Assembly and budding

Assembly and Budding

Assembly and budding are critical stages in the life cycle of many viruses, particularly enveloped viruses. These processes involve the formation of new viral particles and their release from the host cell, allowing the virus to spread and infect new cells.

Overview

The assembly of viral components into a complete virion is a complex process that requires precise coordination of viral and host cell machinery. This process typically occurs in specific regions of the host cell, such as the cytoplasm or the nucleus, depending on the type of virus.

Budding is the mechanism by which newly assembled virions acquire their lipid envelope and exit the host cell. This process is crucial for the maturation and infectivity of enveloped viruses.

Viral Assembly

During viral assembly, the following key steps occur:

• Genome Packaging: The viral genome, which can be either RNA or DNA, is packaged into a protein shell known as the capsid. This process often involves specific signals or sequences within the viral genome that are recognized by viral proteins.

• Capsid Formation: The capsid is assembled from viral proteins, which may form spontaneously into a stable structure. In some viruses, this process is assisted by host cell factors.

• Incorporation of Viral Enzymes: Some viruses, such as retroviruses, incorporate viral enzymes like reverse transcriptase into the assembling virion.

Viral Budding

Budding is the process by which enveloped viruses acquire their lipid bilayer from the host cell membrane. The steps involved in viral budding include:

• Membrane Curvature and Scission: Viral proteins, such as matrix proteins, interact with the host cell membrane to induce curvature. This is followed by membrane scission, which releases the virion from the cell.

• Envelope Acquisition: As the virion buds from the host cell, it acquires a portion of the host cell's lipid membrane, which becomes the viral envelope. This envelope often contains viral glycoproteins that are essential for infectivity.

• Release of Virions: The newly formed virions are released into the extracellular environment, where they can infect new host cells.

Role of Host Cell Factors

Host cell factors play a significant role in both assembly and budding. For example, the Endosomal Sorting Complex Required for Transport (ESCRT) machinery is often hijacked by viruses to facilitate membrane scission during budding.

Clinical Implications

Understanding the mechanisms of viral assembly and budding is crucial for the development of antiviral therapies. Inhibitors that target these processes can potentially block the replication and spread of viruses.

Also see

• Viral life cycle
• Viral entry
• Viral replication
• Viral pathogenesis