Notes on Viral Assembly and Exit (Lecture Transcript)

Overview

• Transition from attachment, entry, genome delivery to assembly and exit. Understanding assembly and exit helps explain replication strategies across diverse viruses and sets up for Baltimore scheme discussions in upcoming lectures.
• Key idea: assembly, maturation, and exit follow principles similar to attachment/entry, but in the reverse direction. Metastability and timing are central: virions are built to be stable, then become metastable and infectious at the right moment.
• Goal for students: describe the challenges to assembly and exit, the mechanisms viruses use to overcome them, differentiate sequential vs concerted assembly, and explain maturation.
• Reminder: one-step growth curve framework applies to viruses; viruses replicate differently from cells and early work (bacteriophage+E. coli) established core concepts.

One-step Growth Curve: Core Concepts

• Experimental setup (classic model): add bacteriophage to a population of E. coli with synchronized attachment; rapidly dilute so no new infections occur; initiate infection.
• Synchronization: attachment and entry events are synchronized across cells; observations reflect a single replication cycle per cell.
• Nature of infection: critically lytic; cells burst at the end of the cycle; no reinfection of cells within the same cycle.
• Eclipse phase: after infection but before infectious virions appear; viruses are destabilized, uncoated, and not infectious. No virions detected in the supernatant.
• Burst phase (burst): a large production of virions occurs almost simultaneously; typical burst sizes are in the hundreds to thousands of virions per infected cell.
• Latent period: the interval during which new virus is produced inside cells but nothing is released yet; once enough virions accumulate, cells lyse and virions are released.
• Production timeline: eclipse → latent period → burst -> extracellular virions accumulate in the medium; then a second phase of virion production and release.
• General pattern applies broadly to viruses; analogous patterns observed (e.g., adenovirus) across diverse families.

Virus vs. Cell Growth: Fundamental Difference

• Viruses assemble with a mix of host-derived components and viral components; the virion is built from multiple newly synthesized components that originate in different cellular locations.
• Viral components can be scattered across the cell (nucleus, cytoplasm, ER, Golgi, plasma membrane); membranes may be cellular in origin and later encapsidated into enveloped virions.
• For enveloped viruses, acquiring a lipid envelope during exit is a key challenge; non-enveloped viruses face different exit constraints.

Initial Inventory of Challenges for Assembly and Exit

• Complexity: virions range from simple tailed or filamentous forms to large, multi-protein shells with large genomes; even simple viruses require coordinated assembly of many components.
• Separation and concentration: synthesis and accumulation occur in different cellular compartments; components must be concentrated locally to overcome dilute-solution inefficiency and unfavorable entropy during assembly.
• Directionality and specificity: assembly must proceed in a defined direction with correct interactions; reversible reactions can allow errors to be corrected, but overall progression must be directed toward the correct final structure.
• Reversible vs irreversible steps: assembly often begins reversibly (e.g., monomer interactions), but some steps become irreversible (e.g., proteolytic cleavage, conformational changes) to enforce directionality.
• Competition with cellular material: viral proteins may interact with abundant cellular proteins; high specificity is needed to preferentially assemble viral structures.
• Packaging of the genome: genomes vary in size and organization; some viruses have segmented genomes (e.g., influenza) requiring precise assembly of multiple genomic pieces; others rely on selective RNA/protein packaging signals.
• Membrane acquisition (enveloped viruses): envelope acquisition necessitates membrane trafficking and targeting to sites of budding; this imposes constraints on timing and location.
• Quality control: ensuring correct folding and assembly is critical; misfolded intermediates must be degraded or recycled; chaperones and scaffolds aid quality control.
• Protection vs exposure: assembled structures must be stable enough to survive the extracellular environment but metastable enough to become infectious upon maturation.

Viral Components and the Cellular Landscape

• Virions are built from viral proteins plus nucleic acid; many viruses also incorporate host-derived membranes (enveloped viruses).
• Viral components are synthesized and accumulate in different places:
  • Nucleus (e.g., replication for some DNA viruses, and some RNA viruses with nuclear phases)
  • Cytosol (soluble internal proteins, polymerases, etc.)
  • Endoplasmic Reticulum (ER) and Golgi (membrane glycoproteins, envelope components, trafficking)
  • Plasma membrane (sites of budding and organelle contact)
• Some viral components originate from cellular structures (membranes, scaffolding) and are co-opted by the virus.
• Directionality and assembly efficiency are enhanced by concentrating components in the same region, not by random diffusion alone.

