Fig. 2: Crystal structures of SAMHD1:oligonucleotide complexes reveal that oligonucleotides compete with nucleotide triphosphates for binding to the allosteric sites.

a Without allosteric ligands, the HD-domain construct of SAMHD1 is monomeric in solution. b In the presence of nucleotide triphosphates, it assembles into a tetramer with almost perfect 222-point symmetry. c Backbone cartoon of the GTP/dCTP-bound SAMHD1 structure¹⁸ (PDB: 4TNP). d In the presence of oligonucleotides SAMHD1 dimerizes. e Packing of two SAMHD1 dimers in the asymmetric unit of the oligonucleotide-bound crystals lacks 222-point symmetry. f Backbone cartoon of the d(C*G*C*C*T)-bound SAMHD1 structure (PDB: 6U6X). Electron-density maps of the d(C*G*C*C*T) (g) and r(CGCCU) (h) oligonucleotides bound in the allosteric sites of SAMHD1 (PDB: 6U6X and PDB: 6U6X, respectively). The mesh shows the composite 2mFo-DFc omit map contoured at 1.5 σ. i Binding of the G nucleobase in the guanine-recognition pocket of A1 is a determinant of the oligonucleotide- binding affinity (n = 2 independent experiments). j Phosphorothioation enhances dNTPase inhibition by oligonucleotide binding. dTTP hydrolysis rates were measured at [GTP] = 50 μM and [dTTP]=1 mM (n = 2 independent experiments). k EC₅₀^GTP but not k_cat of the dNTPase activity is affected by d(C*G*C*C*T) binding, which is consistent with competition between d(C*G*C*C*T) and GTP for the allosteric sites (n = 2 independent experiments). l Increasing GTP concentrations attenuate binding of 6FAM-d(C*G*C*C*T) to SAMHD1 as monitored by fluorescence anisotropy (n = 2 independent experiments). Error bars represent s.d. of two replicate measurements performed on distinct samples. Source data are provided as a Source Data File.