Macrophages and dendritic cells play key roles in viral infections, providing virus reservoirs that frequently resist anti-viral therapies and linking innate virus detection to anti-viral adaptive immune responses1,2. HIV-1 fails to transduce dendritic cells and has a reduced ability to transduce macrophages, due to an as yet uncharacterized mechanism that inhibits infection by interfering with efficient synthesis of viral cDNA3,4. In contrast, HIV-2 and related simian immunodeficiency viruses (SIVsm/mac) transduce myeloid cells efficiently owing to their virion-associated Vpx accessory proteins, which counteract the restrictive mechanism5,6. Here we show that the inhibition of HIV-1 infection in macrophages involves the cellular SAM domain HD domain-containing protein 1 (SAMHD1). Vpx relieves the inhibition of lentivirus infection in macrophages by loading SAMHD1 onto the CRL4DCAF1 E3 ubiquitin ligase, leading to highly efficient proteasome-dependent degradation of the protein. Mutations in SAMHD1 cause Aicardi-Goutieres syndrome (AGS), a disease that produces a phenotype that mimics the effects of a congenital viral infection7,8. Failure to dispose of endogenous nucleic acid debris in AGS results in inappropriate triggering of innate immune responses via cytosolic nucleic acids sensors9,10. Thus, our findings reveal that macrophages are defended from HIV-1 infection by a mechanism that prevents an unwanted interferon response triggered by self nucleic acids, and uncover an intricate relationship between innate immune mechanisms that control response to self and to retroviral pathogens.

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SAMHD1, a deoxyribonucleoside triphosphate triphosphohydrolase (dNTPase), plays a key role in human innate immunity. It inhibits infection of blood cells by retroviruses, including HIV, and prevents the development of the autoinflammatory Aicardi-Goutières syndrome (AGS). The inactive apo-SAMHD1 interconverts between monomers and dimers, and in the presence of dGTP the protein assembles into catalytically active tetramers. Here, we present the crystal structure of the human tetrameric SAMHD1-dGTP complex. The structure reveals an elegant allosteric mechanism of activation via dGTP-induced tetramerization of two inactive dimers. Binding of dGTP to four allosteric sites promotes tetramerization and induces a conformational change in the substrate-binding pocket to yield the catalytically active enzyme. Structure-based biochemical and cell-based biological assays confirmed the proposed mechanism. The SAMHD1 tetramer structure provides the basis for a mechanistic understanding of its function in HIV restriction and the pathogenesis of AGS.The sterile alpha motif and HD-domain containing protein 1 (SAMHD1) dNTPase plays dual roles in human innate immunity. It restricts HIV-1 infection in immune cells of myeloid Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms Accession codesThe coordinates and structure factors have been deposited in PDB, with accession code 4BZC for the wild type and 4BZB for the RN mutant. HHS Public Access Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript lineage and in quiescent CD4-positive T lymphocytes [1][2][3][4][5] . In these non-dividing cells, SAMHD1 reduces cellular dNTP levels to concentrations below the threshold required for reverse transcription of the viral RNA genome into DNA 6-8 . Furthermore, mutations in SAMHD1 are associated with an autoimmune condition, termed Aicardi Goutières Syndrome (AGS) 9,10 , whose clinical manifestations resemble congenital viral infection 11,12 . AGS-associated SAMHD1 mutations appear to disrupt the dNTPase activity of SAMHD1. Thus, SAMHD1's ability to negatively regulate cellular dNTP levels is essential for its roles in innate immunity 13,14 .The dNTPase activity of SAMHD1 resides in its histidine-aspartate (HD) domain, with the N-terminal sterile alpha motif (SAM) domain involved in other activities [13][14][15][16][17] . A recent crystal structure of a dimeric SAMHD1 catalytic core fragment (SAMHD1c1, residues 120-626) suggested an allosteric, dGTP-stimulated mechanism for the promotion of the dNTPase activity of dimeric SAMHD1 14 . However, the SAMHD1c1 structure did not contain substrate or the dGTP cofactor, thus providing limited insight into the mechanism of SAMHD1 activation. Recent biochemical and functional studies revealed that SAMHD1 interconverts between an inactive monomeric or dimeric form and a dGTP-induced tetr...