Structural analysis of a set of proteins resulting from a bacterial genomics project.

The targets of the Structural GenomiX (SGX) bacterial genomics project were proteins conserved in multiple prokaryotic organisms with no obvious sequence homolog in the Protein Data Bank of known structures. The outcome of this work was 80 structures, covering 60 unique sequences and 49 different genes. Experimental phase determination from proteins incorporating Se-Met was carried out for 45 structures with most of the remainder solved by molecular replacement using members of the experimentally phased set as search models. An automated tool was developed to deposit these structures in the Protein Data Bank, along with the associated X-ray diffraction data (including refined experimental phases) and experimentally confirmed sequences. BLAST comparisons of the SGX structures with structures that had appeared in the Protein Data Bank over the intervening 3.5 years since the SGX target list had been compiled identified homologs for 49 of the 60 unique sequences represented by the SGX structures. This result indicates that, for bacterial structures that are relatively easy to express, purify, and crystallize, the structural coverage of gene space is proceeding rapidly. More distant sequence-structure relationships between the SGX and PDB structures were investigated using PDB-BLAST and Combinatorial Extension (CE). Only one structure, SufD, has a truly unique topology compared to all folds in the PDB.

The structure of the signal receiver domain of the Arabidopsis thaliana ethylene receptor ETR1.

BACKGROUND: In Arabidopsis thaliana, ethylene perception and signal transduction into the cell are carried out by a family of membrane-bound receptors, one of which is ethylene resistant 1 (ETR1). The large cytoplasmic domain of the receptor showed significant sequence homology to the proteins of a common bacterial regulatory pathway, the two-component system. This system consists of a transmitter histidine kinase and a response regulator (or signal receiver). We present the crystal structures of the first plant receiver domain ETRRD (residues 604-738) of ETR1 in two conformations. RESULTS: The monomeric form of ETRRD resembles the known structure of the bacterial receiver domain. ETRRD forms a homodimer in solution and in the crystal, an interaction that has not been described previously. Dimerization is mediated by the C terminus, which forms an extended beta sheet with the dimer-related beta-strand core. Furthermore, the loop immediately following the active site adopts an exceptional conformation. CONCLUSIONS: The three-dimensional structure of ETRRD shows the expected conformational conservation to prokaryotic receiver proteins, such as CheY and CheB, both of which are part of the chemotaxis signaling pathway. ETRRD provides the first detailed example of a dimerized receiver domain. Given that the dimer interface of ETRRD coincides with the phosphorylation-dependent interfaces of CheY and CheB, we suggest that the monomerization of ETRRD is phosphorylation-dependent too. In the Mg(2+)-free form of ETRRD, the gamma-loop conformation does not allow a comparable interaction as observed in the active-site architectures of Mg(2+)-bound CheY from Escherichia coli and Salmonella typhimurium.

Subcloning, crystallization and preliminary X-ray analysis of the signal receiver domain of ETR1, an ethylene receptor from Arabidopsis thaliana.

The signal receiver domain of ETR1, an ethylene receptor from Arabidopsis thaliana, has been subcloned and expressed in E. coli and purified by affinity chromatography. Crystals of both native and a selenomethionine-substituted form of the receiver domain have been obtained. Native crystals grew in 1.6 M Li2SO4 and 0.1 M HEPES pH 7. 5 and once flash-frozen diffract to 2.1 A resolution. They belong to space group P41212 with unit-cell dimensions a = b = 48.4, c = 112.3 A.

Substrate specificity and assembly of the catalytic center derived from two structures of ligated uridylate kinase.

Two crystal structures of ligated uridylate kinase from Saccharomyces cerevisiae were determined by X-ray analyses. The ligands were ADP and AMP. Cocrystallization with ATP yielded crystals with ADP at the ATP site and a mixture of AMP and ADP at the NMP site. Cocrystallization with ADP gave rise to a distinct crystal type with ADP at the ATP site, but only AMP at the NMP site. In both cases, the substrates are kept in place by favorable crystal contacts. The structures have been refined to R-factors of 17.8% and 19.6% at resolutions of 2.1 A and 1.9 A, respectively. A comparison with the related cytosolic adenylate kinase from pig disclosed large induced-fit movements on substrate binding and the disassembly of the catalytic center in the absence of substrates. The relatively high side-activity of uridylate kinase for AMP is explained by the finding that the binding pocket is sized for an AMP, but constructed to bind UMP together with a water molecule.

The structure of uridylate kinase with its substrates, showing the transition state geometry.

Uridylate kinase from Saccharomyces cerevisiae is a member of the nucleoside monophosphate (NMP) kinase family and catalyzes the reaction ATP+NMP<==>ADP+NDP with moderate specificity for UMP. The recombinant enzyme crystallized together with two substrate molecules. The structure was solved, by multiple isomorphous replacement and solvent flattening, at 3.0 A and then refined at 2.13 A resolution. The present R-factor is 19%. Superposition onto the structure of a substrate-free adenylate kinase revealed the motions induced by substrate binding. A further superposition onto an adenylate kinase with bound P1,P5-bis(5'-adenosyl)pentaphosphate (Ap5A), a two-substrate-mimicking inhibitor, failed to explain the UMP preference of the uridylate kinase, but superimposed the nucleosides and in particular the non-transferred phosphates at the ATP- and NMP-site rather well. The coincidence of the phosphates indicate strongly that these groups assume their final positions during catalysis. This locates the transition state, which can be modeled with reasonable geometry in agreement with an in-line associative SN2 mechanism.