In many organisms, 3'-phosphoadenosine 5'-phosphate (PAP) is a product of two reactions in the sulfur activation pathway. The sulfurylation of biomolecules, catalyzed by sulfotransferases, uses 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as a sulfate donor, producing the sulfated biomolecule and PAP product. Additionally, the first step in sulfate reduction for many bacteria and fungi reduces the sulfate moiety of PAPS, producing PAP and sulfite, which is subsequently reduced to sulfide. PAP is removed by the phosphatase activity of CysQ, a 3',5'-bisphosphate nucleotidase, yielding AMP and phosphate. Because excess PAP alters the equilibrium of the sulfur pathway and inhibits sulfotransferases, PAP concentrations can affect the levels of sulfur-containing metabolites. Therefore, CysQ, a divalent cation metal-dependent phosphatase, is a major regulator of this pathway. CysQ (Rv2131c) from Mycobacterium tuberculosis (Mtb) was successfully expressed, purified, and crystallized in a variety of ligand-bound states. Here we report six crystal structures of Mtb CysQ, including a ligand-free structure, a lithium-inhibited state with substrate PAP bound, and a product-bound complex with AMP, phosphate, and three Mg(2+) ions bound. Comparison of these structures together with homologues of the superfamily has provided insight into substrate specificity, metal coordination, and catalytic mechanism.

CysQ is part of the sulfur-activation pathway that dephosphorylates 3'-phosphoadenosine 5'-monophosphate (PAP) to regenerate adenosine 5'-monophosphate (AMP) and free phosphate. PAP is the product of sulfate-transfer reactions from sulfotransferases that use the universal sulfate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS). In some organisms PAP is also the product of PAPS reductases that reduce sulfate from PAPS to sulfite. CysQ from Mycobacterium tuberculosis, which plays an important role in the biosynthesis of sulfated glycoconjugates, was successfully purified and crystallized in 24% PEG 1500, 20% glycerol. X-ray diffraction data were collected to 1.7 Å resolution using a synchrotron-radiation source. Crystals grew in the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a=40.3, b=57.9, c=101.7 Å and with one monomer per asymmetric unit.

Adenosine deaminases acting on RNA (ADARs) hydrolytically deaminate adenosines (A) in a wide variety of duplex RNAs and misregulation of editing is correlated with human disease. However, our understanding of reaction selectivity is limited. ADARs are modular enzymes with multiple double-stranded RNA binding domains (dsRBDs) and a catalytic domain. While dsRBD binding is understood, little is known about ADAR catalytic domain/RNA interactions. Here we use a recently discovered RNA substrate that is rapidly deaminated by the isolated human ADAR2 deaminase domain (hADAR2-D) to probe these interactions. We introduced the nucleoside analog 8-azanebularine (8-azaN) into this RNA (and derived constructs) to mechanistically trap the protein-RNA complex without catalytic turnover for EMSA and ribonuclease footprinting analyses. EMSA showed that hADAR2-D requires duplex RNA and is sensitive to 2'-deoxy substitution at nucleotides opposite the editing site, the local sequence and 8-azaN nucleotide positioning on the duplex. Ribonuclease V1 footprinting shows that hADAR2-D protects ∼ 23 nt on the edited strand around the editing site in an asymmetric fashion (∼ 18 nt on the 5' side and ∼ 5 nt on the 3' side). These studies provide a deeper understanding of the ADAR catalytic domain-RNA interaction and new tools for biophysical analysis of ADAR-RNA complexes.