Visualizing Viral RNA Packaging Signals in Action

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Satellite Tobacco Necrosis Virus-1 (STNV-1) forms T=1 virus-like particles (VLPs).
• Genome packaging in viruses is crucial for infection and assembly.

Purpose of the Study:

• To investigate the role of packaging signals (PSs) in STNV-1 genome packaging.
• To elucidate the mechanism of virus-like particle (VLP) assembly and genome extrusion.

Main Methods:

• Genome-scale RNA fragments in assembly competition assays.
• Asymmetric cryo-electron microscopy (cryo-EM) reconstructions.
• Analysis of RNA-protein interactions and VLP structure.

Main Results:

• Multiple packaging signals (PSs) on the STNV-1 genome make sequence-specific contacts with coat proteins (CPs), nucleating VLP formation.
• CP N-terminal residues interact with gRNA PS loop sequences, explaining selective genome packaging.
• Altered PS-CP affinity leads to asymmetric PS distribution within VLPs, influencing genome extrusion.
• Genome folding and CP affinities dictate roles in virion assembly and infection.

Conclusions:

• Differential packaging signal folding and coat protein affinities are critical for viral genome functions in assembly and infection.
• The findings reveal a novel mechanism for genome extrusion regulated by RNA folding and VLP structure.
• Conserved coat protein folds suggest these genome functions are widespread across many viruses.