The Catharanthus roseus 16-hydroxytabersonine O-methyltransferase involved in vindoline biosynthesis /

Abstract

Madagascar periwinkle (Catharanthus roseus) produces the well known and remarkably complex dimeric anticancer alkaloids vinblastine and vincristine that are derived by coupling vindoline and catharanthine monomers. This thesis describes the novel application of carborundum abrasion (CA) technique as a tool for large scale isolation of leaf epidermis enriched proteins. This technique was used to facilitate the purification to apparent homogeneity of 16-hydroxytabersonine-16-0-methyltransferse (l60MT) that catalyses the second step in the 6 step pathway that converts tabersonine into vindoline. This versatile tool was also used to harvest leaf epidermis enriched mRNAs that facilitated the molecular cloning of the 160MT. Functional expression and biochemical characterization of recombinant 160MT enzyme showed that it had a very narrow substrate specificity and high affinity for 16-hydroxytabersonine, since other closely related monoterpene indole alkaloids (MIAs) did not act as substrates. In addition to allowing the cloning of this gene, CA technique clearly showed that 160MT is predominantly expressed in Catharanthus leaf epidermis, in contrast to several other OMTs that appear to be expressed in other Catharanthus tissues. The results provide compelling evidence that most of the pathway for vindoline biosynthesis including the 0- methylation of 16-hydroxytabersonine occurs exclusively in leaf epidermis, with subsequent steps occurring in other leaf cell types. Small molecule O-methyltransferases (OMTs) (E.C. 2.1.1.6.x) catalyze the transfer of the reactive methyl group of S-adenosyl-L-methionine (SAM) to free hydroxyl groups of acceptor molecules. Plant OMTs, unlike their monomeric mammalian homologues, exist as functional homodimers. While the biological advantages for dimer fonnation with plant OMTs remain to be established, studies with OMTs from the benzylisoquinoline producing plant, Thalictrum tuberosum, showed that co-expression of 2 recombinant OMTs produced novel substrate specificities not found when each rOMT was expressed individually (Frick, Kutchan, 1999) . These results suggest that OMTs can fonn heterodimers that confer novel substrate specificities not possible with the homodimer alone. The present study describes a 160MT model based strategy attempting to modify the substrate specificity by site-specific mutagenesis. Our failure to generate altered substrate acceptance profiles in our 160MT mutants has lead us to study the biochemical properties ofhomodimers and heterodimers. Experimental evidence is provided to show that active sites found on OMT dimers function independently and that bifunctional heterodimeric OMTs may be fonned in vivo to produce a broader and more diverse range of natural products in plants.