A Concrete Example: Influenza Virus (Segmented Negative-Sense RNA)

• Genome organization: segmented RNA with 8 pieces collected and packaged together; genome assembly is selective and coordinated.
• Protein synthesis locations: genome transcription and replication occur in the nucleus; viral polymerases and other internal proteins are produced in the cytosol.
• Membrane proteins: membrane glycoproteins synthesized in the ER, processed in the Golgi, and trafficked to the plasma membrane.
• Packaging challenge: must select and assemble the correct eight genomic segments into a single virion, despite abundant cellular RNAs.

Genome Packaging and Specificity in Viral Assembly

• Specificity in packaging is achieved despite a crowded cellular background; viruses can selectively pick viral nucleic acid amid nonviral RNA.
• Segmented and non-segmented genomes each present unique packaging challenges; influenza exemplifies segmented genome assembly and selective packaging.
• Directionality and assembly efficiency arise from local concentration, complementary interfaces, and multistep assembly processes.

Self-assembly Principles: How Capsids Form

• Core idea: viral structural proteins have information to self-assemble via extensive, complementary interaction surfaces; symmetry plays a central role.
• Self-assembly tends to produce regular, symmetric shells (icosahedral, helical) through interactions with nucleic acid and with each other.
• For simple viruses (e.g., tobacco mosaic virus), assembly can be direct from nucleic acid and a single type of structural protein; real viruses are more complex and require additional controls.
• Helical vs icosahedral assembly:
  • Helical: nucleic acid binds proteins that stack sequentially to form a helix; can be illustrated by evolving interactions on a nucleic acid.
  • Icosahedral: involves multiple copies of structural units arranged with icosahedral symmetry; assembly is more complex and often uses intermediate steps.
• Visual metaphor: even simple assembly looks like slotting surfaces together; a movie or animation can illustrate how surface complementarity drives ordered assembly (not shown due to copyright in some resources).

Stepwise Assembly and Production Lines

• Real viruses rely on stepwise assembly (production line) rather than mixing all pieces together at once.
• Structural subunits are formed first, often as monomers or small oligomers; these form larger subassemblies, which then assemble into the full capsid.
• Subassemblies, intermediates, and finally mature virions are produced in a controlled, ordered sequence.
• Car analogy: building a car from components (brakes, engine parts, doors) that are assembled stepwise yields a correctly formed product; random assembly would be a mess and fail.
• For complex viruses (e.g., bacteriophage T4), a large number of proteins participate; assembly uses discrete subassemblies that progressively build toward the mature particle.
• Directionality in production line assembly arises from complementary interactions and symmetry; once intermediates are correctly formed, the next step proceeds.
• Quality control is essential: if intermediates are not correctly formed, they are degraded or recycled until proper assembly is achieved.

Monomeric vs Polyprotein Assembly: Two Key Pathways

• Pathway 1: Monomeric proteins assemble into structural units and then into larger shells.
  • Example: SV40 (a DNA virus).
  • SV40 forms pentameric structural units from monomeric proteins via specific interaction surfaces; these pentamers assemble into the capsid.
  • Assembly is largely self-assembly driven by primary sequence and interlocking interfaces; cellular proteins can complicate this process due to nonspecific interactions.
  • Reversibility: initial assembly is reversible; enough production of monomers drives forward assembly toward completion.
• Pathway 2: Polyprotein processing and proteolytic maturation.
  • Example: positive-sense ssRNA viruses (e.g., HIV and others discussed later).
  • A single large polyprotein is translated and folded into domains; proteases cleave the polyprotein to yield individual viral proteins (e.g., VP1, VP2, VP3 in some viruses; larger proteins later in the genome).
  • In early folding, intermediates (e.g., folded P1) form a structural unit; proteolysis converts and stabilizes the mature units (e.g., 5s structural unit) and irreversible steps drive maturation.
  • Intermediates may be non-covalently associated; disassembly is possible during entry; interactions are designed to be reversible up to the point of irreversible maturation.

