title: A Highly Active Isoform of Lentivirus Restriction Factor SAMHD1 in Mouse.
abstract: The triphosphohydrolase SAMHD1 (sterile α motif and histidine-aspartate domain-containing protein 1) restricts HIV-1 replication in nondividing myeloid cells by depleting the dNTP pool, preventing reverse transcription. SAMHD1 is also reported to have ribonuclease activity that degrades the virus genomic RNA. Human SAMHD1 is regulated by phosphorylation of its carboxyl terminus at Thr-592, which abrogates its antiviral function yet has only a small effect on its phosphohydrolase activity. In the mouse, SAMHD1 is expressed as two isoforms (ISF1 and ISF2) that differ at the carboxyl terminus due to alternative splicing of the last coding exon. In this study we characterized the biochemical and antiviral properties of the two mouse isoforms of SAMHD1. Both are antiviral in nondividing cells. Mass spectrometry analysis showed that SAMHD1 is phosphorylated at several amino acid residues, one of which (Thr-634) is homologous to Thr-592. Phosphomimetic mutation at Thr-634 of ISF1 ablates its antiviral activity yet has little effect on phosphohydrolase activity in vitro dGTP caused ISF1 to tetramerize, activating its catalytic activity. In contrast, ISF2, which lacks the phosphorylation site, was significantly more active, tetramerized, and was active without added dGTP. Neither isoform nor human SAMHD1 had detectable RNase activity in vitro or affected HIV-1 genomic RNA stability in newly infected cells. These data support a model in which SAMHD1 catalytic activity is regulated through tetramer stabilization by the carboxyl-terminal tail, phosphorylation destabilizing the complexes and inactivating the enzyme. ISF2 may serve to reduce the dNTP pool to very low levels as a means of restricting virus replication.
authors.affiliation: From the Department of Microbiology, New York School of Medicine, New York, New York 10016.
authors.forename: Nicolin
authors.initials: N
authors.lastname: Bloch
authors.affiliation: From the Department of Microbiology, New York School of Medicine, New York, New York 10016.
authors.forename: Sabine
authors.initials: S
authors.lastname: Gläsker
authors.affiliation: From the Department of Microbiology, New York School of Medicine, New York, New York 10016.
authors.forename: Poojitha
authors.initials: P
authors.lastname: Sitaram
authors.affiliation: From the Department of Microbiology, New York School of Medicine, New York, New York 10016.
authors.forename: Henning
authors.initials: H
authors.lastname: Hofmann
authors.affiliation: the Center for Drug Discovery, Department of Pediatrics, Emory School of Medicine, Atlanta, Georgia 30322, and.
authors.forename: Caitlin N
authors.initials: CN
authors.lastname: Shepard
authors.affiliation: From the Department of Microbiology, New York School of Medicine, New York, New York 10016.
authors.forename: Megan L
authors.initials: ML
authors.lastname: Schultz
authors.affiliation: the Center for Drug Discovery, Department of Pediatrics, Emory School of Medicine, Atlanta, Georgia 30322, and.|the School of Pharmacy, Kyunghee University, Seoul 02447, South Korea.
authors.forename: Baek
authors.initials: B
authors.lastname: Kim
authors.affiliation: From the Department of Microbiology, New York School of Medicine, New York, New York 10016, nathaniel.landau@med.nyu.edu.
authors.forename: Nathaniel R
authors.initials: NR
authors.lastname: Landau
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D015658:HIV Infections / Q000201:enzymology* / Q000235:genetics; D015497:HIV-1 / Q000502:physiology*; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D020559:Monomeric GTP-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020125:Mutation, Missense; D010766:Phosphorylation; D055503:Protein Multimerization; D012367:RNA, Viral / Q000235:genetics / Q000378:metabolism*; D000076106:SAM Domain and HD Domain-Containing Protein 1; D020298:U937 Cells; D014779:Virus Replication / Q000502:physiology*
chemical_list: D007527:Isoenzymes; D012367:RNA, Viral; D000076106:SAM Domain and HD Domain-Containing Protein 1; C484281:Samhd1 protein, mouse; D020559:Monomeric GTP-Binding Proteins
keywords: RNA degradation; SAM domain and HD domain-containing protein 1 (SAMHD1); dGTP; dNTP; human immunodeficiency virus (HIV); hydrolase; phosphorylation; restriction factor; reverse transcription