engleski

Contrasting effect of evolutionary conserved cysteines on the stability of plant seryl-tRNA synthetase

In general, cytosolic proteins do not contain disulfide bonds due to high reducing power of the cytosol. Previously, we have identified a unique disulfide bond between Cys213 and Cys244 in the crystal structure of cytosolic seryl-tRNA synthetase (SerRS) from plant Arabidopsis thaliana. SerRS belongs to family of aminoacyl-tRNA synthetases, essential houskeeping enzymes that covalently link amino acid to cognate tRNA, preparing aminoacylated tRNA substrates for translation on the ribosome. Cysteines involved in disulfide link are evolutionary conserved in all green plants, including unicellular green algae. In order to decipher the role of disulfide link in plant SerRS we have prepared mutant variants comprising substitution of cysteines with serine residues. Aminoacylation activity of all variants were comparable to the activity of the wild type (wt) protein. C213S mutant showed lower thermal stability compared to the wt which can be atributed to the fact that it can not form disulfide link. Unexpectedly, C244S mutant showed higher stability than wt, although it can not form disulfide link either. Interestingly, double mutant had the same stability as the wt, indicating that mutation C213S conferring lower stability and mutation C244S conferring higher stability compensate each other in the double mutant. We have solved the crystal structure of C244S variant and modeled 3D- structures of the C213S and C213S/C244S variants. The major differences in crystal structure of C244S variant with respect to the wt SerRS is the orientation of the region Phe136-Leu143 in the catalytic core and a slightly shifted coiled-coil region in the tRNA binding domain. Ser244 forms a hydrogen bond network with the surrounding residues Cys213 and Ala214 of the loop region. Consequently, loop Ala214-Asp220 that is flexible in wt SerRS is now structured in C244S variant which is in accordance with observed higher stability of the variant. We conclude that conserved cysteines involved in the disulfide bond have contrasting effect on the stability of plant SerRS. Considering that disulfide bonds in cytosolic proteins are usually linked to cellular response mechanisms to oxidative stress, disulfide link in plant SerRS may be involved in regulation of translation during oxidative stress.

aminoacyl-tRNA synthetase ; seryl-tRNA synthetase ; plant ; Arabidopsis ; disulfide bond ; evolutionary conserved cysteines ; thermal stability

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