Questions and Answers
How are envelope glycoproteins synthesized and inserted into the plasma membrane?
Envelope glycoproteins have large glycosylated ectodomains, hydrophobic transmembrane anchor domains, and short internal tails, synthesized in the ER and inserted into the plasma membrane via standard export pathways.
What is the role of glycosylation in preventing viral aggregation?
Glycosylation of viral surfaces prevents dehydration and reduces protein-protein interactions to prevent viral aggregation.
How do capsid assembly and packaging signals contribute to genome incorporation into virions?
Capsid assembly varies based on size, shape, and genomic composition and may involve scaffolding proteins and concatemer formation to fill the genome. Packaging signals on viral genomes interact with capsid proteins to direct the specificity of genome incorporation into virions.
What are the six categories of organisms that can serve as hosts for viruses, as mentioned in the text?
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What are the criteria used to group virus species into genera?
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How are bacteriophages named?
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What distinguishes satellite viruses from satellite nucleic acids?
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What are the components of a viral envelope and how do they form?
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How does the lipid content of the lipid bilayer reflect the composition of the membrane it was derived from? Provide an example.
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What is the role of glycoproteins (spikes) in forming the viral envelope?
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Study Notes
Viral Envelope Glycoproteins and Capsid Assembly
- Envelope glycoproteins have large glycosylated ectodomains, hydrophobic transmembrane anchor domains, and short internal tails, synthesized in the ER and inserted into the plasma membrane via standard export pathways.
- There are two types of integral membrane proteins, Type I and Type II, each with different orientations and signal sequences for membrane insertion.
- Hemagglutinin (HA) from influenza virus has an ectodomain for cell receptor binding, a transmembrane anchor domain, and a cytoplasmic tail.
- Glycosylation of viral surfaces prevents dehydration and reduces protein-protein interactions to prevent viral aggregation.
- Capsid assembly varies based on size, shape, and genomic composition and may involve scaffolding proteins and concatemer formation to fill the genome.
- Packaging signals on viral genomes interact with capsid proteins to direct the specificity of genome incorporation into virions.
- Core proteins associated with the viral genome inside the capsid neutralize negative charges on DNA and condense the viral DNA for optimal packaging, resembling chromatin.
- Interactions between viral proteins drive the formation of viral envelopes by budding, involving membrane curvature and localized protein aggregates.
- Budding can be driven by interactions between envelope glycoproteins, matrix proteins, or nucleocapsids, and may involve different mechanisms depending on the virus type.
- Virions can release their genomes through proteolytic cleavage of capsid proteins, unspooling of the genome, or interaction with cytoplasmic components.
- Virions are energetically metastable and can easily dissociate with the right trigger, such as binding to a receptor or protein on the cell surface or in the cytoplasm.
- Virus classification is based on molecular architecture, genetic relatedness, and host organism, with criteria including nucleic acid genome type, strandedness, topology, capsid symmetry, and presence or absence of an envelope. Comparing genomic and amino acid sequences helps determine genetic relatedness.
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