Glycoside-specific glycosyltransferases catalyze regio-selective sequential glucosylations for a sesame lignan, sesaminol triglucoside

Sesame (Sesamum indicum) seeds contain a large number of lignans, phenylpropanoid-related plant specialized metabolites. (+)-Sesamin and (+)-sesamolin are major hydrophobic lignans, whereas (+)-sesaminol primarily accumulates as a water-soluble sesaminol triglucoside (STG) with a sugar chain branched via beta 1 -> 2 and beta 1 -> 6-O-glucosidic linkages [i.e. (+)-sesaminol 2-O-beta-d-glucosyl-(1 -> 2)-O-beta-d-glucoside-(1 -> 6)-O-beta-d-glucoside]. We previously reported that the 2-O-glucosylation of (+)-sesaminol aglycon and beta 1 -> 6-O-glucosylation of (+)-sesaminol 2-O-beta-d-glucoside (SMG) are mediated by UDP-sugar-dependent glucosyltransferases (UGT), UGT71A9 and UGT94D1, respectively. Here we identified a distinct UGT, UGT94AG1, that specifically catalyzes the beta 1 -> 2-O-glucosylation of SMG and (+)-sesaminol 2-O-beta-d-glucosyl-(1 -> 6)-O-beta-d-glucoside [termed SDG(beta 1 -> 6)]. UGT94AG1 was phylogenetically related to glycoside-specific glycosyltransferases (GGTs) and co-ordinately expressed with UGT71A9 and UGT94D1 in the seeds. The role of UGT94AG1 in STG biosynthesis was further confirmed by identification of a STG-deficient sesame mutant that predominantly accumulates SDG(beta 1 -> 6) due to a destructive insertion in the coding sequence of UGT94AG1. We also identified UGT94AA2 as an alternative UGT potentially involved in sugar-sugar beta 1 -> 6-O-glucosylation, in addition to UGT94D1, during STG biosynthesis. Yeast two-hybrid assays showed that UGT71A9, UGT94AG1, and UGT94AA2 were found to interact with a membrane-associated P450 enzyme, CYP81Q1 (piperitol/sesamin synthase), suggesting that these UGTs are components of a membrane-bound metabolon for STG biosynthesis. A comparison of kinetic parameters of these UGTs further suggested that the main beta-O-glucosylation sequence of STG biosynthesis is beta 1 -> 2-O-glucosylation of SMG by UGT94AG1 followed by UGT94AA2-mediated beta 1 -> 6-O-glucosylation. These findings together establish the complete biosynthetic pathway of STG and shed light on the evolvability of regio-selectivity of sequential glucosylations catalyzed by GGTs.