Nicked multifunctional loop of glutathione synthetase still protects the catalytic intermediate

CtrA activates the expression of glutathione S-transferase conferring oxidative stress resistance to Ehrlichia chaffeensis

Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis (HME), is a Gram-negative obligatory intracellular bacterium, which infects and multiplies in human monocytes and macrophages. Host immune cells produce reactive oxygen species (ROS) to eliminate E. chaffeensis upon infection. E. chaffeensis global transcriptional regulator CtrA activates the expression of GshA and GshB to synthesize glutathione (GSH), the most potent natural antioxidant, upon oxidative stress to combat ROS damage. However, the mechanisms exploited by E. chaffeensis to utilize GSH are still unknown. Here, we found that in E. chaffeensis CtrA activated the expression of glutathione S-transferase (GST) upon oxidative stress, and E. chaffeensis GST utilizes GSH to eliminate ROS and confers the oxidative stress resistance to E. chaffeensis. We found that CtrA bound to the promoter regions of 211 genes, including gst, in E. chaffeensis using chromatin immunoprecipitation coupled to deep sequencing (ChIP-seq). Recombinant E. chaffeensis CtrA directly bound to the gst promoter region determined with electrophoretic mobility shift assay (EMSA), and activated the gst expression determined with reporter assay. Recombinant GST showed GSH conjugation activity towards its typical substrate 2,4-dinitrochlorobenzene (CDNB) in vitro and peptide nucleic acid (PNA) transfection of E. chaffeensis, which can knock down the gst transcription level, reduced bacterial survival upon oxidative stress. Our results demonstrate that E. chaffeensis CtrA regulates GSH utilization, which plays a critical role in resistance to oxidative stress, and aid in the development of new therapeutics for HME.

Molecular mechanism underlying substrate recognition of the peptide macrocyclase PsnB

Graspetides, also known as ω-ester-containing peptides (OEPs), are a family of ribosomally synthesized and post-translationally modified peptides (RiPPs) bearing side chain-to-side chain macrolactone or macrolactam linkages. Here, we present the molecular details of precursor peptide recognition by the macrocyclase enzyme PsnB in the biosynthesis of plesiocin, a group 2 graspetide. Biochemical analysis revealed that, in contrast to other RiPPs, the core region of the plesiocin precursor peptide noticeably enhanced the enzyme-precursor interaction via the conserved glutamate residues. We obtained four crystal structures of symmetric or asymmetric PsnB dimers, including those with a bound core peptide and a nucleotide, and suggest that the highly conserved Arg213 at the enzyme active site specifically recognizes a ring-forming acidic residue before phosphorylation. Collectively, this study provides insights into the mechanism underlying substrate recognition in graspetide biosynthesis and lays a foundation for engineering new variants.

Characterization of a cold-adapted glutathione synthetase from the psychrophile Pseudoalteromonas haloplanktis

Glutathione (GSH) biosynthesis occurs through two ATP-dependent reactions, usually involving distinct enzymes; in the second step of this process, catalysed by glutathione synthetase (GshB), GSH is formed from γ-glutamylcysteine and glycine. A recombinant form of GshB from the cold-adapted source Pseudoalteromonas haloplanktis (rPhGshB) was purified and characterised. The enzyme formed a disulfide adduct with β-mercaptoethanol, when purified in the presence of this reducing agent. The homotetrameric form of rPhGshB observed at high protein concentration disassembled into two homodimers at low concentration. A new method for directly determining the rPhGshB activity was developed, based on [γ-(32)P]ATP hydrolysis coupled to the GSH synthesis. The ATPase activity required the presence of both γ-glutamylcysteine and glycine and its optimum was reached in the 7.4-8.6 pH range; a divalent cation was absolutely required for the activity, whereas monovalent cations were dispensable. rPhGshB was active at low temperatures and had a similar affinity for ATP (K(m) 0.26 mM) and γ-glutamylcysteine (K(m) 0.25 mM); a lower affinity was measured for glycine (K(m) 0.75 mM). The oxidised form of glutathione (GSSG) acted as an irreversible inhibitor of rPhGshB (K(i) 10.7 mM) and formed disulfide adducts with the enzyme. rPhGshB displayed a great temperature-dependent increase in its activity with an unusually high value of energy of activation (75 kJ mol(-1)) for a psychrophilic enzyme. The enzyme was moderately thermostable, its half inactivation temperature being 50.5 °C after 10 min exposure. The energy of activation of the heat inactivation process was 208 kJ mol(-1). To our knowledge, this is the first contribution to the characterization of a GshB from cold-adapted sources.

Tetramerization and ATP binding by a protein comprising the A, B, and C domains of rat synapsin I

Synapsins are multidomain proteins that are critical for regulating neurotransmitter release in vertebrates. In the present study, two crystal structures of the C domain of rat synapsin I (rSynI-C) in complex with Ca(2+) and ATP reveal that this protein can form a tetramer and that a flexible loop (the "multifunctional loop") contacts bound ATP. Further experiments were carried out on a protein comprising the A, B, and C domains of rat synapsin I (rSynI-ABC). An ATP-stabilized tetramer of rSynI-ABC is observed during velocity sedimentation and size-exclusion chromatographic experiments. These hydrodynamic results also indicate that the A and B domains exist in an extended conformation. Calorimetric measurements of ATP binding to wild-type and mutant rSynI-ABC demonstrate that the multifunctional loop and a cross-tetramer contact are important for ATP binding. The evidence supports a view of synapsin I as an ATP-utilizing, tetrameric protein made up of monomers that have a flexible, extended N terminus.