The wide diversity of plant secondary products results from different modifications undergone during biosynthesis, including glucosylation. These modification reactions result in production of the compounds actually found in plants and to unique chemical and biochemical properties, including some bitter compounds in grapefruit. While the presence of a PSPG box motif allows for identification of a clone as a putative glucosyltransferase (PGT), diversity of GT primary structures makes it difficult to accurately assign specific function. Our approach is to identify and isolate putative GT clones, express them heterologously, and biochemically characterize the proteins. Eleven putative GT clones have been isolated from Citrus paradise and some have been biochemically characterized. The current hypothesis being tested is that PGT3 and PGT9 clones are plant secondary product GTs. Due to issues with inclusion bodies when using E. coli, proteins were expressed in Pichia pastoris using the pPICZA vector. Recombinant protein expression was confirmed by Western blot and proteins were enriched by IMAC. Over 30 flavonoid and simple phenolic substrates, representing many compounds found in grapefruit, were screened for activity with PGT3 and PGT9 proteins. No significant activity was found and the biochemical function of the proteins encoded by these clones will be further investigated.

Plant secondary products such as flavonoids have a variety of roles in plants including UV protection, antifeedant activity, pollinator attraction, stress response, flavor, and many more. These compounds also have effects on human physiology. Glucosylation is an important modification of many flavonoids and other plant secondary products. In grapefruit, glucosylation is important in the synthesis of the bitter compound naringin and several flavonoid glucosyltransferase (GT) enzymes have been characterized from young grapefruit leaf tissue. To study structure and function of flavonoid GTs, it is necessary to isolate cDNA’s that can be cloned and manipulated. In prior work, the plant secondary product glucosyltransferase (PSPG) box was used to identify putative GT clones. We report on results from experiments to test the hypothesis that PGT clones 4 and 11 are plant secondary product GTs, specifically flavonoid GTs. Previously, PGT 4 was cloned into a bacterial expression system, however all protein was localized into inclusion bodies and GT activity could not be tested. For this work, recombinant PGT 4 and PGT 11 were transformed into yeast and the proteins expressed and screened for glucosyltransferase activity with a variety of flavonoid substrates including flavanones, flavones, and flavonols.