Cauliflower mosaic virus: a 420 subunit (T = 7), multilayer structure.

Area of Science:

• Structural biology
• Virology
• Biophysics

Background:

• Cauliflower mosaic virus (CaMV) is a plant pathogen with an icosahedral structure.
• Understanding CaMV's structure is crucial for comprehending its replication and assembly processes.
• Previous studies suggested CaMV's T=7 icosahedral symmetry, but high-resolution structural data were limited.

Purpose of the Study:

• To determine the high-resolution three-dimensional structure of Cauliflower Mosaic Virus (CaMV).
• To investigate the arrangement of capsomeric units and overall viral architecture.
• To provide structural insights for understanding CaMV replication and assembly.

Main Methods:

• Cryo-electron microscopy of unstained, frozen-hydrated CaMV particles (Cabb-B and CM1841 strains).
• Low-irradiation imaging to minimize sample damage.
• Three-dimensional image reconstruction techniques to generate high-resolution structural models.

Main Results:

• The CaMV structure was resolved to approximately 3 nm resolution.
• Particles are spherical (53.8 nm diameter) with three concentric layers surrounding a central cavity.
• The outer layer exhibits T=7 icosahedral symmetry, composed of 420 subunits arranged as 72 capsomeric units (12 pentamers, 60 hexamers).
• Hexameric capsomers showed deviations from regular sixfold symmetry, despite T=7 arrangement.

Conclusions:

• CaMV is the first T=7 virus conforming to Caspar and Klug's stoichiometry rules.
• The determined structure is consistent with previous neutron diffraction and replication-assembly models.
• High agreement between Cabb-B and CM1841 strain structures suggests a conserved basic architecture across CaMV strains.