Structural basis for HIV-1 capsid adaption to a deficiency in IP6 packaging

Area of Science:

• Virology
• Structural Biology
• Biochemistry

Background:

• Inositol hexakisphosphate (IP6) is crucial for HIV-1 assembly and maturation.
• IP6 stabilizes the Gag lattice and is essential for mature capsid formation within virions.
• Previous studies identified Gag mutants with low IP6 packaging, leading to non-infectious particles with defective capsids.

Purpose of the Study:

• To investigate compensatory mechanisms in HIV-1 assembly under IP6-deficient conditions.
• To characterize a mutation in the capsid protein (CA) that restores infectivity in IP6-deficient HIV-1 mutants.
• To elucidate the structural and functional role of the CA C-terminus in viral assembly.

Main Methods:

• Genetic analysis of HIV-1 Gag mutants.
• In vitro assembly assays of capsid protein (CA).
• Cryo-electron microscopy (CryoEM) for structural determination.
• Molecular dynamics (MD) simulations.

Main Results:

• A compensatory mutation, G225R, in the CA C-terminus restores capsid assembly and infectivity in IP6-deficient HIV-1 mutants.
• G225R enhances in vitro CA assembly at significantly lower IP6 concentrations compared to wild-type CA.
• CryoEM structures reveal that G225R induces a structured CA C-terminus, stabilizing hexamer-hexamer interfaces.
• MD simulations support the stabilizing mechanism of the G225R mutation.

Conclusions:

• HIV-1 can adapt to IP6 deficiency through mutations like G225R in the capsid protein.
• The CA C-terminus plays a critical, previously unrecognized structural role in stabilizing HIV-1 capsid assembly.
• The findings provide insights into viral adaptation and offer tools for studying HIV-1 capsid formation.