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

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

Animal viruses can enter host cells through two primary mechanisms: membrane fusion and endocytosis. Understanding these processes is crucial for grasping how viral infections occur.

Enveloped viruses, which possess an outer lipid envelope, can utilize both methods for entry. In the case of membrane fusion, the virus's lipid envelope merges with the host cell's cytoplasmic membrane. This fusion allows the virus to enter the cell while leaving its envelope embedded in the host membrane. This process is unique to enveloped viruses due to their lipid layer, which facilitates the fusion with the host cell.

The second method, endocytosis, specifically receptor-mediated endocytosis, is also available to enveloped viruses. In this process, the virus binds to specific receptors on the host cell's surface, triggering the invagination of the membrane and the formation of an endocytic vesicle that engulfs the virus. Non-enveloped viruses, lacking a lipid envelope, can only enter host cells through this endocytosis mechanism, as they cannot fuse with the membrane.

In summary, enveloped viruses can enter host cells via either membrane fusion or endocytosis, while non-enveloped viruses rely solely on endocytosis. Understanding these entry mechanisms is essential for developing strategies to combat viral infections and for further studies on viral pathogenesis.

Entry & Uncoating of Enveloped Viruses by Membrane Fusion

The entry and encoding of envelope viruses through membrane fusion is a complex process that unfolds in several key steps. Initially, the viral spike proteins play a crucial role in binding the virus to the host cell receptors, marking the first step in viral infection. This binding is essential for the subsequent fusion of the viral envelope with the host cell's cytoplasmic membrane.

Once the viral envelope fuses with the host cell membrane, only the nucleocapsid enters the cell. The nucleocapsid consists of a protein coat that encases the viral nucleic acid. Importantly, while the nucleocapsid is able to enter the host cell, the outer lipid envelope remains integrated within the cell's membrane, contributing components derived from the viral envelope to the host cell.

After the nucleocapsid has entered the cell, it undergoes a process known as uncoating. During uncoating, the protein coat of the nucleocapsid sheds, releasing the viral nucleic acid. This released nucleic acid is now free to proceed to the next stages of the viral infection cycle, allowing the virus to hijack the host's cellular machinery for replication.

This overview of the entry and encoding of envelope viruses highlights the significance of membrane fusion and the role of the nucleocapsid in initiating viral replication within host cells. Understanding these mechanisms is crucial for further exploration of viral entry strategies, including endocytosis.

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.

Answer: (A)

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.

Answer: (B)

Entry & Uncoating by Endocytosis

Animal viruses enter host cells through a process known as receptor-mediated endocytosis, which involves several key steps. The initial step is the binding of the virus to the host cell, facilitated by spike proteins on the virus that attach to specific receptors on the cell's surface. This interaction is crucial for the virus to gain entry.

Once the virus is bound, the host cell membrane begins to envelop the virus, forming an endocytic vesicle. This vesicle then transports the virus into the cell. Inside the cell, the virus must exit the endocytic vesicle, which may also contain lipid layers from the virus itself. This process is known as uncoating, where the virus's nucleocapsid is released from the vesicle.

Following uncoating, the viral nucleic acid is freed from its protein coat and enters the cytoplasm of the host cell. This release of the viral genome is a critical step, as it allows the virus to hijack the host's cellular machinery for replication and production of new viral particles. Understanding these steps is essential for comprehending how viral infections initiate and progress within host organisms.

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.

Answer: (C)

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.

Answer: (C)

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.