Mycobacterium tuberculosis caseinolytic protease ClpP1 (Mt ClpP1) is a self-compartmentalized protease consisting of two heptameric rings stacked on top of each other, thus enclosing a catalytic chamber. Within the chamber, which can be reached through two axial pores, each of the 14 identical monomers possesses a serine protease active site. The unfolding and translocation of substrates into the chamber are mediated by associated hexameric ATPases covering the axial pores. Three crystal structures of Mt ClpP1, determined by molecular replacement, are presented in this study. Two of the models were refined to a resolution of 2.6 A and the third to 3.0 A. It was found that disorder in the handle domain affects the formation and configuration of the tetradecamer and results in condensed structures with larger equatorial pores when compared with ClpPs from other species. Additionally, this disorder accompanies conformational changes of the residues in the catalytic triad. The models also reveal structural differences within the N-terminal hairpin-loop domain, which possibly reflect the significant differences in amino-acid sequence between Mt ClpP1 and other ClpP homologues in this region.

Two structures of monomeric methionyl‐tRNA synthetase, from Mycobacterium smegmatis, in complex with the ligands methionine/adenosine and methionine, were analyzed by X‐ray crystallography at 2.3 Å and at 2.8 Å, respectively. The structures demonstrated the flexibility of the multidomain enzyme. A new conformation of the structure was identified in which the connective peptide domain bound more closely to the catalytic domain than described previously. The KMSKS(301‐305) loop in our structures was in an open and inactive conformation that differed from previous structures by a rotation of the loop of about 90° around hinges located at Asn297 and Val310. The binding of adenosine to the methionyl‐tRNA synthetase methionine complex caused a shift in the KMSKS domain that brought it closer to the catalytic domain. The potential use of the adenosine‐binding site for inhibitor binding was evaluated and a potential binding site for a specific allosteric inhibitor was identified.

Methionyl-tRNA synthetase (MetRS) from Mycobacterium smegmatis was recombinantly expressed in Escherichia coli and purified using Ni(2+)-affinity and size-exclusion chromatography. Crystals formed readily in the presence of the ligands methionine and adenosine. These two ligands are components of an intermediate in the two-step catalytic mechanism of MetRS. The crystals were produced using the vapour-diffusion method and a full data set to 2.1 A resolution was collected from a single crystal. The crystal belonged to the monoclinic space group C2, with unit-cell parameters a = 155.9, b = 138.9, c = 123.3 A, beta = 124.8 degrees . The presence of three molecules in the asymmetric unit corresponded to a solvent content of 60% and a Matthews coefficient of 3.1 A(3) Da(-1). Structure determination is in progress.

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