Protein Sorting - Online Tutor, Practice Problems & Exam Prep

Protein sorting occurs through three mechanisms: gated transport, transmembrane transport, and vesicle transport, all utilizing sorting signals—short amino acid sequences directing proteins to specific organelles. Gated transport involves nuclear localization signals (NLS) allowing proteins to enter the nucleus via nuclear pores. Transmembrane transport requires proteins to unfold for passage through translocators into organelles like the ER and mitochondria, where chaperone proteins assist in refolding. Vesicle transport involves membrane-enclosed compartments that bud off from organelles, transporting proteins to their destinations, crucial for cellular function.

Gated Transport

Hi. In this video, we're going to talk about protein sorting. First, there are three mechanisms through which proteins are sorted called gated transport, transmembrane transport, and vesicle transport. We're going to go over each one of these individually. However, they all have similar components, which are sorting signals. So what are sorting signals? They are short amino acid sequences located on the protein that direct the protein to different regions of the cell or different organelles. Once the protein with the sorting signal, such as an ER sorting signal, a Golgi sorting signal, or a plasma membrane sorting signal, reaches its destination, that signal can be removed, leaving the protein there. If the protein does not have any sorting signals, it is actually left in the cytosol.

In this video, let's first go over the first type of transport, gated transport. Gated transport refers to the transport of proteins between the cytosol and the nucleus. The nuclear pore is responsible for this type of transport. The nuclear pore extends through the nuclear envelope, and this internal pore has all of these unstructured regions that prevent large molecules or molecules that shouldn't get into the nucleus from passing through. Proteins that need to get into the nucleus contain nuclear localization signals. This signal is recognized by import receptors. When that signal is recognized by the receptor, it helps the protein move through the pore, using energy from GTP to facilitate this process. Here we have the nuclear pore. It has all these different regions that are unstructured. Normally, it blocks things from getting through. But if a protein comes along and has what's known as a nuclear localization signal (NLS), it can then bind to different receptors and travel through the nuclear envelope to reach the nucleus. That's gated transport. Let's now move on.

Translocators

So in this video, we're going to talk about transmembrane transport, which occurs through what are known as protein translocators. Protein translocators are proteins that translocate, which makes complete sense, proteins across different types of organelle membranes. Typically, translocators translocate a protein from the cytosol into membranes like the ER, mitochondria, or chloroplasts. In order to pass, proteins must unfold, and then the translocator can transport them across. They are directed here very similarly. They have signal sequences that direct them to this organelle and to these organelles, where you now have this unfolded protein, so it now needs to be folded. Typically, there are chaperone proteins that are present within the organelle itself and help fold the protein again once it is translocated across. So here we have the mitochondria. This is mitochondria and we have our translocator here. And so we have this protein with this signal sequence. This is the mitochondrial signal sequence, and it is unfolded and passed into the mitochondria. The signal sequence is removed, and then chaperone proteins come in and help refold this protein back up into its normal form. So that's protein translocation. Let's now move on.

Vesicular Transport

Okay. So now we're gonna talk about vesicle transport. And vesicle transport, we're gonna talk about through transport vesicles. Now, what transport vesicles are, they're just really small membrane-enclosed compartments that transport things. Vesicles form and how they end up fusing is they actually pinch off on one compartment, say, the endoplasmic reticulum, and then when they get to the place where they're going to let's say there's a protein that's being synthesized in the ER, but that protein isn't meant for the ER, it's meant for the plasma membrane. Well, what would happen is after that protein is synthesized, it's going to be in the membrane or attached somehow to the membrane so that it's incorporated into the vesicle. Right? Then it's going to pinch off. Now we have our little protein here attached to the vesicle. I'll make it red so you can see it. And, what we get is we get a vesicle here. It's going to be double membrane, and we have our little protein. Now that protein can go on and travel anywhere else in the cell. And when it gets there, this membrane is going to fuse with whatever compartment it's trying to release this protein into. So it can be the ER to the plasma membrane. It can be the Golgi to the ER. It can be, I don't know, anywhere to the mitochondria. Now, inside of a transport vesicle, that environment can be very different, depending on the transport vesicle, so it can differ from vesicle to vesicle. It also can differ from the cytoplasm. So you can have a really high concentration of a protein or a molecule in a Vesicle and a very low concentration of it outside the Vesicle. You can have 3 vesicles, each with different proteins going to different places. So the content of a vesicle is really just so independent of other vesicles and the cytoplasm itself. And it can really hold most proteins and molecules, so anything soluble, insoluble, membrane attached can be transported this way. So you can imagine how important this type of transport is for the entire cell.

