Question

The envelope of EBOV contains many copies of a single viral glycoprotein (GP) that is required for cell entry. Following attachment, the virion is brought into the cell via endocytosis, but fusion with the endosomal membrane occurs only after the GP is cleaved by a host protease. Compare and contrast this uncoating step with what you have learned about uncoating of HIV and influenza.

Short answer

In EBOV and Influenza, the host cell entry is initiated through the binding of a glycoprotein (EBOV-GP and Influenza-HA) to a host cell receptor and subsequent endocytosis. EBOV requires cleavage of GP by a host protease before fusion with the endosomal membrane, while the fusion in Influenza is mediated by a conformational change in the HA protein due to the acidic environment of the endosome. In contrast, HIV enters the host cell through direct fusion with the cell membrane, initiated by the binding of the viral envelope to the CD4 receptor on the cell surface. While both EBOV and Influenza rely on endocytosis and fusion with the endosomal membrane for uncoating, they differ in the requirement of host proteases for cleavage of the viral glycoproteins that facilitate fusion. In contrast, HIV uncoating involves direct membrane fusion and capsid disassembly, which distinguishes it from EBOV and Influenza.

Step-by-step solution

Understanding EBOV Uncoating

EBOV enters the host cell through the binding of its glycoprotein (GP) to the cell surface receptor. Following attachment, the virus is taken into the cell via endocytosis, and fusion with the endosomal membrane occurs only after the GP is cleaved by a host protease.

Understanding HIV Uncoating

HIV enters the host cell by first binding to the CD4 receptor on the cell surface. The viral envelope then fuses with the cell membrane, and the viral capsid is released into the cell. The uncoating process consists of the disassembly of the HIV capsid, exposing the viral RNA and reverse transcriptase enzyme, which then produces a DNA copy of the viral genome.

Understanding Influenza Uncoating

Influenza enters the host cell through the binding of its hemagglutinin (HA) protein to the sialic acid receptor on the host cell surface. The virus is then taken into the cell via endocytosis. The acidic environment of the endosome triggers a conformational change in the HA protein, which then mediates the fusion of the viral envelope with the endosomal membrane. This fusion leads to the release of the viral genome into the host cell's cytoplasm for replication.

Comparing the Uncoating Steps

In EBOV and Influenza, the host cell entry is initiated through the binding of a glycoprotein (EBOV-GP and Influenza-HA) to a host cell receptor and subsequent endocytosis. However, EBOV requires cleavage of GP by a host protease before fusion with the endosomal membrane, while the fusion in Influenza is mediated by a conformational change in the HA protein due to the acidic environment of the endosome. In contrast, HIV enters the host cell through direct fusion with the cell membrane, initiated by the binding of the viral envelope to the CD4 receptor on the cell surface.

Contrasting the Uncoating Steps

While both EBOV and Influenza rely on endocytosis and fusion with the endosomal membrane for uncoating, they differ in the requirement of host proteases for cleavage of the viral glycoproteins that facilitate fusion. In contrast, HIV uncoating involves direct membrane fusion and capsid disassembly, which distinguishes it from EBOV and Influenza.

Key concepts

Glycoprotein Function

Glycoproteins play a crucial role in the viral infection process. They are embedded in the viral envelope and are responsible for facilitating the virus's attachment and entry into the host cell. For example, in the Ebola virus (EBOV), the glycoprotein, or GP, binds to specific receptors on the surface of the host cell. This binding initiates the process of viral entry through endocytosis.

Glycoproteins not only determine the host range, but they also contribute to the fusion of the viral envelope with the host cell membrane. This fusion is essential for the release of the viral genome into the host cell.

Thus, glycoproteins serve multiple functions:

• Attachment to host receptors
• Facilitation of membrane fusion
• Mediation of host immune evasion

Each virus has a unique glycoprotein structure that influences its infectivity and specificity.

Endocytosis in Viruses

Many viruses, including EBOV and Influenza, utilize endocytosis to enter host cells. This process involves the virus being engulfed by the host cell's membrane, forming an endosome. The viral particles are essentially trapped inside these vesicles, allowing them to be transported into the cell's interior.

Endocytosis is a passive entry strategy but crucial for successful infection. Viruses take advantage of this mechanism to gain access to the nutrient-rich environment of the cell. Once inside, viruses must escape the endosome to continue the replication process.

Endocytosis includes several steps:

• Binding of viral particles to cell surface receptors.
• Internalization through membrane invagination.
• Formation of endosomal vesicles around the virus.
• Transport of the endosome into the cell.

This method protects the virus from host defenses during the initial infection phase.

Viral Genome Release

After successful entry into the host cell by endocytosis or direct fusion, viruses must release their genetic material (genome) into the host's cytoplasm or nucleus to initiate replication. This crucial step is often tightly regulated and involves complex interactions between the viral and host cell components.

For EBOV, the release of the viral genome occurs after the glycoprotein is cleaved by host proteases and fusion with the endosomal membrane. This process allows the viral RNA to enter the host's cytoplasmic environment for replication.

Different viruses use different strategies for genome release:

• Acidic pH-induced conformational changes (Influenza)
• Host protease-mediated cleavage (EBOV)
• Direct membrane fusion (HIV)

These varied mechanisms highlight the adaptability of viruses to differing cellular environments and conditions.

Host Protease Role

Host proteases are enzymes that play a pivotal role in the viral life cycle by facilitating the cleavage of viral proteins, which is essential for viral infectivity and replication. In the context of EBOV, the host protease cleaves the glycoprotein after the virus enters the host cell through endocytosis.

This cleavage is a prerequisite for the fusion of the viral envelope with the endosomal membrane, enabling the release of the viral genome. Without this cleavage step, the viral replication cycle would be halted.

The role of host proteases in viral infection includes:

• Cleavage of viral glycoproteins
• Activation of fusion proteins
• Facilitation of viral entry

Understanding the interaction between host proteases and viral proteins helps in the development of antiviral therapies designed to impede viral entry and proliferation.

Membrane Fusion in Viruses

Membrane fusion is a key step in the viral entry process, enabled by the interaction between viral proteins and host cell membranes. This event allows the mixing of the viral envelope with the host membrane, resulting in the release of the viral contents into the host cell.

In Influenza, the low pH within the endosome induces a change in the hemagglutinin protein structure, which triggers membrane fusion. Meanwhile, EBOV utilizes host proteases to cleave GP, facilitating fusion and subsequent viral genome release.

The key steps in viral membrane fusion generally include:

• Viral attachment to the host membrane
• Conformational changes in viral proteins (triggered by pH or cleavage)
• Fusion of viral and host membranes

Membrane fusion not only determines the efficiency of the viral infection process but also affects tropism and pathogenesis across different host cell types.