Biosynthesis of Phytosterol Esters: Identification of a Sterol O-acyltransferase in Arabidopsis

Qilin Chen , Lee Steinhauer , Joe Hammerlindl , Wilf Keller , and Jitao Zou ^*

Plant Biotechnology Institute, National Research Council Canada, 110 Gymnasium Place, Saskatoon, SK, S7N 0W9, Canada

^* Corresponding author; email: jitao.zou{at}nrc-cnrc.gc.ca.

Fatty acyl esters of phytosterols are a major form of sterolconjugates distributed in many parts of plants. In this studywe report an Arabidopsis gene, AtSAT1 (At3g51970), which encodesfor a novel sterol O-acyltransferase. When expressed in yeast,AtSAT1 mediated production of sterol esters enriched with lanosterol.Enzyme property assessment using yeast cell-free lysate expressingAtSAT1 suggested the enzyme preferred cycloartenol as acyl-acceptor and saturated fatty acyl-CoAs as acyl-donor. Takinga transgenic approach, we showed that Arabidopsis seeds over-expressingAtSAT1 accumulated fatty acyl esters of cycloartenol, accompaniedby substantial decreases in ester content of campesterol and ${beta}$ -sitosterol. Furthermore, fatty acid components of sterol estersfrom the transgenic lines were enriched with saturated and longchain fatty acids. The enhanced AtSAT1 expression resulted indecreased level of free sterols, but the total sterol contentin the transgenic seeds increased by up to 60% compared to thatin wild type. We conclude that AtSAT1 mediates phytosterol esterbiosynthesis, alternative to the route previously describedfor phospholipid : sterol acyltransferase, and provides themolecular basis for modification of phytosterol ester levelin seeds.