Assessing Rattus norvegicus recombinant syntaxin 18 for endonucleolytic function.

Recently a native endonuclease, purified from Rattus norvegicus liver, having the ability to cleave c-myc mRNA within the coding region, was tentatively identified as syntaxin 18. The objective of this research was to generate, purify and test recombinant R. norvegicus syntaxin 18 for endonucleolytic activity. To further investigate the role of syntaxin 18, homologs to R. norvegicus syntaxin 18 in a range of eukaryotes were selected for endonucleolytic testing: Mus musculus, Xenopus laevis, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana and Saccharomyces cerevisiae. M. musculus, X. laevis, D. melanogaster and A. thaliana were expressed but not tested for endonucleolytic activity. C. elegans and S. cerevisiae were not expressed. R. norvegicus syntaxin 18 cDNA was amplified and subcloned into protein expression vector pHTT7K. E. coli BL21 were transformed with the modified vector and induced. R. norvegicus recombinant protein did express. However purification of recombinant protein was unobtainable in the pHTT7K vector. A Western blot analysis revealed that the recombinant protein was missing the 6X His-tag at the amino terminus. R. norvegicus was subcloned into new protein expression vector pET28a where the recombinant protein expressed with an attached 6X His-tag. Upon removal of the trans membrane domain, the recombinant protein was expressed, purified and tested for endonucleolytic activity against a portion of c-myc mRNA using an endonuclease assay. In contrast to the original report, a R. norvegicus recombinant syntaxin 18 preparation demonstrated only weak endonucleolytic activity. Analysis of the preparation showed although the vast majority of the yield was recombinant syntaxin 18, a number of undetermined proteins co-purified. The results shed doubt on R. norvegicus recombinant syntaxin 18 protein's function as an endoribonuclease. --P.ii.