GTP is the primary activator of the anti-HIV restriction factor SAMHD1.

SAMHD1 (SAM domain- and HD domain-containing protein 1) is a dGTP-dependent dNTP triphosphohydrolase that converts dNTPs into deoxyribonucleosides and triphosphates. Therefore, SAMHD1 expression, particularly in non-dividing cells, can restrict retroviral infections such as HIV and simian immunodeficiency virus by limiting cellular dNTPs, which are essential for reverse transcription. It has previously been established that dGTP acts as both an activator and a substrate of this enzyme, suggesting that phosphohydrolase activity of SAMHD1 is regulated by dGTP availability in the cell. However, we now demonstrate biochemically that the NTP GTP is equally capable of activating SAMHD1, but GTP is not hydrolyzed by the enzyme. Activation of SAMHD1 phosphohydrolase activity was tested under physiological concentrations of dGTP or GTP found in either dividing or non-dividing cells. Because GTP is 1000-fold more abundant than dGTP in cells, GTP was able to activate the enzyme to a greater extent than dGTP, suggesting that GTP is the primary activator of SAMHD1. Finally, we show that SAMHD1 has the ability to hydrolyze base-modified nucleotides, indicating that the active site of SAMHD1 is not restrictive to such modifications, and is capable of regulating the levels of non-canonical dNTPs such as dUTP. This study provides further insights into the regulation of SAMHD1 with regard to allosteric activation and active site specificity.