Animal Viruses: 2. Entry & Uncoating in the Host Cell: Study with Video Lessons, Practice Problems & Examples

Animal viruses enter host cells primarily through two mechanisms: membrane fusion and endocytosis. Enveloped viruses utilize membrane fusion, where the viral envelope fuses with the host cell membrane, allowing only the nucleocapsid to enter. In contrast, non-enveloped viruses rely solely on receptor-mediated endocytosis, forming an endocytic vesicle around the virus. Following entry, the nucleocapsid undergoes uncoating, releasing its nucleic acid into the cytoplasm, which is crucial for subsequent viral replication and infection processes.

Animal Viruses: 2. Entry & Uncoating in the Host Cell

In this video, we're going to continue to talk about animal viruses and animal virus infections by focusing on the second step of an animal virus infection, which is entry into the host cell and uncoating within the host cell. And so after a virus has attached to the surface of a host cell, it can then begin the process of entry and encoding. And so it turns out that enveloped viruses, which recall from our previous lesson videos, are viruses that have an outer lipid envelope, can actually enter into their host cell in one of two ways. The first way is through membrane fusion, and the second way is through endocytosis. Now entry via membrane fusion is going to be when the outer lipid envelope of an enveloped virus fuses with the cell's cytoplasmic membrane. And so the cell's cytoplasmic membrane ends up containing pieces of the virus' outer lipid envelope. And so if we take a look at our image down below, notice that the top half of our image here is focused on entry via membrane fusion. And so notice that this only occurs here with enveloped viruses because enveloped viruses have an outer lipid layer. And so the outer lipid layer here of the enveloped virus is going to be able to fuse with the host cell cytoplasmic membrane. And so here you can see the fusion beginning to happen, and again, the virus' outer lipid envelope is going to remain embedded within the cell's cytoplasmic membrane as the virus enters into the cell without its outer lipid envelope. And so that is one way by which enveloped viruses can enter into a host cell. Now once again, the second way by which enveloped viruses can enter into a host cell is via endocytosis. And this is going to involve the mechanism of receptor mediated endocytosis, which recall we had covered receptor mediated endocytosis in some of our previous lesson videos. And so if you don't remember much about receptor mediated endocytosis, be sure to go back to our older lesson videos and check that out. Now envelope viruses can enter via either membrane fusion or endocytosis. However, non-enveloped viruses can only enter by endocytosis. And this is because non-enveloped viruses do not have an envelope, a lipid layer, and so they are not able to enter by membrane fusion. And so notice down below on the bottom half of the image here, we're focusing on entry via endocytosis. And we're focusing specifically on a non-enveloped virus because non-enveloped viruses can only enter via endocytosis. However, again, enveloped viruses can enter via either membrane fusion or endocytosis. And so what you'll notice is we have our non-enveloped virus here which does not have its outer lipid envelope. And so it is going to be able to enter into the host cell through endocytosis, receptor mediated endocytosis. And notice that it is going to become invaginated by the cell's cytoplasmic membrane. And so it enters into the cell as an endocytic vesicle. And so notice that here the virus is within the host cell as an endocytic vesicle, whereas up above here the virus is within the host cell, not within an endocytic vesicle. And so those are some of the differences between membrane fusion and endocytosis. And so we'll be able to actually talk more details about each of these entry processes as we move forward in our course. And we'll start off talking about entry via membrane fusion, and then later, we'll talk about entry via endocytosis. And so that being said, I'll see you all in our next video.

Entry & Uncoating of Enveloped Viruses by Membrane Fusion

This video will discuss the entry and encoding of enveloped viruses by membrane fusion, which occurs in a series of steps, as shown in the image below. We will start analyzing this image from the left and progress to the right. Notice on the far left, the spike proteins on the virus are essential for binding the virus to the host cell receptor. This binding is the first step in a viral infection. As a result, the spike proteins bind to the receptors on the host cell. Following this, during membrane fusion, the viral envelope fuses with the cell’s membrane. Notice the viral envelope fusing with the host cell cytoplasmic membrane. When this fusion occurs, only the nucleocapsid of the virus enters the cell. Remember, the nucleocapsid is merely the protein coat containing nucleic acid. Only the nucleocapsid enters the cell; however, the outer lipid envelope fuses with the cell’s membrane but does not enter, so the viral envelope remains within the cell's cytoplasmic membrane. Components of the cell's membrane are derived from that viral envelope. The nucleocapsid that enters the cell begins to uncoat, meaning that the virus genome is released. Notice here the nucleocapsid, which includes the protein coat and nucleic acid inside. The protein coat uncoats and sheds, releasing the nucleic acid inside, allowing the nucleic acid to proceed to the next step in an animal virus infection. This concludes our brief lesson on the entry and encoding of enveloped viruses by membrane fusion. We will practice applying these concepts as we discuss endocytosis entry in upcoming videos. See you in our next video.

There are two ways a virus can enter an animal cell. Which method is unique to enveloped viruses and why?

• A. Entry via membrane fusion. Enveloped viruses have an outer member which fuses with the host cell's membrane.
• B. Entry via endocytosis. Enveloped viruses have surface proteins which start endocytosis while naked viruses do not.
• C. Both entry methods are unique to enveloped viruses, naked viruses cannot enter animal cells.

Which method of entry is used by non-enveloped or 'naked' viruses to enter animal cells?

