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

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.

SAMHD1以细胞类型特异性的方式限制1型人类免疫缺陷病毒（HIV - 1）感染。除了原代单核细胞衍生细胞和静息CD4 + T细胞外，据报道SAMHD1介导的对HIV - 1的阻断仅发生在佛波醇12 - 肉豆蔻酸酯13 - 乙酸酯（PMA）分化的THP - 1和U937单核细胞系中。我们之前报道过，除了这些细胞系外，SAMHD1还限制TE671横纹肌肉瘤细胞中的HIV - 1感染。在本研究中，我们比较了U937、TE671和HeLa细胞中全长SAMHD1及其缺失突变体、含SAM结构域的N末端片段（残基1 - 119，SAMHD1n）和含HD结构域的C末端片段（120 - 626，SAMHD1c）的量。结果显示，在TE671和分化的U937细胞中，全长SAMHD1和SAMHD1n蛋白比SAMHD1c蛋白明显更丰富。蛋白酶体抑制剂MG132增加了SAMHD1c和与SAMHD1c融合的绿色荧光蛋白（GFP）的量。相反，在TE671和分化的U937细胞中，SAMHD1n与APOBEC3G蛋白的融合抑制了Vif诱导的蛋白酶体降解。这些结果表明，含SAMHD1 C末端HD结构域的区域导致SAMHD1发生蛋白酶体降解，而含SAMHD1 N末端SAM结构域的区域使该蛋白稳定。我们的研究表明，SAMHD1蛋白的表达在翻译后受到调控，以及SAM和HD结构域对全长SAMHD1蛋白稳定性的重要性。此外，我们认为含SAM结构域的N末端区域通过稳定蛋白参与了SAMHD1对HIV - 1感染的细胞类型特异性限制功能。