Virus Assembly, Release, and Maturation

Introduction

  • Course Title: Virus Assembly, Release, and Maturation
  • Instructor: Dr. Amy Ott
    • Email: ac3482@drexel.edu
    • Institution: Drexel University College of Medicine
  • Purpose: Discuss processes of virus life cycle, focusing on assembly, release, and maturation.
  • Prerequisites: Understanding of virus structure, infection mechanisms, transcription, and replication.
  • Course Structure: Divided into six sections:
    1. Assembly of Protein Shells
    2. Selective Packaging of the Viral Genome
    3. Acquisition of an Envelope
    4. Release of Virus Particles
    5. Maturation and Cell-to-Cell Spread
    6. Summary and Conclusions

Assembly of Protein Shells

  • Introduction to how viruses assemble their protein shells.
  • All viruses have a protein capsid or nucleocapsid, which encases and protects the viral genome.
    • Capsid: The protein shell.
    • Nucleocapsid: Nucleic acid and protein assembly.
    • Importance: Must disintegrate to release the viral genome into new host cells.
  • Learning Objectives:
    • Understand the life cycle of the influenza virus as an example.
  • Recognize the roles of chaperones, scaffolding proteins, and self-assembly in protein shell assembly.

Viral Life Cycle

  • Key Components: Understanding the life cycle stages aids in grasping the assembly process.
    • Influenza virus: 8 segments of single-stranded RNA; enveloped with helical nucleocapsid.
  • Common Themes:
    1. Assembly requires sufficient concentration of viral components.
    2. Assembly occurs at different cellular locations for different viruses.
    3. Viruses rely on host proteins and structures for assembly and release.

Formation of Protein Shells

  • Formation of structural units from viral proteins:
    • Individual proteins can self-assemble into a capsid or nucleocapsid.
    • Example: The structural units can include VP1 pentamers of SV40.
  • Assembly processes can be categorized as:
    1. Individual Proteins: Assemble from a small number of proteins.
    2. Polyprotein Precursor: Forms linked proteins that assemble more efficiently.
    3. Chaperones: Assist in the correct folding and assembly of proteins, preventing non-specific interactions.

Assembly Reactions

  • Self-Assembly Reactions:

    • Viral structural proteins direct their own assembly based on sequence information.
    • Example: SV40 VP1 forms capsid-like structures without assistance in vitro.

  • Assisted Assembly Reactions:

    • Other proteins or factors help the assembly process by organizing viral proteins or increasing local concentrations.
    • This includes binding of structural proteins to genomes (retrovirus example) and modifications like phosphorylation.

Scaffolding Proteins

  • Definition: Scaffolding proteins aid in the assembly of capsids but are not included in the final structure.
  • Examples:
    • HSV-1 VP22a and its role in nucleocapsid assembly and protease activity post-assembly.

Summary of Assembly Section

  • Viral capsids are generated from proteins.
  • Viral chaperones and scaffolding proteins facilitate assembly but are not retained in final capsid structures.

Selective Packaging of the Viral Genome

  • Definition: Selective incorporation of viral genomes into assembling virion to differentiate from host nucleic acids.
  • Mechanisms of packaging:
    • Concerted incorporation: Packaging occurs simultaneously with protein assembly (e.g., retroviruses).
    • Sequential incorporation: Packaging in pre-formed shells (e.g., herpesvirus).

Packaging Signals

  • Packaging signals are unique nucleic acid sequences that facilitate viral genome recognition.
  • Various virus examples:
    • DNA Viruses: Adenovirus and their specific repeated sequences.
    • RNA Viruses: Retroviral dimerization processes related to HIV packaging.

Acquisition of an Envelope

  • Objective: Discuss how some viruses acquire a lipid envelope from host membranes.
  • Envelopes provide benefits and disadvantages:
    • Advantages: Protects from immune response, aids in infection.
    • Disadvantages: Sensitive to environmental factors.
  • Acquisition Mechanisms:
    • Sequential: After assembly (e.g., influenza).
    • Coordinated: Combined assembly and acquisition (e.g., retroviruses like HIV).

Release of Virus Particles

Outlining Virus Release Mechanisms

  • Possible outcomes for host cells vary based on the virus (abortive, cytolytic, persistent).
  • Cytolytic infection: Non-enveloped viruses often lead to cell death.
  • ESCRT-dependent pathways: Essential for some enveloped viruses to release effectively.
  • L- Domains: Amino acid motifs promoting viral budding through interaction with host proteins.

Non-Enveloped Virus Release Mechanisms

  • Lysis: Many non-enveloped viruses are released through cell lysis.
  • Viroporins: Proteins that form pores in host membranes facilitating virus release.

Maturation and Cell-to-Cell Spread

  • Maturation often involves conformational changes making the virion infectious.
  • Examples include different maturation processes for HIV, poliovirus, and influenza, highlighting the import of proteolytic cleavage and structural modifications.

Strategies for Cell-to-Cell Spread

  1. Budding from infected cells exposes viruses to immune responses.
  2. Direct Cell-to-Cell Transmission, bypassing defenses, seen with retroviruses and certain synaptic connections like in neuronal tissues.

Summary of Maturation and Spread

  • Viral maturation ensures infectivity via structural adjustments and completion of protein processing; spread is achieved either through exposure to immune responses or direct cell fusion mechanisms, incorporating strategies that vary based on virus types.