Characterization Of The Interaction Between Host Restriction Factor SAMHD1 And HIV/SIV Accessory Protein Vpx

Viruses are intracellular parasites. They do not carry all the necessary machineries to replicate on their own. Instead, viruses have to exploit the machineries and resources in host cells to complete their life cycle. To protect from virus infection, host cells often encode factors that inhibit viruses and are thus called restriction factors. This "arms race" process leaves positive selection signatures in these host restriction factors. The outcome of this process often modulates the susceptibility of a species to a specific virus.HIV-1 is poorly infectious in macrophages and dendritic cells. In contrast, HIV-2 and certain SIV strains encode an accessory protein called Vpx that counteracts a restriction factor that exists in macrophages and dendritic cells and thus allow HIV-2 and SIV to infect these two cell types. This restriction factor was subsequently identified as SAMHD1 (Sterile a motif and HD domain-containing protein-1). SAMHD1 depletes cellular dNTP pool with its triphosphohydrolase activity and thus blocks HIV-1 reverse transcription. SAMHD1 restricts HIV-1 infection also by degrading viral RNA genome.HIV-2/SIV Vpx mediates polyubiquitination of SAMHD1 by recruiting E3 ubiquitin ligase complex, which triggers SAMHD1 degradation at proteasomes. Out of the ten lineages of SIV, only two encode Vpx. One is the HIV-2/SIVmac (macaque)/SIVsm (sooty mangabey) lineage and the other is the SIVrcm (red-capped mangabey)/SIVmnd2 (mandrill) lineage. Our results and that of others all showed that Vpx proteins from these two linages mediate species-specific degradation of SAMHD1, i.e. SIVsm/SIVmac/HIV-2 Vpx target human SAMHD1 and that of their own host for degradation, but not SAMHD1 from Red-capped mangabey, whereas SIVrcm does the opposite. This phenomenon is due to the different recognition mode used by SIVsm/SIVmac/HIV-2 Vpx and SIVrcm Vpx. It has already been elucidated how SIVsm Vpx mediates SAMHD1 degradation. SIVsm Vpx binds SAMHD1 C-terminal sequence using its N-terminal amino acids N12, E15,E16, T17, and loads SAMHD1 onto the adaptor protein of E3 ligase complex for ubiquitination and degradation. In contrast, SIVrcm Vpx uses a different mode to recognize SAMHD1. Instead of binding the C-terminal region of SAMHD1, SIVrcm Vpx recognizes the N-terminal sequence of SAMHD1. However, details of this latter interaction are not known.These two different modes of SAMHD1 recognition are probably due to the competition between host restriction factor and its viral antagonist. SAMHD1 forms head to tail dimer or tetramer. It has been proposed that the ancestral Vpx may bind both ends of SAMHD1 in the light of the spatial proximity of the two termini. When SAMHD1 changes to escape from the interaction with Vpx, Vpx may acquire mutations to bind only one terminus of SAMHD1.In this study, we have investigated SIVrcm Vpx mediated degradation of RCM SAMHD1 using mutagenesis and molecular modeling approaches. We have identified the key role of the 23-WLHR-26 peptide of SIVrcm Vpx in recognizing rcmSAMHD1. The amino acids F15, L36, F52, R55 and R56 at the N-terminal region of rcmSAMHD1 were found to participate in the interaction with Vpxrcm. The molecular model of the N-terminal region of rcmSAMHD1 (rcmSAMHD1-NtD), Vpxrcm and C-terminal region of DCAF1 (DCAF1-CtD) complex further reveals that rcmSAMHD1-NtD and Vpxrcm utilize an interaction interface that is different from that used by human SAMHD1-CtD and Vpxsm.Moreover, we observed that SAMHD1 might have a truncated version that is likely a result of cleavage by a cellular protease at the N-terminal region of SAMHD1. This cleavage event leads to the loss of the nuclear localization signal in SAMHD1 and renders SAMHD1 resistant to Vpx mediated degradation. Together, these findings provide further insights into the different modes of interaction between Vpx and SAMHD1 as a result of the ’arms race’ between virus and host.