HK97 fold - Stephen Harrison (Harvard/HHMI)

TL;DR

Viruses have assembly units with scaffolds ensuring accurate structure formation.

Transcript

in all of these structures the papilloma viruses the adenoviruses the picornoviruses on the the plant viruses such as tomato bushy stunt we see a simple construction principle at work that is a little bit like an assembly line like a factory assembly there is in all cases a fixed assembly unit happens to be a dimer in the case of the code protein o... Read More

Key Insights

• Viruses like papilloma, adenoviruses, and picornaviruses use a mass-produced assembly unit for constructing their structures, similar to a factory assembly line.
• A scaffold or framework ensures the precise placement of these assembly units, preventing errors in constructing larger or smaller particles.
• The HK97 fold is a distinct architectural motif found in many bacteriophages, differing from the common assembly units of other viruses.
• The HK97 fold involves protein subunits forming hexamers and pentamers, creating a shell structure in bacteriophages.
• In some bacteriophages, the inner scaffold is discarded or reused, playing a role in DNA injection during the next assembly stage.
• Herpes viruses utilize a more complex version of the HK97 fold, with additional loops and decorations on the protein subunits.
• The complex surface loops in herpes viruses contribute to their larger and more intricate structures compared to phage particles.
• Current herpes virus structures are based on electron microscopy, which provides lower resolution compared to x-ray structures.

Questions & Answers

Q: What is the common construction principle in virus structures?

The common construction principle in virus structures involves a mass-produced assembly unit, similar to a factory assembly line. This unit forms the coat of the virus, with a scaffold or framework ensuring the accurate placement of these units, preventing errors in constructing larger or smaller particles.

Q: What is the HK97 fold and where is it found?

The HK97 fold is a distinct architectural motif found in many bacteriophages. Unlike the common assembly units of other viruses, the HK97 fold involves protein subunits forming hexamers and pentamers, creating a shell structure. It was first discovered in the bacteriophage HK97 and is prevalent in double-strand DNA bacteriophages.

Q: How does the scaffold function in bacteriophages?

In bacteriophages, the scaffold plays a crucial role in assembling the virus structure. It ensures the accurate placement of protein subunits, forming a shell. After assembly, the scaffold is either discarded through proteolytic digestion or reused. This process is essential for the subsequent DNA injection stage, where motor proteins insert DNA into the particle.

Q: What differentiates herpes viruses from other viruses in terms of structure?

Herpes viruses differ from other viruses due to their use of a more complex version of the HK97 fold. Their protein subunits have additional loops and decorations, making them larger and more intricate compared to simpler phage particles. These complex surface loops are crucial for the virus's structural integrity and interactions.

Q: What role do motor proteins play in virus assembly?

Motor proteins are responsible for inserting DNA into the virus structure during the assembly process. After the scaffold is discarded or reused, these proteins facilitate the DNA injection stage, ensuring that the genetic material is correctly positioned within the assembled shell. This step is critical for the virus's functionality and replication.

Q: How are current herpes virus structures studied?

Current herpes virus structures are studied using electron microscopy, which provides a relatively low-resolution representation compared to x-ray structures. Despite this limitation, electron microscopy allows researchers to visualize the fundamental fold and complex surface loops, essential for understanding the virus's structural integrity and interactions.

Q: What is the significance of surface loops in herpes viruses?

Surface loops in herpes viruses are significant because they contribute to the virus's larger and more intricate structure compared to simpler phage particles. These loops aid in additional interactions and decorations on the protein subunits, playing a crucial role in the virus's structural integrity and its ability to interact with host cells.

Q: Why are there always 12 pentamers in icosahedral structures?

There are always 12 pentamers in icosahedral structures because this arrangement is mathematically necessary to form a closed, stable geometric shape. The combination of pentamers and hexamers allows for the construction of a symmetrical, icosahedral virus particle, which is a common structural form in many viruses, providing stability and efficiency in packaging genetic material.

Summary & Key Takeaways

• Virus structures like papilloma and adenoviruses use a standardized assembly unit with a scaffold ensuring accurate construction. The HK97 fold, found in many bacteriophages, represents a different architectural motif. It involves protein subunits forming hexamers and pentamers, creating a shell structure around the viral DNA.

• In bacteriophages, the inner scaffold can be discarded or reused, aiding in DNA injection during the assembly process. Herpes viruses have a more complex version of the HK97 fold, with additional loops and decorations, making them larger and more intricate compared to phage particles.

• Herpes virus structures are currently studied using electron microscopy, offering lower resolution than x-ray techniques. Despite this, the fundamental fold and complex surface loops are crucial for the virus's structural integrity and interactions, differentiating them from simpler phage particles.