tRNA aminoacylation by arginyl-tRNA synthetase: induced conformations during substrates binding.

The 2.2 A crystal structure of a ternary complex formed by yeast arginyl-tRNA synthetase and its cognate tRNA(Arg) in the presence of the L-arginine substrate highlights new atomic features used for specific substrate recognition. This first example of an active complex formed by a class Ia aminoacyl-tRNA synthetase and its natural cognate tRNA illustrates additional strategies used for specific tRNA selection. The enzyme specifically recognizes the D-loop and the anticodon of the tRNA, and the mutually induced fit produces a conformation of the anticodon loop never seen before. Moreover, the anticodon binding triggers conformational changes in the catalytic center of the protein. The comparison with the 2.9 A structure of a binary complex formed by yeast arginyl-tRNA synthetase and tRNA(Arg) reveals that L-arginine binding controls the correct positioning of the CCA end of the tRNA(Arg). Important structural changes induced by substrate binding are observed in the enzyme. Several key residues of the active site play multiple roles in the catalytic pathway and thus highlight the structural dynamics of the aminoacylation reaction.

In vivo selection of lethal mutations reveals two functional domains in arginyl-tRNA synthetase.

Using random mutagenesis and a genetic screening in yeast, we isolated 26 mutations that inactivate Saccharomyces cerevisiae arginyl-tRNA synthetase (ArgRS). The mutations were identified and the kinetic parameters of the corresponding proteins were tested after purification of the expression products in Escherichia coli. The effects were interpreted in the light of the crystal structure of ArgRS. Eighteen functional residues were found around the arginine-binding pocket and eight others in the carboxy-terminal domain of the enzyme. Mutations of these residues all act by strongly impairing the rates of tRNA charging and arginine activation. Thus, ArgRS and tRNA(Arg) can be considered as a kind of ribonucleoprotein, where the tRNA, before being charged, is acting as a cofactor that activates the enzyme. Furthermore, by using different tRNA(Arg) isoacceptors and heterologous tRNA(Asp), we highlighted the crucial role of several residues of the carboxy-terminal domain in tRNA recognition and discrimination.

Crystallization and preliminary X-ray crystallographic analysis of yeast arginyl-tRNA synthetase-yeast tRNAArg complexes.

Three different crystal forms of complexes between arginyl-tRNA synthetase from the yeast Saccharomyces cerevisae (yArgRS) and the yeast second major tRNA(Arg) (tRNA(Arg)(ICG)) isoacceptor have been crystallized by the hanging-drop vapour-diffusion method in the presence of ammonium sulfate. Crystal form II, which diffracts beyond 2.2 A resolution at the European Synchrotron Radiation Facility ID14-4 beamline, belongs to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 129.64, b = 107.47, c = 71. 38 A. This crystal form presents the highest resolution obtained for an active form of an aminoacyl-tRNA synthetase-tRNA complex. The estimated V(m) of 2.6 A(3) Da(-1) indicates one molecule of complex in the asymmetric unit. The three crystal forms were solved by the molecular-replacement method using the coordinates of the free yArgRS.

L-arginine recognition by yeast arginyl-tRNA synthetase.

The crystal structure of arginyl-tRNA synthetase (ArgRS) from Saccharomyces cerevisiae, a class I aminoacyl-tRNA synthetase (aaRS), with L-arginine bound to the active site has been solved at 2.75 A resolution and refined to a crystallographic R-factor of 19.7%. ArgRS is composed predominantly of alpha-helices and can be divided into five domains, including the class I-specific active site. The N-terminal domain shows striking similarity to some completely unrelated proteins and defines a module which should participate in specific tRNA recognition. The C-terminal domain, which is the putative anticodon-binding module, displays an all-alpha-helix fold highly similar to that of Escherichia coli methionyl-tRNA synthetase. While ArgRS requires tRNAArg for the first step of the aminoacylation reaction, the results show that its presence is not a prerequisite for L-arginine binding. All H-bond-forming capability of L-arginine is used by the protein for the specific recognition. The guanidinium group forms two salt bridge interactions with two acidic residues, and one H-bond with a tyrosine residue; these three residues are strictly conserved in all ArgRS sequences. This tyrosine is also conserved in other class I aaRS active sites but plays several functional roles. The ArgRS structure allows the definition of a new framework for sequence alignments and subclass definition in class I aaRSs.

The structure of Staphylococcus aureus epidermolytic toxin A, an atypic serine protease, at 1.7 A resolution.

BACKGROUND: Staphylococcal epidermolytic toxins A and B (ETA and ETB) are responsible for the staphylococcal scalded skin syndrome of newborn and young infants; this condition can appear just a few hours after birth. These toxins cause the disorganization and disruption of the region between the stratum spinosum and the stratum granulosum--two of the three cellular layers constituting the epidermis. The physiological substrate of ETA is not known and, consequently, its mode of action in vivo remains an unanswered question. Determination of the structure of ETA and its comparison with other serine proteases may reveal insights into ETA's catalytic mechanism. RESULTS: The crystal structure of staphylococcal ETA has been determined by multiple isomorphous replacement and refined at 1.7 A resolution with a crystallographic R factor of 0.184. The structure of ETA reveals it to be a new and unique member of the trypsin-like serine protease family. In contrast to other serine protease folds, ETA can be characterized by ETA-specific surface loops, a lack of cysteine bridges, an oxyanion hole which is not preformed, an S1 specific pocket designed for a negatively charged amino acid and an ETA-specific specific N-terminal helix which is shown to be crucial for substrate hydrolysis. CONCLUSIONS: Despite very low sequence homology between ETA and other trypsin-like serine proteases, the ETA crystal structure, together with biochemical data and site-directed mutagenesis studies, strongly confirms the classification of ETA in the Glu-endopeptidase family. Direct links can be made between the protease architecture of ETA and its biological activity.