Structural basis for spumavirus GAG tethering to chromatin

Area of Science:

• Molecular Biology
• Virology
• Structural Biology
• Epigenetics

Background:

• Retroviral integration into host cell chromatin and subsequent provirus expression mechanisms remain largely unknown.
• The prototype foamy virus (PFV) structural protein GAG binds to chromosomes through a specific chromatin-binding sequence (CBS) in its C-terminal region.

Purpose of the Study:

• To elucidate the role of the PFV CBS in nucleosome interaction and its impact on viral integration site selection.
• To determine the structural basis of the CBS-nucleosome interaction.
• To investigate how mutations in the GAG CBS affect viral integration and expression.

Main Methods:

• Crystallography to determine the structure of the CBS bound to a mononucleosome.
• Site-directed mutagenesis of the PFV GAG CBS.
• Analysis of viral integration site distribution and proviral expression in cells with mutated GAG.

Main Results:

• The PFV CBS directly binds to nucleosomes, interacting with the histone octamer's acidic patch, similar to LANA.
• Mutations in the GAG CBS alter integration sites of PFV and simian foamy virus (SFVmac) towards centromeres.
• Altered integration sites lead to reduced proviral expression without affecting overall integration efficiency.

Conclusions:

• The PFV GAG protein's CBS is crucial for direct nucleosome binding and dictates viral integration site selection.
• Retroviral structural proteins play a significant role in targeting integration and potentially avoiding detrimental genomic regions.
• Understanding these interactions provides insights into retroviral pathogenesis and host-genome dynamics.