HIV-1 and HIV-2 lentiviruses are derived from interspecies transmissions of lentiviruses infecting simian species to humans. For millions of years, restriction factors have been engaged into a molecular conflict with the viral auxiliary proteins of these viruses. Their role as a barrier in the interspecies transmission of lentiviruses has been demonstrated for some of them. The laboratory's work focuses on two restriction factors: SAMHD1, which blocks the viral cycle at the level of reverse transcription and and HUSH (a complex composed of TASOR, MPP8 and Periphilin), which blocks expression of the integrated provirus. Vpx and Vpr from some HIV and SIV lineages have evolved to inactivate these restriction factors via the hijacking of the Ubiquitin Ligase Cul4A-DDB1.
The African Green Monkey (AGM) model of infection with SIV viruses offers a unique opportunity to study the dynamics of the molecular interactions between restriction factors and viral proteins. Seven haplotypes of SAMHD1 have been heterogeneously identified in different AGM species. Each species (Vervet, Grivet, Tantalus, Sabeus) is infected with a particular SIVagm virus, which uses its Vpr protein to primarily recognize the predominant haplotype of the species (Spragg and Emerman, 2013). SAMHD1 recognition involves N- or C-terminal residues of the protein, suggesting that different SIV Vpr use different unknown viral determinants to target SAMHD1. The viral and cellular determinants involved in HUSH antagonism are also not known. I therefore wondered how the different SIVagm Vpr proteins could inactivate both SAMHD1 and HUSH. In addition, I wondered whether divergent lentiviral proteins could degrade HUSH from different species.

Thus, the objectives of my thesis were:

• To identify viral determinants of SIVagm Vpr involved in SAMHD1 and HUSH antagonism. Using chimeric constructs between SIVagm.Vervet and SIVagm.Grivet Vpr, I characterized viral determinants essential for the degradation of some SAMHD1 haplotypes. These determinants differ between viral proteins, in line with SAMHD1 being targeted either at the N or C-terminus. Having demonstrated that Vervet's Vpr protein, but not Grivet's, induced HUSH degradation (lentiviral species-specificity), the same chimeras allowed me to characterize the viral determinants involved in HUSH degradation. Thus, Vpr from SIVagm.Vervet targets TASOR through a distinct interface from that used for SAMHD1 antagonism. Vpr from SIVagm.Grivet targets another haplotype of SAMHD1 differently. These results highlight the molecular plasticity of viral proteins to adapt to host restrictions.
• To analyze the ability of Vpr and Vpx of HIV and SIV lineages to degrade HUSH in different species. Using cell models from divergent primates, I was able to show that TASOR antagonism is species-specific. The viral protein Vpx from HIV-2 and related SIV does not appear to be able to inactivate HUSH in owl monkeys. These data support a model in which HUSH, like SAMHD1, could be a player in viral adaptation during natural infection.
• To clarify the mechanism of Vpx-induced HUSH degradation. I contributed to a study showing that Vpx uses slightly different mechanisms to induce HUSH and SAMHD1 degradation (Martin et al., 2021)

Taken together, my results provide a better understanding of how the same viral protein antagonizes two restriction factors with the involvement of different interfaces from one viral protein to the other. Futhermore, my results show that HUSH antagonism is species-specific and support the hypothesis that HUSH is involved in a molecular conflict with lentiviruses during primate evolution.