A key interaction between the alphavirus envelope proteins responsible for initial dimer dissociation during fusion

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Alphaviruses, like Semliki Forest virus (SFV), feature an E2-E1 heterodimer envelope protein structure essential for viral fusion.
• E2 protein processing by furin and subsequent dissociation from E1 at low pH are critical for virus entry.
• Previous studies identified an acid-sensitive region in E2, with E2 H170 potentially interacting with E1 S57.

Purpose of the Study:

• To experimentally investigate the role of the E1 S57-E2 H170 interaction in regulating E2-E1 dimer dissociation.
• To determine the impact of this interaction on pH-dependent fusion and viral entry mechanisms.

Main Methods:

• Site-directed mutagenesis was used to create alanine substitutions at E1 S57 and E2 H170.
• Assays were performed to assess dimer stability, E1 fusion loop exposure, and low-pH-triggered fusion.
• Analysis included studying the effects of mutations on viral transport, assembly, and lethality, with rescue experiments.

Main Results:

• Mutations in E1 S57 and E2 H170 destabilized the E2-E1 heterodimer, increasing the pH threshold for fusion.
• Specific mutations (E1 S57K/D) were lethal, causing transport and assembly defects, partially rescued by exocytic pathway neutralization.
• Second-site mutations at E2 H170/M171 rescued the lethal phenotype of E1 S57K, highlighting the interaction's importance.

Conclusions:

• The E1 S57-E2 H170 interaction plays a pivotal role in maintaining E2-E1 dimer stability and regulating pH-dependent fusion.
• This interaction is essential for the stepwise dissociation of the E2-E1 dimer, a key event during alphavirus entry.
• Understanding this interaction provides insights into alphavirus fusion mechanisms and potential therapeutic targets.