Asymmetric analysis reveals novel virus capsid features.

Area of Science:

• Structural Biology
• Virology
• Biophysics

Background:

• Cryo-electron microscopy (cryo-EM) and single-particle image analysis are standard for macromolecular structure determination.
• Conventional analysis often applies symmetry (e.g., icosahedral) to viral structures, simplifying calculations but losing asymmetric details.
• Many viruses possess icosahedral symmetry, making symmetry imposition a common practice in structure determination.

Purpose of the Study:

• To provide an overview of methods for investigating non-symmetric capsid features in viruses.
• To highlight the importance of asymmetric analysis in uncovering novel viral structural information.
• To demonstrate how advanced image reconstruction can yield new discoveries from existing data.

Main Methods:

• Focussed classification: a recently developed method for analyzing specific regions.
• Symmetry-free analysis: conventional methods that do not impose symmetry constraints.
• Asymmetric single-particle image analysis: a technique to resolve non-symmetric components.

Main Results:

• Asymmetric analysis reveals previously hidden viral features, such as partial occupancy of rotavirus VP4 spikes.
• Identified unique viral components like the single maturation protein in bacteriophage MS2.
• Discovered portal or portal-like structures involved in genome packaging and release in some viruses.

Conclusions:

• Asymmetric single-particle image analysis significantly expands our understanding of virus structure and function.
• This approach enables novel discoveries from existing cryo-EM datasets, advancing virology.
• Findings contribute to a deeper comprehension of fundamental viral processes like entry and egress.