5ao4
Crystal structure of in vitro phosphorylated human SAMHD1 (amino acid residues 115-626) bound to GTP
Structural highlights
DiseaseSAMH1_HUMAN Defects in SAMHD1 are the cause of Aicardi-Goutieres syndrome type 5 (AGS5) [MIM:612952. A form of Aicardi-Goutieres syndrome, a genetically heterogeneous disease characterized by cerebral atrophy, leukoencephalopathy, intracranial calcifications, chronic cerebrospinal fluid (CSF) lymphocytosis, increased CSF alpha-interferon, and negative serologic investigations for common prenatal infection. Clinical features as thrombocytopenia, hepatosplenomegaly and elevated hepatic transaminases along with intermittent fever may erroneously suggest an infective process. Severe neurological dysfunctions manifest in infancy as progressive microcephaly, spasticity, dystonic posturing and profound psychomotor retardation. Death often occurs in early childhood.[1] [2] Defects in SAMHD1 are the cause of chilblain lupus type 2 (CHBL2) [MIM:614415. A rare cutaneous form of lupus erythematosus. Affected individuals present with painful bluish-red papular or nodular lesions of the skin in acral locations precipitated by cold and wet exposure at temperatures less than 10 degrees centigrade.[3] FunctionSAMH1_HUMAN Putative nuclease involved in innate immune response by acting as a negative regulator of the cell-intrinsic antiviral response. May play a role in mediating proinflammatory responses to TNF-alpha signaling.[4] [5] Publication Abstract from PubMedSAMHD1 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. Phospho-dependent Regulation of SAMHD1 Oligomerisation Couples Catalysis and Restriction.,Arnold LH, Groom HC, Kunzelmann S, Schwefel D, Caswell SJ, Ordonez P, Mann MC, Rueschenbaum S, Goldstone DC, Pennell S, Howell SA, Stoye JP, Webb M, Taylor IA, Bishop KN PLoS Pathog. 2015 Oct 2;11(10):e1005194. doi: 10.1371/journal.ppat.1005194., eCollection 2015 Oct. PMID:26431200[6] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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