Is Sindbis a simple picornavirus with an envelope?

Area of Science:

• Structural virology
• Molecular evolution
• Cryo-electron microscopy

Background:

• Sindbis virus (SNV) is an enveloped alphavirus.
• Understanding viral capsid structure is crucial for virology.
• Previous studies have characterized various viral capsid folds.

Purpose of the Study:

• To determine the three-dimensional structure of isolated Sindbis virus nucleocapsids.
• To compare SNV capsid protein structure with other viral proteins.
• To investigate the evolutionary origins of viral capsid proteins.

Main Methods:

• Three-dimensional image reconstruction from cryo-electron micrographs.
• Structural comparison of SNV nucleocapsids with whole virions.
• Sequence alignment of SNV capsid proteins with picornavirus VP3 proteins.

Main Results:

• Isolated SNV nucleocapsids exhibit a smooth, fenestrated T=3 structure.
• The SNV capsid structure resembles swollen forms of non-enveloped viruses.
• Sequence alignment predicts an eight-stranded beta-barrel fold for SNV capsid proteins, similar to picornaviruses.

Conclusions:

• SNV capsid proteins share structural and potentially evolutionary links with picornavirus VP3 proteins.
• Alphavirus and picornavirus capsid proteins may have diverged from a common vp3-like ancestor.
• This suggests a shared evolutionary pathway for enveloped and non-enveloped viruses.