Tomoyuki Numata

Snapshots of tRNA sulphuration via an adenylated intermediate

Uridine at the first anticodon position (U34) of glutamate, lysine and glutamine transfer RNAs is universally modified by thiouridylase into 2-thiouridine (s2U34), which is crucial for precise translation by restricting codon–anticodon wobble during protein synthesis on the ribosome. However, it remains unclear how the enzyme incorporates reactive sulphur into the correct position of the uridine base. Here we present the crystal structures of the MnmA thiouridylase–tRNA complex in three discrete forms, which provide snapshots of the sequential chemical reactions during RNA sulphuration. On enzyme activation, an α-helix overhanging the active site is restructured into an idiosyncratic β-hairpin-containing loop, which packs the flipped-out U34 deeply into the catalytic pocket and triggers the activation of the catalytic cysteine residues. The adenylated RNA intermediate is trapped. Thus, the active closed-conformation of the complex ensures accurate sulphur incorporation into the activated uridine carbon by forming a catalytic chamber to prevent solvent from accessing the catalytic site. The structures of the complex with glutamate tRNA further reveal how MnmA specifically recognizes its three different tRNA substrates. These findings provide the structural basis for a general mechanism whereby an enzyme incorporates a reactive atom at a precise position in a biological molecule.

Structural Basis of RNA-Dependent Recruitment of Glutamine to the Genetic Code

Glutaminyl–transfer RNA (Gln-tRNAGln) in archaea is synthesized in a pretranslational amidation of misacylated Glu-tRNAGln by the heterodimeric Glu-tRNAGln amidotransferase GatDE. Here we report the crystal structure of the Methanothermobacter thermautotrophicus GatDE complexed to tRNAGln at 3.15 angstroms resolution. Biochemical analysis of GatDE and of tRNAGln mutants characterized the catalytic centers for the enzymes three reactions (glutaminase, kinase, and amidotransferase activity). A 40 angstrom–long channel for ammonia transport connects the active sites in GatD and GatE. tRNAGln recognition by indirect readout based on shape complementarity of the D loop suggests an early anticodon-independent RNA-based mechanism for adding glutamine to the genetic code.

Purification, crystallization and preliminary X-ray diffraction of SecDF, a translocon-associated membrane protein, from Thermus thermophilus

Thermus thermophilus has a multi-path membrane protein, TSecDF, as a single-chain homologue of Escherichia coli SecD and SecF, which form a translocon-associated complex required for efficient preprotein translocation and membrane-protein integration. Here, the cloning, expression in E. coli, purification and crystallization of TSecDF are reported. Overproduced TSecDF was solubilized with dodecylmaltoside, chromatographically purified and crystallized by vapour diffusion in the presence of polyethylene glycol. The crystals yielded a maximum resolution of 4.2 angstroms upon X-ray irradiation, revealing that they belonged to space group P4(3)2(1)2. Attempts were made to improve the diffraction quality of the crystals by combinations of micro-stirring, laser-light irradiation and dehydration, which led to the eventual collection of complete data sets at 3.74 angstroms resolution and preliminary success in the single-wavelength anomalous dispersion analysis. These results provide information that is essential for the determination of the three-dimensional structure of this important membrane component of the protein-translocation machinery.

Amino Acids Conserved at the C-Terminal Half of the Ribonuclease T2 Family Contribute to Protein Stability of the Enzymes

The ribonuclease MC1 (RNase MC1) from the seeds of the bitter gourd belongs to the RNase T2 family. We evaluated the contribution of 11 amino acids conserved in the RNase T2 family to protein folding of RNase MC1. Thermal unfolding experiments showed that substitution of Tyr101, Phe102, Ala105, and Phe190 resulted in a significant decrease in themostability; the Tm values were 47–58 °C compared to that for the wild type (64 °C). Mutations of Pro125, Gly127, Gly144, and Val165 caused a moderate decrease in thermostability (Tm: 60–62 °C). In contrast, mutations of Asp107 and Gly173 did little effect on thermostability. The contribution of Tyr101, Phe102, Pro125, and Gly127 to protein stability was further corroborated by means of Gdn–HCl unfolding and protease digestions. Taken together, it appeared that Tyr101, Phe102, Ala105, Pro125, Gly127, Gly144, Leu162, Val165, and Phe190 conserved in the RNase T2 family play an important role in the stability of the proteins.