Quality Control, Chaperones, and Scaffolds in Assembly

• Molecular chaperones facilitate folding and assembly, prevent non-specific interactions, and can be energy-dependent (often ATP-driven).
• Cellular chaperones are used by viruses in the cytoplasm, ER, and Golgi; examples include HSC70 (human chaperone) aiding polio assembly.
• Viral chaperones (virus-encoded) can also assist folding and assembly; viruses may rely on both cellular and viral chaperones depending on the system.
• Chaperones are sometimes essential; without them, certain viruses fail to assemble properly.
• Examples of chaperone involvement:
  • Polio assembly involves HSC70; a second chaperone collaborates to complete the process.
  • SV40 assembly into pentamers and capsids requires a human chaperone and a viral chaperone.
  • Adenovirus hexon trimer assembly is dependent on a chaperone; the fiber attaches to the hexon and its formation requires chaperone assistance.
• Scaffold proteins provide a framework for assembling large, complex shells (not part of the final virion).
  • Example: herpes simplex virus nucleocapsid assembly relies on scaffold proteins and a protease-containing precursor (which becomes activated) to organize through a portal and other structural features.
  • VP24–VP21 region contains protease activity that participates in scaffold processing and activation; scaffolds guide correct assembly and are later removed to allow DNA insertion through the portal.
• Portal proteins: a notable feature in some herpesviruses; a portal is a specialized channel through which DNA enters the capsid.
• The role of proteases: proteolytic processing is a critical irreversible step that enables conformational changes and maturation of the capsid structures.

Nomenclature and Concepts You’ll Encounter

• Subunit: a single folded polypeptide chain used in building the capsid.
• Structural unit: the basic building block of the capsid; can be a single protein or a small assembly (sometimes called an asymmetric unit or protomer).
• Capsid: the protein shell of the virion.
• Nucleocapsid: the capsid associated with nucleic acid (genome packaged with the protein shell).
• Envelope: lipid membrane acquired by enveloped viruses during exit; essential for infectivity and entry in many viruses.
• Structural unit assembly: repeated use of identical or related subunits to build larger shells; assembly often proceeds via intermediate subassemblies.
• Production line: a stepwise, ordered process by which viral components assemble into mature virions, rather than random aggregation.
• Chaperone: proteins (cellular or viral) that assist folding and assembly of virion components.
• Scaffold: non-structural proteins that facilitate large complex shell assembly and are removed before or during maturation.
• Monomeric vs polyprotein pathways: two distinct routes to generate structural units; monomeric proteins assemble into units, while polyprotein processing yields mature subunits after proteolysis.
• Directionality and symmetry: assembly is guided by surfaces that fit together complementarily and by symmetry; this drives forward progression and ensures correct final architecture.

Enveloped vs Non-enveloped: Exit and Maturation Considerations

• Enveloped viruses:
  • Must acquire a lipid envelope during exit, typically by budding through a cellular membrane.
  • Envelope acquisition imposes constraints on timing, site of assembly, and maturation steps.
• Non-enveloped viruses:
  • Exit typically involves lysis or other release mechanisms; their assembly must avoid premature disassembly while inside the cell.
• Predicting exit strategies: the presence or absence of an envelope predicts different exit routes and maturation requirements.
• Maturation: after assembly, virions often require maturation steps (metastable to infectious) to become capable of initiating infection.

Key Takeaways for Exam Preparation

• Virion assembly is a carefully choreographed, highly regulated, multistep process that often relies on self-assembly motifs, stepwise assembly lines, and quality control mechanisms.
• Local concentration and directed interactions overcome dilution and inefficiency of reactions in the cellular milieu.
• Two major assembly pathways exist: monomeric protein assembly into structural units and polyprotein processing with proteolytic maturation; each has distinct control points and chaperone requirements.
• Chaperones and scaffolds are frequently essential for correct folding and assembly, with some being incorporated into virions (directly or indirectly) or removed during maturation.
• The lifecycle timing (eclipse, latent period, burst) is tightly linked to assembly and maturation processes, and to the release strategy (envelope acquisition vs direct release).
• Terminology (subunit, structural unit, asymmetric unit, protomer, capsid, nucleocapsid, envelope) is used variably across virology texts; be flexible and understand the functional role rather than memorize one fixed table.
• Real-world examples highlighted in the lecture:
  • SV40: monomeric proteins assemble into pentamers, then into the capsid; requires cellular and viral chaperones.
  • Polio: chaperone-assisted folding and assembly; polyprotein context with proteolytic processing to mature proteins.
  • Adenovirus: hexon trimer assembly dependent on chaperones; fiber attachment involves interactions with hexon.
  • Herpes simplex virus: scaffold-driven assembly and portal-based genome packaging; protease activity within scaffolds enables maturation.
  • Bacteriophage T4: illustrates production line assembly with discrete structural units and stepwise growth.
• Mathematical/quantitative notes (conceptual):
  • Burst size per infected cell is typically in the range
    $B \approx 10^2 \text{ to } \ 10^3$
    virions.
  • Latent period defines the interval before virion release; eclipse phase precedes detectable infectious virions and represents disassembly and uncoating.
  • Assembly challenges involve balancing enthalpic gains from specific interactions with entropic costs; local concentration and intermediate stabilization help overcome diffusion limits in dilute cellular environments.