SAM domain-containing N-terminal region of SAMHD1 plays a crucial role in its stabilization and restriction of HIV-1 infection

SAMHD1 restricts human immunodeficiency virus type 1 (HIV-1) infection in a cell-type specific manner. Other than primary monocyte derived cells and resting CD4+ T cells, the SAMHD1-mediated HIV-1 block was reported only in phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 and U937 monocyte cell lines. We previously reported that SAMHD1 restricted HIV-1 infection in TE671 rhabdomyosarcoma cells in addition to these cell lines. In this study, we compared the amounts of the full-length SAMHD1 and its deletion mutants, SAM domain containing N-terminal fragment (residues 1-119, SAMHD1n) and HD domain containing C-terminal fragment (120-626, SAMHD1c) in U937, TE671, and HeLa cells. The results showed that the full-length SAMHD1 and SAMHD1n proteins were significantly more abundant than the SAMHD1c protein in TE671 and differentiated U937 cells. The proteasome inhibitor MG132 increased the amount of the SAMHD1c and the SAMHD1c-fused GFP proteins. In contrast, the fusion of the SAMHD1n to the APOBEC3G protein inhibited Vif-induced proteasomal degradation in TE671 and in differentiated U937 cells. These results indicated that the SAMHD1 C-terminal HD domain-containing region leads the SAMHD1 to proteasomal degradation, and the SAMHD1 N-terminal SAM domain-containing region stabilizes the protein. Our study showed that the SAMHD1 protein expression is post-translationally regulated and the significance of SAM and HD domains for the full-length SAMHD1 protein stability. Further, we suggest that the SAM domain-containing N-terminal region participate in the cell-type specific restrictive function of SAMHD1 against HIV-1 infection, by protein stabilization.