Abstract
The human HD domain protein SAMHD1 is implicated in the Aicardi-Goutières autoimmune syndrome and in the restriction of HIV-1 replication in myeloid cells. Recently, this protein has been shown to possess dNTP triphosphatase activity, which is proposed to inhibit HIV-1 replication and the autoimmune response by hydrolyzing cellular dNTPs. Here, we show that the purified full-length human SAMHD1 protein also possesses metal-dependent 3'→5' exonuclease activity against single-stranded DNAs and RNAs in vitro. In double-stranded substrates, this protein preferentially cleaved 3'-overhangs and RNA in blunt-ended DNA/RNA duplexes. Full-length SAMHD1 also exhibited strong DNA and RNA binding to substrates with complex secondary structures. Both nuclease and dNTP triphosphatase activities of SAMHD1 are associated with its HD domain, but the SAM domain is required for maximal activity and nucleic acid binding. The nuclease activity of SAMHD1 could represent an additional mechanism contributing to HIV-1 restriction and suppression of the autoimmune response through direct cleavage of viral and endogenous nucleic acids. In addition, we demonstrated the presence of dGTP triphosphohydrolase and nuclease activities in several microbial HD domain proteins, suggesting that these proteins might contribute to antiviral defense in prokaryotes.
Highlights
The human SAMHD1 protein has dNTP triphosphatase activity and is involved in HIV-1 restriction and autoimmune syndrome
Nuclease and dGTP Hydrolase Activities of SAMHD1 and Its Isolated Domains—Full-length human SAMHD1 was overexpressed in E. coli and analyzed for the presence of nuclease activity using 5Ј-32P-labeled ssDNAs and ssRNAs as substrates
The elution profile of SAMHD1 from the size-exclusion and anion-exchange columns correlated with the peaks of DNase and dGTP triphosphatase activities, confirming the association of both activities with this protein
Summary
The human SAMHD1 protein has dNTP triphosphatase activity and is involved in HIV-1 restriction and autoimmune syndrome. Full-length SAMHD1 exhibited strong DNA and RNA binding to substrates with complex secondary structures Both nuclease and dNTP triphosphatase activities of SAMHD1 are associated with its HD domain, but the SAM domain is required for maximal activity and nucleic acid binding. The nuclease activity of SAMHD1 could represent an additional mechanism contributing to HIV-1 restriction and suppression of the autoimmune response through direct cleavage of viral and endogenous nucleic acids. Our data suggest that the biochemical function of SAMHD1 might not be limited to dGTP hydrolysis and that its nuclease activity could contribute to HIV-1 restriction and autoimmune response through a direct cleavage of viral and endogenous nucleic acids
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