Allosteric targeting with antiviral nucleotide analogs allows fine-tuning of SAMHD1 dNTPase activity

SAM and HD domain-containing protein 1 (SAMHD1) is a dNTP hydrolase that controls intracellular dNTP pools and plays diverse roles in human health and disease. Notably, this enzymatic activity also confers chemotherapy resistance by hydrolyzing the active triphosphate forms of nucleoside analog drugs, thereby reducing their efficacy and contributing to worse treatment outcomes in cancer patients. The dNTPase activity of SAMHD1 is tightly regulated by allosteric activation and oligomerization through binding of (d)NTPs to two allosteric sites (ASs), the first of which-AS1-requires binding of a guanine nucleotide. In the present study, we investigated strategies to pharmacologically modulate SAMHD1 dNTPase activity via AS1. Using a variety of biochemical and biophysical assays, we demonstrate that the antiviral guanine nucleotide analogs, acyclovir- and ganciclovir-triphosphate, are potent AS1 binders that induce the formation of enzymatically competent SAMHD1 tetramers, however with reduced enzymatic activity. Furthermore, we show that AS1 activator identity can fine-tune dNTPase activity toward different dNTP substrates, providing a new avenue to pharmacologically control SAMHD1. This differential activity of acyclovir- and ganciclovir-triphosphate-activated SAMHD1 can be explained by distinct kinetic profiles that deviate from Michaelis-Menten kinetics. Furthermore, based on an apparent synergistic activation between these nucleotide analogs and the physiological AS1 activator GTP, we also propose the existence of mixed-occupancy SAMHD1 tetramers. Our work therefore provides new insights into the allosteric activation and oligomerization process of SAMHD1 and opens new avenues to pharmacologically control the dNTPase activity utilizing non-natural allosteric ligands.