Intercapsomeric disulfide bonds in papillomavirus assembly and disassembly

Area of Science:

• Virology
• Structural Biology
• Biochemistry

Background:

• Papillomaviruses, including bovine papillomavirus (BPV) and human papillomavirus (HPV), are non-enveloped viruses with icosahedral capsids.
• The capsid is primarily composed of the L1 major capsid protein, which is essential for virion assembly.
• Understanding the molecular interactions stabilizing the capsid is key to viral assembly and uncoating mechanisms.

Purpose of the Study:

• To investigate the role of bonding contacts in stabilizing bovine papillomavirus (BPV) virions and promoting capsid assembly.
• To analyze the effects of specific buffer conditions on BPV virion integrity and L1 protein stability.
• To elucidate the function of conserved cysteine residues in papillomavirus capsid formation.

Main Methods:

• BPV virions were subjected to buffer conditions including dithiothreitol (DTT) and EGTA.
• Virion integrity was assessed by electron microscopy, trypsin and DNase I digestion sensitivity, and analytical ultracentrifugation.
• Recombinant HPV type 11 L1 protein with a cysteine mutation was expressed in E. coli and analyzed for in vitro assembly.

Main Results:

• DTT treatment, but not EGTA, sensitized BPV virions to trypsin and DNase I digestion, indicating capsid destabilization.
• DTT-treated BPV virions showed reduced sedimentation velocity (230S vs. 273S), suggesting capsid expansion.
• A mutant HPV L1 protein with a Cys-to-Gly change at a conserved position failed to form capsid-like structures in vitro, unlike the wild-type.

Conclusions:

• Interpentamer disulfide bonds play a critical role in papillomavirus capsid assembly and disassembly.
• The findings suggest a mechanism for virus uncoating occurring in the reducing environment of the host cell cytoplasm.
• Conserved cysteine residues in the L1 protein are essential for maintaining capsid structure and facilitating assembly.