title: The deoxynucleotide triphosphohydrolase SAMHD1 is a major regulator of DNA precursor pools in mammalian cells.
abstract: Sterile alpha motif and HD-domain containing protein 1 (SAMHD1) is a triphosphohydrolase converting deoxynucleoside triphosphates (dNTPs) to deoxynucleosides. The enzyme was recently identified as a component of the human innate immune system that restricts HIV-1 infection by removing dNTPs required for viral DNA synthesis. SAMHD1 has deep evolutionary roots and is ubiquitous in human organs. Here we identify a general function of SAMHD1 in the regulation of dNTP pools in cultured human cells. The protein was nuclear and variably expressed during the cell cycle, maximally during quiescence and minimally during S-phase. Treatment of lung or skin fibroblasts with specific siRNAs resulted in the disappearence of SAMHD1 accompanied by loss of the cell-cycle regulation of dNTP pool sizes and dNTP imbalance. Cells accumulated in G1 phase with oversized pools and stopped growing. Following removal of the siRNA, the pools were normalized and cell growth restarted, but only after SAMHD1 had reappeared. In quiescent cultures SAMHD1 down-regulation leads to a marked expansion of dNTP pools. In all cases the largest effect was on dGTP, the preferred substrate of SAMHD1. Ribonucleotide reductase, responsible for the de novo synthesis of dNTPs, is a cytosolic enzyme maximally induced in S-phase cells. Thus, in mammalian cells the cell cycle regulation of the two main enzymes controlling dNTP pool sizes is adjusted to the requirements of DNA replication. Synthesis by the reductase peaks during S-phase, and catabolism by SAMHD1 is maximal during G1 phase when large dNTP pools would prevent cells from preparing for a new round of DNA replication.
authors.affiliation: Department of Biology, University of Padova, 35131 Padua, Italy.
authors.forename: Elisa
authors.initials: E
authors.lastname: Franzolin
authors.forename: Giovanna
authors.initials: G
authors.lastname: Pontarin
authors.forename: Chiara
authors.initials: C
authors.lastname: Rampazzo
authors.forename: Cristina
authors.initials: C
authors.lastname: Miazzi
authors.forename: Paola
authors.initials: P
authors.lastname: Ferraro
authors.forename: Elisa
authors.initials: E
authors.lastname: Palumbo
authors.forename: Peter
authors.initials: P
authors.lastname: Reichard
authors.forename: Vera
authors.initials: V
authors.lastname: Bianchi
mesh_terms: D002453:Cell Cycle; D002460:Cell Line; D049109:Cell Proliferation; D004261:DNA Replication; D020868:Gene Silencing; D006801:Humans; D020559:Monomeric GTP-Binding Proteins / Q000235:genetics*; D009694:Nucleic Acid Precursors / Q000235:genetics*; D034741:RNA, Small Interfering; D000076106:SAM Domain and HD Domain-Containing Protein 1
chemical_list: D009694:Nucleic Acid Precursors; D034741:RNA, Small Interfering; D000076106:SAM Domain and HD Domain-Containing Protein 1; C484280:SAMHD1 protein, human; D020559:Monomeric GTP-Binding Proteins
keywords: cell cycle arrest; dGTP pool; dNTP regulation