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Virology Lecture: Virus Assembly and Release
0:00:15 Introduction: The lecture focuses on virus particle assembly, covering the steps from protein synthesis to release, and prepares for a discussion of pathogenesis.
0:00:39 Virus Structure and Assembly: The structure of a virus particle (icosahedral, enveloped, helical nucleocapsids) often provides clues about its assembly process.
0:01:25 Common Assembly Reactions: All virus particles undergo a similar set of assembly reactions:
Formation of individual structural units (proteins).
Assembly of the protein shell (capsid or nucleocapsid).
Selective packaging of the nucleic acid genome.
0:02:18 Selective Packaging: Key concept. The goal is to only pack viral nucleic acids, excluding cellular nucleic acids.
0:03:01 Alternatives After Release: Some viruses, unlike most, undergo maturation after release from the host cell. Others may acquire an envelope, sometimes concurrently with release.
0:03:56 Host Dependence: Assembly, like other viral processes, heavily relies on the host:
Chaperones: (Both viral and cellular) ensure correct protein folding and capsid assembly.
Transport Systems: Move components and particles within the cell.
Secretory Pathway: Used for releasing many enveloped viruses.
Nuclear Import/Export Machinery: Crucial for viruses replicating in the nucleus.
0:05:00 Cell is Compact: Viruses rely on specific transport mechanisms (short-distance pores, long-distance motor proteins on cytoskeletal tracks) because the cell is crowded.
0:06:13 VSV Nucleocapsid Transport: Vesicular stomatitis virus (VSV) nucleocapsids are transported along microtubules to the plasma membrane for budding, as shown by experiments with microtubule inhibitors.
0:08:39 Viral Factories/Inclusions: Virus components are concentrated in specific areas within the cell (factories or inclusions) to facilitate efficient assembly.
Poliovirus: Uses internal membranes for concentration.
Rabies Virus: Forms Negri bodies in the cytoplasm.
0:11:41 Protein Targeting: Viral proteins have "address labels" (membrane-targeting sequences, fatty acid modifications, nuclear localization signals) to reach their correct destinations.
0:17:30 The Assembly Line Analogy Like assembling cars, Virus assemblies can be broken down into sub-assemblies, with quality checks.
0:24:02 Sequential Capsid Assembly: Step-by-step assembly (e.g., poliovirus, herpesvirus). Herpesvirus is more complex, involving a scaffold protein within the capsid that is later degraded.
0:27:54 Concerted Assembly: Genome and structural proteins assemble simultaneously (e.g., influenza virus budding at the plasma membrane).
0:29:32 Hemagglutinin (HA) A complex, important protein found on influenza and some other viruses.
0:33:49 Genome Packaging: Mechanisms ensure only viral genomes are packaged, using "packaging signals" on the viral nucleic acid.
DNA Viruses: (e.g., adenovirus, SV40, herpesvirus) Packaging signals are DNA sequences recognized by viral proteins. Herpesvirus uses a motor to pull DNA into the capsid.
RNA Viruses: (e.g., retroviruses, influenza virus) Packaging signals are RNA sequences, often with specific secondary structures. Retroviruses use the nucleocapsid protein to bind the RNA. Influenza virus has signals to ensure all eight RNA segments are packaged.
0:46:42 Serial Dependence: Some viruses (e.g., bacteriophage Ξ¦6) package their genome segments in a specific order, ensuring the correct number is included.
0:49:47 Budding Drivers: Viral proteins drive budding; the entire particle isn't always necessary. Some viruses bud using only envelope proteins, while others require additional proteins.
0:52:05 Retrovirus Assembly. Slightly different, where internal and external structures are not fully assembled at budding.
0:55:51 Mistro alation Targeting of proteins to the cell membrane is a common mechanism.
0:57:06 Late Domains and Escort Pathway: Retroviruses (and others) use "late domains" in their structural proteins to recruit the cellular ESCRT pathway, facilitating budding and release.
01:00:26 Budding Sites: Budding can occur at various cellular membranes (nuclear membrane, ER, Golgi, plasma membrane), but it's consistent for a given virus.
01:00:59 Coronavirus Example: Buds into the ER-Golgi intermediate compartment (ERGIC), then vesicles transport the virus to the plasma membrane for release.
01:03:51 Herpesvirus Complexity: Herpesvirus has a complex egress pathway, involving multiple budding and fusion events.
01:05:06 Transport, Secretory Pathways Other things, such as PH changes and proteases, may alter virus structure and function during movement.