So let's look at a picture of what this looks like. We have our nucleus, we have our ER here, and you can see that there's some type of protein that's been made, this green purple protein here, and it needs to get to the plasma membrane, which we can see it up here is where it ends up. So what happens is the vesicle buds off the ER. We can see it here, then goes into the Golgi, and you can see this protein traveling through the Golgi. There's actually a couple of different ways this can travel through the Golgi, but that's not for this video. We'll talk about that in another video. But, one of the ways potentially it could transport is through vesicles, so that is a possibility here. But, once it gets to the transphase of the Golgi, it's then gonna bud off into another vesicle and then make its way to the plasma membrane where that membrane is gonna fuse with the plasma membrane. And then we finally have this protein where it's supposed to be, which is in the plasma membrane. So that is vesicular transport through transport vesicles.

With that, let's turn the page.

Which of the following is not a type of transport?

• A. Gated transport
• B. Organelle transport
• C. Vesicular transport
• D. Transmembrane transport

The nuclear pore is classified as which of the following types of transport?

• A. Gated transport
• B. Organelle transport
• C. Vesicular transport
• D. Transmembrane transport

Here’s what students ask on this topic:

What are the three mechanisms of protein sorting?

Protein sorting occurs through three primary mechanisms: gated transport, transmembrane transport, and vesicle transport. Gated transport involves the movement of proteins between the cytosol and the nucleus via nuclear pores, utilizing nuclear localization signals (NLS). Transmembrane transport requires proteins to unfold and pass through translocators into organelles like the endoplasmic reticulum (ER) and mitochondria, where chaperone proteins assist in refolding. Vesicle transport involves membrane-enclosed compartments that bud off from organelles, transporting proteins to their specific destinations within the cell. Each mechanism relies on sorting signals, which are short amino acid sequences that direct proteins to their appropriate locations.

What is the role of sorting signals in protein sorting?

Sorting signals are short amino acid sequences located on proteins that direct them to specific regions within the cell. These signals are crucial for the proper localization of proteins, ensuring they reach their intended organelles or cellular compartments. For example, a nuclear localization signal (NLS) directs a protein to the nucleus, while an ER signal sequence targets a protein to the endoplasmic reticulum. Without these sorting signals, proteins would remain in the cytosol and fail to perform their designated functions, disrupting cellular processes.

How does gated transport work in protein sorting?

Gated transport refers to the movement of proteins between the cytosol and the nucleus through nuclear pores. Proteins destined for the nucleus contain nuclear localization signals (NLS), which are recognized by import receptors. These receptors facilitate the transport of the protein through the nuclear pore complex, utilizing energy from GTP. The nuclear pore complex has unstructured regions that prevent the passage of large or unauthorized molecules, ensuring only proteins with the correct NLS can enter the nucleus.

What is the function of chaperone proteins in transmembrane transport?

In transmembrane transport, proteins must unfold to pass through translocators into organelles like the endoplasmic reticulum (ER) or mitochondria. Once inside the organelle, chaperone proteins assist in refolding the protein into its functional three-dimensional structure. Chaperone proteins ensure that the newly translocated proteins achieve proper folding, preventing misfolding and aggregation, which could lead to cellular dysfunction or disease.

How do transport vesicles function in vesicle transport?

Transport vesicles are small, membrane-enclosed compartments that facilitate the movement of proteins and other molecules within the cell. They form by budding off from donor organelles, such as the endoplasmic reticulum (ER), and carry their cargo to target destinations like the Golgi apparatus or plasma membrane. The vesicle's membrane fuses with the target compartment, releasing its contents. This process is essential for distributing proteins and maintaining cellular organization and function.