Structural basis for spumavirus GAG tethering to chromatin

The interactions between a retrovirus and host cell chromatin that underlie integration and provirus expression are poorly understood. The prototype foamy virus (PFV) structural protein GAG associates with chromosomes via a chromatin-binding sequence (CBS) located within its C-terminal region. Here, we show that the PFV CBS is essential and sufficient for a direct interaction with nucleosomes, and present a crystal structure of the CBS bound to a mono-nucleosome. The CBS interacts with the histone octamer, engaging the H2A-H2B acidic patch, in a manner similar to other acidic patch binding proteins such as herpesvirus latency-associated nuclear antigen (LANA). Substitutions of the invariant arginine anchor residue in GAG result in global re-distribution of PFV and macaque simian foamy virus (SFVmac) integration sites towards centromeres, dampening the resulting proviral expression without affecting the overall efficiency of integration. Our findings underscore the importance of retroviral structural proteins for integration site selection and the avoidance of genomic junkyards. Overall design: Integration sites obtained by infection of cells with single-cycle prototype foamy virus (PFV) or macaque simian foamy virus (SFV) or by incubation of deproteinised human DNA with recombinant PFV intasome were amplified by linker-mediated PCR and sequenced using Illumina technology.