Date Published: October 2, 2015
Publisher: Public Library of Science
Author(s): Laurence H. Arnold, Harriet C. T. Groom, Simone Kunzelmann, David Schwefel, Sarah J. Caswell, Paula Ordonez, Melanie C. Mann, Sabrina Rueschenbaum, David C. Goldstone, Simon Pennell, Steven A. Howell, Jonathan P. Stoye, Michelle Webb, Ian A. Taylor, Kate N. Bishop, Thomas J. Hope.
SAMHD1 restricts HIV-1 infection of myeloid-lineage and resting CD4+ T-cells. Most likely this occurs through deoxynucleoside triphosphate triphosphohydrolase activity that reduces cellular dNTP to a level where reverse transcriptase cannot function, although alternative mechanisms have been proposed recently. Here, we present combined structural and virological data demonstrating that in addition to allosteric activation and triphosphohydrolase activity, restriction correlates with the capacity of SAMHD1 to form “long-lived” enzymatically competent tetramers. Tetramer disruption invariably abolishes restriction but has varied effects on in vitro triphosphohydrolase activity. SAMHD1 phosphorylation also ablates restriction and tetramer formation but without affecting triphosphohydrolase steady-state kinetics. However phospho-SAMHD1 is unable to catalyse dNTP turnover under conditions of nucleotide depletion. Based on our findings we propose a model for phosphorylation-dependent regulation of SAMHD1 activity where dephosphorylation switches housekeeping SAMHD1 found in cycling cells to a high-activity stable tetrameric form that depletes and maintains low levels of dNTPs in differentiated cells.
HIV-1 replicates poorly in cells of the myeloid lineage and resting T cells through blocks that occur early in infection [1–3]. However, in HIV-2 and related simian viruses (SIVs) the lentiviral accessory protein Vpx overcomes this restriction by targeting the cellular protein SAMHD1 for proteasomal degradation [4–6].