Identification of key amino acid residues determining product specificity of 2,3-oxidosqualene cyclase in Oryza species

• Published in: The New phytologist 2018-05, Vol.218 (3), p.1076-1088
• Main Authors: Xue, Zheyong, Tan, Zhengwei, Huang, Ancheng, Zhou, Yuan, Sun, Juncong, Wang, Xiaoning, Thimmappa, Ramesha B., Stephenson, Michael J., Osbourn, Anne, Qi, Xiaoquan
• Format: Article
• Language: English
• Subjects: Acids; Amino Acid Sequence; Amino acids; Amino Acids - metabolism; Boats; Cations; chemical diversity; conformation; Conversion; Cyclization; Genetic Variation; Grain cultivation; Intramolecular Transferases - chemistry; Intramolecular Transferases - genetics; Lanosterol - analogs & derivatives; Lanosterol - chemistry; Lanosterol - metabolism; Orientation; orysatinol; Oryza - enzymology; Oryza - genetics; Oryza sativa indica; Oryza sativa japonica; oxidosqualene cyclase (OSC); parkeol; Phylogeny; product specificity; Proteins; Residues; rice; Specificity; Steroid hormones; Steroids; Substrate Specificity; Synthesis; triterpene; Triterpenoids
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.15080

Triterpene synthases, also known as 2,3-oxidosqualene cyclases (OSCs), synthesize diverse triterpene skeletons that form the basis of an array of functionally divergent steroids and triterpenoids. Tetracyclic and pentacyclictriterpene skeletons are synthesized via protosteryl and dammarenylcations, respectively. The mechanism of conversion between two scaffolds is not well understood. Here, we report a promiscuous OSC from rice (Oryza sativa) (OsOS) that synthesizes a novel pentacyclictriterpeneorysatinol as its main product. The OsOS gene is widely distributed in indica subspecies of cultivated rice and in wild rice accessions. Previously, we havecharacterized a different OSC, OsPS, a tetracyclic parkeol synthase found in japonica subspecies. Phylogenetic and protein structural analyses identified three key amino acid residues (#732, #365, #124) amongst 46 polymorphic sites that determine functional conversion between OsPS and OsOS, specifically, the chair–semi(chair)–chair and chair–boat–chair interconversions. The different orientation of a fourth amino acid residue Y257 was shown to be important for functional conversion The discovery of orysatinol unlocks a new path to triterpene diversity in nature. Our findings also reveal mechanistic insights into the cyclization of oxidosqualene into tetra- and pentacyclic skeletons, and provide a new strategy to identify key residues determining OSC specificity.