• B. False

A single tubulin subunit is composed of which of the following components?

• A. An alpha tubulin
• B. A beta tubulin
• C. A dimer of alpha and beta tubulin
• D. A tetramer of alpha and beta tubulin

Here’s what students ask on this topic:

What are microtubules and what are they made of?

Microtubules are components of the cytoskeleton that act as cellular tracks for transporting vesicles and organelles within the cell. They are composed of tubulin dimers, which consist of two subunits: α-tubulin and β-tubulin. These dimers assemble in a highly organized manner, giving microtubules their characteristic polarity, with a plus end (β-tubulin) and a minus end (α-tubulin). This polarity is crucial for their function, as it influences the direction and rate of microtubule growth and disassembly.

How do microtubules contribute to cell division?

During cell division, microtubules play a critical role in organizing and segregating chromosomes. Centrosomes, which contain centrioles, act as microtubule-organizing centers. Microtubules nucleate at these centrosomes, with their minus ends anchored and plus ends extending outward. They attach to chromosomes at the kinetochores, helping to align and separate them into daughter cells. This ensures that each daughter cell receives an equal distribution of genetic material and organelles, which is essential for proper cell function and viability.

What is the significance of the GTP cap in microtubule dynamics?

The GTP cap is crucial for microtubule stability and growth. Tubulin dimers bind to GTP, and when they are added to the growing microtubule, they form a GTP cap at the plus end. This cap stabilizes the microtubule and promotes further polymerization. If the GTP is hydrolyzed to GDP before the next dimer is added, the microtubule becomes unstable and prone to depolymerization. The presence of a GTP cap thus facilitates rapid and controlled growth, while its loss can lead to microtubule shrinkage, a process known as dynamic instability.

What is dynamic instability in microtubules?

Dynamic instability refers to the rapid switching between growth and shrinkage at the ends of microtubules. This phenomenon is primarily observed at the plus end, where the presence of a GTP cap promotes growth. If the GTP is hydrolyzed to GDP, the microtubule becomes unstable and begins to depolymerize, leading to shrinkage. This process allows microtubules to rapidly reorganize in response to cellular needs, which is essential for functions like mitosis and intracellular transport.

What is the role of centrosomes in microtubule organization?

Centrosomes serve as the primary microtubule-organizing centers in animal cells. They contain centrioles, which act as nucleation sites for microtubule growth. During cell division, centrosomes help organize the microtubule arrays that segregate chromosomes and distribute organelles evenly between daughter cells. The minus ends of microtubules are anchored at the centrosomes, while the plus ends extend outward to interact with chromosomes and other cellular structures, ensuring proper cell division and function.