Animal Viruses: 5. Release from Host Cell - Online Tutor, Practice Problems & Exam Prep

Enveloped viruses are released from host cells primarily through budding, utilizing the cell's cytoplasmic membrane to form their lipid envelope. This process does not immediately kill the host cell. In contrast, non-enveloped viruses trigger apoptosis, leading to cell death and subsequent release of viral particles. Both types can initiate new infections in neighboring cells, restarting the infection cycle. Understanding these mechanisms is crucial for grasping viral pathogenesis and developing antiviral strategies.

Release of Enveloped Viruses

In this video, we're going to continue to talk about animal viruses and animal virus infections by specifically focusing on the 5th and final step of an animal virus infection, which is the release of the new viruses from the host cell. Now it turns out that this 5th step is actually a little bit different in enveloped viruses and non-enveloped viruses. And so in this video, we're going to focus on the release of enveloped viruses, and then later in a different video, we'll talk about the release of non-enveloped viruses. Once again, recall from our previous lesson videos that enveloped viruses are going to have an outer lipid envelope surrounding their nucleocapsid. It's important to note that most enveloped viruses are going to be released from the host cell by a process that's known as budding. Budding is the process that allows for the release of enveloped viruses by using the cell's cytoplasmic membrane to actually form the envelope itself. As we'll see down below in our image, the lipid envelope of an enveloped virus is actually derived from the host cell's cytoplasmic membrane. Now budding does not immediately kill the cell, the host cell, and so that is also important to note. If we take a look at our example image down below, we can get a better understanding of the release of an enveloped animal virus from a host cell. In this image, we're going to start over here on the far left, and we're going to make our way from left to right in that direction. So starting over here on the far left, notice that we're showing you a nucleocapsid of a virus that has assembled in the host cell cytoplasm. In order for this enveloped virus to eventually exit from the host cell, what needs to happen is the viral spike proteins are going to insert themselves into the cell's cytoplasmic membrane. You can see here these little yellow structures that are the spike proteins that are going to insert themselves into the cell's cytoplasmic membrane. Then what's going to happen is, the matrix proteins are going to begin to bind and the viral envelope itself is going to begin to coat the virion or the viral particle via those matrix proteins. Recall that the matrix proteins, which are shown here in pink, are the proteins that link the nucleocapsid to the viral envelope that is forming here. Notice that the viral envelope is forming from the cell's cytoplasmic membrane and budding of the cell's cytoplasmic membrane. Over here in this third image that you see, you can see that the nucleocapsid is going to be fully coated with the matrix protein. Notice that there is matrix protein all the way around here. Notice that the viral envelope, which has viral spike proteins, is also going to be surrounding the nucleocapsid. You can see the viral envelope with the viral spike proteins here as well. Ultimately what happens is the virion or the viral particle is going to be completely released from the host cell. You can see here we have the release of the virion or release of the viral particle, and it has its nucleocapsid along with its lipid envelope. It is a fully mature and a fully infectious virus. It is also important to note that occasionally, some viruses, some enveloped viruses, can actually obtain their lipid envelope from the cell's organelles. The envelopes can sometimes develop from a cell's organelle, like for example, it could develop from the Golgi apparatus membrane, or it could develop from the rough endoplasmic reticulum membrane, rather than the cytoplasmic membrane. The way that that would work is that the viral particle, the nucleocapsid, would bud into either the Golgi apparatus or the rough ER, and then through vesicles, it would be released into the environment. But either way, the enveloped viruses are going to be obtaining their lipid envelope upon being released from the cell. This here concludes our brief lesson on the release of enveloped viruses from the host cell, and we'll be able to get some practice applying these concepts, and then we'll talk about the release of non-enveloped viruses from the host cell. So I'll see you all in our next video.

Release of Non-Enveloped Viruses

This video, we're going to briefly talk about the release of non-envelope viruses from the host cell. Recall that non-envelope viruses do not have an outer lipid envelope, and they are going to be released from the host cell after the host cell has died by triggering a process known as apoptosis. Apoptosis is really just a complex mechanism of programmed cell death that can be initiated by the host cell as the host cell is carrying out an immune response against the infecting virus. Once the virus has been released from the host cell, the viral particles, of course, can then go on to infect other nearby healthy cells, restarting the entire infection process, starting with attachment, then shifting into entry and assembly, then synthesis and replication, assembly, and finally ending off with release.

