Investigation into the biological function of the highly conserved GTPase LepA

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Copyright: Sinan, Canan P.
LepA is a highly conserved GTP-binding protein of unknown function. Its amino acid sequence reveals that it is a GTPase with homology to elongation factor G (EF-G). Previous data led to the hypothesis that LepA negatively regulates a posttranslational process such as protein folding. To examine this possibility, two sets of strains carrying mutated alleles encoding molecular chaperones in E. coli were transformed with a lepA expression vector. LepA had a dominant negative effect specifically in a dnaK25 strain whose product exhibits a 20-fold lower ATPase activity compared to wild-type DnaK. The expression of DnaK and other heat-shock proteins is repressed following temperature downshift. Aptly, it was found that temperature shift from 37 degrees Celcius to 15 degrees Celcius in cells harboring a lepA expression vector led to the induction of lepA and downstream lepB. Furthermore, like cold-shock genes, lepA and lepB are induced by sublethal doses of chloramphenicol, although it appears that lep operon induction is related to the antibiotic's action on the 50S ribosome. Due to LepA's insolubility, it could not be confirmed whether it interacts with DnaK, DnaJ or which other proteins it interacts with. Two-dimensional gel electrophoretic analysis revealed the absence of an isoform of OmpA in two lepA deletion strains. It is possible that LepA is involved in a folding pathway that is responsible for the conformation of this isoform. Phylogenetic analysis showed that while LepA is extremely well conserved and has been identified in all completed Bacterial and Eukaryal genomes, it is not present in the completed genomes of any Archaea. Sequence analysis revealed the existence of N-terminus mitochondrial import sequences in Eukaryal LepA orthologues. Additionally, A. thaliana contains a second LepA orthologue that clusters phylogenetically with Synechocystis LepA and has a chloroplastic import sequence. This indicates that plastidal LepA was acquired in A. thaliana (and probably in all plants) through endosymbiosis of an ancestral cyanobacterium. In constrast, mitochondrial LepA are not closely related to those of a- proteobacteria, believed to be the precursors of mitochondria. These findings imply that in sharp contrast to mitochondrial LepA, chloroplastic LepA is under strong evolutionary pressure to remain conserved.
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Sinan, Canan P.
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PhD Doctorate
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