The Sg-1 Glycosyltransferase Locus Regulates Structural Diversity of Triterpenoid Saponins of Soybean

Abstract

Triterpene saponins are a diverse group of biologically functional products in plants. Saponins usually are glycosylated, which gives rise to a wide diversity of structures and functions. In the group A saponins of soybean (Glycine max), differences in the terminal sugar species located on the C-22 sugar chain of an aglycone core, soyasapogenol A, were observed to be under genetic control. Further genetic analyses and mapping revealed that the structural diversity of glycosylation was determined by multiple alleles of a single locus, Sg-1, and led to identification of a UDP-sugar-dependent glycosyltransferase gene (Glyma07g38460). Although their sequences are highly similar and both glycosylate the nonacetylated saponin A0-ag, the Sg-1 a allele encodes the xylosyltransferase UGT73F4, whereas Sg-1 b encodes the glucosyltransferase UGT73F2. Homology models and site-directed mutagenesis analyses showed that Ser-138 in Sg-1 a and Gly-138 in Sg-1 b proteins are crucial residues for their respective sugar donor specificities. Transgenic complementation tests followed by recombinant enzyme assays in vitro demonstrated that sg-1 0 is a loss-of-function allele of Sg-1. Considering that the terminal sugar species in the group A saponins are responsible for the strong bitterness and astringent aftertastes of soybean seeds, our findings herein provide useful tools to improve commercial properties of soybean products.

Volatile Glycosylation in Tea Plants: Sequential Glycosylations for the Biosynthesis of Aroma β-Primeverosides Are Catalyzed by Two Camellia sinensis Glycosyltransferases

“…Therefore, our findings indicate that the typical sugar donor specificity of CsGT2 and Sg-1 a for UDP-Xyl results from convergent evolution, because (1) Sg-1 a and CsGT2 are phylogenetically remote GGTs specialized for structurally different substrates (glycosides of large triterpenes versus small volatile monoterpenes, respectively) but with the same sugar donor specificity for UDP-Xyl, and (2) replacement of Ile-141 with a Ser residue in the CsGT2-I141S mutant resulted in a significant decrease in specificity for UDPXyl (Fig. 5C), whereas replacement of Ser-138 with Gly in Sg-1 a (S138G) converted the xylosylating activity into a glucosylating activity (Sayama et al, 2012). These findings not only highlight the plasticity of the sugar donor specificity of UGTs but demonstrate that the metabolic specialization is a consequence of the lineage-specific differentiation of the enzymes.…”

“…These crucial residues for sugar donor specificity are located in three distinguishable regions, the N-terminal, middle, and C-terminal regions (Sayama et al, 2012). Mutagenesis experiments (Ile-141→Ser-141) revealed that the unique Ile-141 of CsGT2 located in the middle region is a residue determining the specificity toward UDP-Xyl.…”

“…Instead, Sg-1 a has a unique Ser residue (Ser-138) in the middle domain found to be essential for its sugar donor specificity toward UDP-Xyl (Sayama et al, 2012). Therefore, our findings indicate that the typical sugar donor specificity of CsGT2 and Sg-1 a for UDP-Xyl results from convergent evolution, because (1) Sg-1 a and CsGT2 are phylogenetically remote GGTs specialized for structurally different substrates (glycosides of large triterpenes versus small volatile monoterpenes, respectively) but with the same sugar donor specificity for UDP-Xyl, and (2) replacement of Ile-141 with a Ser residue in the CsGT2-I141S mutant resulted in a significant decrease in specificity for UDPXyl (Fig.…”

“…The residues are located in three distinct domains: N-terminal, middle, and C-terminal (PSPG box) domains (Sayama et al, 2012). To gain insights into the molecular mechanism of the UDP-Xyl specificity of CsGT2, we constructed a structural model of CsGT2 by homology modeling (Discovery Studio 3.5 [DS3.5]; Accelrys).…”

Comparison of saponin composition and content in wild soybean (Glycine soja Sieb. and Zucc.) before and after germination

“…4) The Sg-1 locus adds xylose and glucose at the terminal position of the C-22 sugar chain of SS-A by co-dominant alleles Sg-1 a and Sg-1 b , respectively. 21) The Sg-4 locus attaches the second sugar (arabinose) of the C-3 sugar chain of all soyasapogenols. [4][5][6] The Sg-6 locus is assumed to directly and/or indirectly control the presence of SS-H, SS-I, and SS-J.…”