Abstract
SAM and HD domain-containing protein 1 (SAMHD1) is a host factor that restricts reverse transcription of lentiviruses such as HIV in myeloid cells and resting T cells through its dNTP triphosphohydrolase (dNTPase) activity. Lentiviruses counteract this restriction by expressing the accessory protein Vpx or Vpr, which targets SAMHD1 for proteasomal degradation. SAMHD1 is conserved among mammals, and the feline and bovine SAMHD1 proteins (fSAM and bSAM) restrict lentiviruses by reducing cellular dNTP concentrations. However, the functional regions of fSAM and bSAM that are required for their biological functions are not well-characterized. Here, to establish alternative models to investigate SAMHD1 in vivo, we studied the restriction profile of fSAM and bSAM against different primate lentiviruses. We found that both fSAM and bSAM strongly restrict primate lentiviruses and that Vpx induces the proteasomal degradation of both fSAM and bSAM. Further investigation identified one and five amino acid sites in the C-terminal domain (CTD) of fSAM and bSAM, respectively, that are required for Vpx-mediated degradation. We also found that the CTD of bSAM is directly involved in mediating bSAM's antiviral activity by regulating dNTPase activity, whereas the CTD of fSAM is not. Our results suggest that the CTDs of fSAM and bSAM have important roles in their antiviral functions. These findings advance our understanding of the mechanism of fSAM- and bSAM-mediated viral restriction and might inform strategies for improving HIV animal models.
Highlights
sterile alpha motif (SAM) and HD domain-containing protein 1 (SAMHD1) is a host factor that restricts reverse transcription of lentiviruses such as HIV in myeloid cells and resting T cells through its dNTP triphosphohydrolase activity
These results demonstrated that C-terminal domain (CTD) of Human SAMHD1 (hSAM) and fSAM and Leu580 of bSAM are not required for the antiviral function of these WT SAM and HD domain-containing protein 1 (SAMHD1) proteins, and Thr-619 of fSAM and Leu-575, Lys-577, Arg-578 and Lys-579 of bSAM are partially or fully necessary for feline and bovine SAMHD1 in restriction of different viral strains
Because significant differences in the dNTP triphosphohydrolase (dNTPase) activity were observed between many mutants and their WT SAMHD1 proteins but the antiviral activity of two of the mutants were not impacted (Fig. 6, A–C), we proposed that a 2-fold reduction in the dNTPase activity in vitro (Fig. 7, D–F) was the threshold of unaffected antiviral activity in our assays, suggesting that the antiviral activity of fSAM and bSAM CTD mutants largely depends on their dNTPase activity and the CTD of SAMHD1, especially of bovine SAMHD1, contributes to the maintenance of dNTPase activity of SAMHD1
Summary
SAM and HD domain-containing protein 1 (SAMHD1) is a host factor that restricts reverse transcription of lentiviruses such as HIV in myeloid cells and resting T cells through its dNTP triphosphohydrolase (dNTPase) activity. The viral protein X (Vpx) of HIV-2 and some strains of SIV antagonize the antiviral function of SAMHD1 by recruiting SAMHD1 to the E3 ubiquitin ligase complex CRL4DCAF1 consisting of DCAF1, DDB1, CUL4, and RBX1 (16 –20) This recruitment leads to proteasomal degradation of SAMHD1 through which Vpx relieves SAMHD1-mediated restriction of lentiviral replication in myeloid-lineage cells and resting T cells (18 –24). We identified key sites in the C-terminal regions of fSAM and bSAM which are required for Vpx-mediated degradation and revealed the role of bovine SAMHD1 CTD in regulating the catalytic function and antiviral activity. Our findings will provide insights into the establishment of alternative models to investigate SAMHD1 in vivo
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