• A. Entry via membrane fusion.
• B. Entry via endocytosis.
• C. Entry via exocytosis.
• D. Entry via viral absorption.

Entry & Uncoating by Endocytosis

This video we're going to talk a little bit more about animal virus entry and uncoating by endocytosis, which actually occurs in a series of steps that we're showing you down below in this image. And so we're actually going to analyze this image from the far left over here and making our way towards the right in that direction. And so, of course, over here on the far left, we're showing you the very first step of an animal virus infection, which is the animal virus binding to the host cell. And so once again, the spike proteins on the animal virus are going to help bind the animal virus to the host cell receptors. And so we're seeing the spike proteins here and the spike proteins will bind to receptors on the host cell. After this, in endocytosis, the process of receptor-mediated endocytosis will actually begin. And so what you'll notice is that the host cell is going to form a membrane and an endocytic vesicle around the virus. And so you can see that here we have the virus and the host cell's membrane is forming an endocytic vesicle around it. Then the endocytic vesicle actually enters into the cell and the endocytic vesicle contains the animal virus. Then at this point, the virus is going to exit the endocytic vesicle along with any lipid layers that it has. And then, the virus genome is going to be uncoded, which basically means it will be released. And so you can see here in this image that the virus nucleocapsid here is going to exit from the endocytic vesicle and its lipid layers. And then, after the nucleocapsid has exited, the nucleic acid is going to exit from the protein coat. And so you can see the protein coat here is uncoating and the nucleic acid is being released into the environment. And so once this nucleic acid is released and is available here within the cell's cytoplasm, then the next stage of animal cell infection, animal virus infection, can take place. And so this here concludes our brief overview of animal virus entry and uncoating by endocytosis. And once again we'll be able to get a little practice applying these concepts as we move forward. So I'll see you all in our next video.

All of the following are major differences between the entry of viruses into animal cells via membrane fusion and via endocytosis except which of these answers?

• A. During entry via endocytosis the virus enters the host cell within an endocytic vesicle.
• B. Non-enveloped viruses can only enter animal cells via endocytosis.
• C. During entry via membrane fusion the viral DNA is released into the cytoplasm in a step called uncoating.
• D. During entry via membrane fusion the viral envelope fuses with the host cell's membrane.

Once a virus has entered an animal cell, what step must occur before the viral DNA is replicated and new viruses are created within the host cell?

• A. The virus must bind with the host cell's surface receptors to initiate endocytosis.
• B. The viral envelope must fuse with the host cell's plasma membrane.
• C. The capsid proteins around the viral DNA must be removed in a process called uncoating.

Here’s what students ask on this topic:

What are the main mechanisms by which animal viruses enter host cells?

Animal viruses primarily enter host cells through two mechanisms: membrane fusion and endocytosis. Enveloped viruses, which have an outer lipid envelope, can utilize membrane fusion. In this process, the viral envelope fuses with the host cell membrane, allowing the nucleocapsid to enter the cell. Non-enveloped viruses, lacking this lipid envelope, rely solely on receptor-mediated endocytosis. Here, the virus binds to host cell receptors, and the host cell membrane forms an endocytic vesicle around the virus, which then enters the cell. Both mechanisms ultimately lead to the uncoating of the nucleocapsid, releasing the viral nucleic acid into the cytoplasm for subsequent replication and infection processes.

How do enveloped viruses enter host cells via membrane fusion?

Enveloped viruses enter host cells via membrane fusion through a series of steps. First, the spike proteins on the viral envelope bind to specific receptors on the host cell membrane. This binding triggers the fusion of the viral envelope with the host cell membrane. As a result, the viral nucleocapsid, which contains the viral genome, is released into the host cell's cytoplasm. The viral envelope remains part of the host cell membrane. Once inside, the nucleocapsid undergoes uncoating, releasing the viral nucleic acid, which is essential for the next stages of viral replication and infection.

What is receptor-mediated endocytosis in the context of viral entry?

Receptor-mediated endocytosis is a process by which viruses enter host cells by binding to specific receptors on the cell surface. This binding triggers the host cell to engulf the virus in an endocytic vesicle. The vesicle then transports the virus into the cell. Once inside, the virus exits the vesicle, and the nucleocapsid is released into the cytoplasm. The nucleocapsid then undergoes uncoating, releasing the viral nucleic acid, which is crucial for the virus to replicate and continue its infection cycle. This mechanism is used by both enveloped and non-enveloped viruses.

What happens during the uncoating process of a virus?

During the uncoating process, the viral nucleocapsid, which consists of the viral genome enclosed within a protein coat, is disassembled. This disassembly releases the viral nucleic acid into the host cell's cytoplasm. Uncoating is a critical step in the viral life cycle because it allows the viral genome to be accessible for replication and transcription. The specific mechanisms of uncoating can vary depending on the type of virus and the entry method, but it generally involves the breakdown of the protein coat, either through host cell enzymes or viral proteins.

How do non-enveloped viruses enter host cells?

Non-enveloped viruses enter host cells exclusively through receptor-mediated endocytosis. These viruses bind to specific receptors on the host cell surface, triggering the host cell to engulf the virus in an endocytic vesicle. The vesicle then transports the virus into the cell. Once inside, the virus exits the vesicle, and the nucleocapsid is released into the cytoplasm. The nucleocapsid then undergoes uncoating, releasing the viral nucleic acid, which is essential for the virus to replicate and continue its infection cycle.