The primate lentivirus auxiliary protein Vpx counteracts an unknown restriction factor that renders human dendritic and myeloid cells largely refractory to HIV-1 infection. Here we identify SAMHD1 as this restriction factor. SAMHD1 is a protein involved in Aicardi-Goutières syndrome, a genetic encephalopathy with symptoms mimicking congenital viral infection, that has been proposed to act as a negative regulator of the interferon response. We show that Vpx induces proteasomal degradation of SAMHD1. Silencing of SAMHD1 in non-permissive cell lines alleviates HIV-1 restriction and is associated with a significant accumulation of viral DNA in infected cells. Concurrently, overexpression of SAMHD1 in sensitive cells inhibits HIV-1 infection. The putative phosphohydrolase activity of SAMHD1 is probably required for HIV-1 restriction. Vpx-mediated relief of restriction is abolished in SAMHD1-negative cells. Finally, silencing of SAMHD1 markedly increases the susceptibility of monocytic-derived dendritic cells to infection. Our results demonstrate that SAMHD1 is an antiretroviral protein expressed in cells of the myeloid lineage that inhibits an early step of the viral life cycle.

The ubiquitin-proteasome pathway regulates gene expression through protein degradation. Here we show that the F-box protein ␤TrCP, the receptor component of the SCF E3 ubiquitin ligase responsible for IB␣ and ␤-catenin degradation, is colocalized in the nucleus with ATF4, a member of the ATF-CREB bZIP family of transcription factors, and controls its stability. Association between the two proteins depends on ATF4 phosphorylation and on ATF4 serine residue 219 present in the context of DSGXXXS, which is similar but not identical to the motif found in other substrates of ␤TrCP. ATF4 ubiquitination in HeLa cells is enhanced in the presence of ␤TrCP. The F-box-deleted ␤TrCP protein behaves as a negative transdominant mutant that inhibits ATF4 ubiquitination and degradation and, subsequently, enhances its activity in cyclic AMP-mediated transcription. ATF4 represents a novel substrate for the SCF ␤TrCP complex, which is the first mammalian E3 ubiquitin ligase identified so far for the control of the degradation of a bZIP transcription factor.Proteasome-mediated protein degradation requires the covalent attachment of polyubiquitin to the substrate proteins (11,25,38). The cascade of ubiquitin transfer reactions involves the ubiquitin-activating enzyme E1, an E2 ubiquitinconjugating enzyme that operates with specificity factor E3. The selectivity of the reaction is due to the E3 ubiquitin ligase, which interacts with both E2 and the substrate.SCF (Skp1/Cullin/F-box protein) complexes were initially shown to function as E3 ubiquitin ligases for a variety of phosphorylated proteins involved in the yeast cell cycle (1,15,35,52,60). The core components of these complexes include Skp1, Cul-1 (Cdc53), and the newly identified protein Rbx1 (Roc1 or Hrt1), which is thought to stabilize the interaction between Cul-1 and the E2 enzyme Cdc34 (12,14,30,61). The SCF complexes also contain a variable receptor subunit, an F-boxcontaining protein, that provides substrate specificity. The Fbox motif serves to anchor the receptor subunit to the SCF complex by its interaction with Skp1 (12,14,52,60). For ubiquitination, substrate proteins are recruited to the SCF E3 ubiquitin ligase complexes through interaction with the substrate binding domain (WD-40 or leucine-rich repeats) of the F-box receptor subunits. SCF complexes are the largest and most versatile class of E3 ubiquitin ligases. To date, only two human SCF complexes, SCF Skp2 and SCF ␤TrCP , have been analyzed in detail and have had some of their substrates characterized (6, 7, 23, 36, 39, 41-44, 49, 62, 63, 67, 68). We made the first identification of human ␤TrCP (beta-transducin repeat-containing protein) as the F-box receptor component of the E3 ubiquitin ligase SCF ␤TrCP responsible for the degradation of CD4 induced by the human immunodeficiency virus type 1 (HIV-1) protein Vpu (43). Subsequently, we and others showed that SCF ␤TrCP is also responsible for phosphorylationdependent ubiquitination and then for the degradation of 36,39,42,62,67,68). Vpu, IB␣, and ␤-catenin shar...