UC Riverside

Phytosterols that plays important roles in regulating membrane properties and serves as the precursor to multiple specialized metabolites such as the phytohormone brassinosteroids. Brassinosteroids are essential and universal plant hormones, regulating plant growth and development, also involved in biotic and abiotic defense. With the promising potential of agriculture application, the biosynthesis of brassinolide in plants remains elusive although it has been intensively studied in the past decades. The promiscuity of brassinosteroid biosynthetic enzymes and the low abundance of brassinosteroids in nature is likely the reasons. In this project, to decipher the biosynthesis of brassinosteroids and pave the way for other steroid nature products (e.g. withanolides, saponins) , we firstly reconstituted the biosynthetic pathway of phytosterols in a heterologous host Saccharomyces cerevisiae and engineered the yeast to produce the free phytosterols and incorporate the sterols in the cell membrane. The resulted phytosterol-producing yeast strains provided a platform for enhanced reconstitution of plant membrane enzymes. Secondly, we optimized the growth of the strain by engineering sterol esterification mechanism, tuning upstream FPP pathway, and performing retro genetic engineering. Thirdly, based on the phytosterol-producing yeast platform, a plant scaffold protein MSBP1 was characterized to play an important role in BR biosynthesis, which expanded our understanding of this multifaced scaffold protein, and more importantly, it brought up a possibility that brassinosteroid biosynthesis takes a place in the metabolome. This finding highlighted the role of non-enzymatic proteins in nature product biosynthesis and broaden our vision when analyze the biosynthetic pathway in nature products.