In the image below, we're showing you an image of the release of non-enveloped viruses from the host cell. Notice over here on the far left, we're showing you these non-enveloped viruses. Again, these non-enveloped viruses are going to assemble on the inside of the cell in the cell's cytoplasm. Unlike enveloped viruses, which will be released and acquire an envelope through the cell's cytoplasmic membrane or another one of the host cell's membranes, here with non-enveloped viruses, the host cell is going to undergo changes in shape. It changes in shape during the early stages of the process of apoptosis. You can see the cell changes in shape right here. Ultimately, apoptosis, the process of programmed cell death, is going to occur. When the cell is programmed to die, the viral particles are able to escape and be released into the environment. Once again, the released viruses can go on to initiate a new infection in another neighboring cell.

This here concludes our brief lesson on the release of non-envelope viruses from the host cell, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Which of these answers is a major difference between the release of enveloped viruses and the release of non-enveloped viruses?

• A. Enveloped viruses assemble after release while non-enveloped viruses assemble before release.
• B. Enveloped viruses when released cause the host cell to burst while non-enveloped viruses do not.
• C. Enveloped viruses do not kill the host cell upon release, while non-enveloped viruses trigger host cell apoptosis.
• D. All of the above answers are major differences between the release of enveloped and non-enveloped viruses.

All of the following descriptions of viral multiplication and viral nucleic acids are true except which of these answers?

• A. Viruses replicate only in living cells.
• B. Viruses contain DNA or RNA, not both.
• C. Viruses use the cell's biosynthetic machinery to synthesize copies of themselves.
• D. The nucleic acid of a virus is surrounding by a protein coat.
• E. Viral mRNA, viral tRNA, and viral ribosomes are used in viral replication.

All viruses must be able to do which of the following?

1. Kill the host cell.
2. Inject their viral genome into the host cell.
3. Lyse the host cell.
4. Have their viral genome replicated by the host cell.
5. Be able to reproduce in the absence of living cells.

• A. 1 and 2.
• B. 2 and 3.
• C. 4 and 5.
• D. 1 and 5.
• E. 2 and 4.

Here’s what students ask on this topic:

What is the process of budding in enveloped viruses?

Budding is the process by which enveloped viruses are released from the host cell. During this process, the viral nucleocapsid assembles in the host cell's cytoplasm. Viral spike proteins insert themselves into the host cell's cytoplasmic membrane, and matrix proteins link the nucleocapsid to the forming viral envelope. The viral envelope, derived from the host cell's cytoplasmic membrane, surrounds the nucleocapsid. The virion is then released from the host cell, acquiring its lipid envelope in the process. This method does not immediately kill the host cell, allowing it to continue producing more viral particles.

How do non-enveloped viruses get released from the host cell?

Non-enveloped viruses are released from the host cell through a process called apoptosis, which is a form of programmed cell death. The host cell undergoes changes in shape and structure as it initiates apoptosis, often as part of an immune response. Once the cell dies, the viral particles are released into the environment. These released viruses can then infect neighboring healthy cells, restarting the infection cycle. Unlike enveloped viruses, non-enveloped viruses do not acquire a lipid envelope from the host cell membrane.

What role do matrix proteins play in the release of enveloped viruses?

Matrix proteins play a crucial role in the release of enveloped viruses by linking the viral nucleocapsid to the forming viral envelope. These proteins bind to the nucleocapsid and help in the assembly of the viral envelope, which is derived from the host cell's cytoplasmic membrane. The matrix proteins ensure that the viral envelope, containing viral spike proteins, properly surrounds the nucleocapsid, facilitating the budding process and the eventual release of a fully mature and infectious virion from the host cell.

Can enveloped viruses obtain their lipid envelope from organelles other than the cytoplasmic membrane?

Yes, enveloped viruses can sometimes obtain their lipid envelope from organelles other than the cytoplasmic membrane. For example, the viral nucleocapsid can bud into the membranes of the Golgi apparatus or the rough endoplasmic reticulum (ER). The viral particles are then transported via vesicles to the cell surface, where they are released into the environment. This alternative method still results in the virus acquiring a lipid envelope, but it originates from the membranes of these specific organelles rather than the cytoplasmic membrane.

What is apoptosis and how does it relate to the release of non-enveloped viruses?

Apoptosis is a form of programmed cell death that is often initiated by the host cell as part of an immune response to viral infection. In the context of non-enveloped viruses, apoptosis leads to the death of the host cell, allowing the viral particles to be released into the environment. The process involves changes in the cell's shape and structure, ultimately resulting in cell death. Once the cell dies, the non-enveloped viruses are freed and can go on to infect neighboring healthy cells, continuing the infection cycle.