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

Abstract

Fatty acyl esters of phytosterols are a major form of sterol conjugates distributed in many parts of plants. In this study we report an Arabidopsis (Arabidopsis thaliana) gene, AtSAT1 (At3g51970), which encodes for a novel sterol O-acyltransferase. When expressed in yeast (Saccharomyces cerevisiae), AtSAT1 mediated production of sterol esters enriched with lanosterol. Enzyme property assessment using cell-free lysate of yeast expressing AtSAT1 suggested the enzyme preferred cycloartenol as acyl acceptor and saturated fatty acyl-Coenyzme A as acyl donor. Taking a transgenic approach, we showed that Arabidopsis seeds overexpressing AtSAT1 accumulated fatty acyl esters of cycloartenol, accompanied by substantial decreases in ester content of campesterol and b-sitosterol. Furthermore, fatty acid components of sterol esters from the transgenic lines were enriched with saturated and long-chain fatty acids. The enhanced AtSAT1 expression resulted in decreased level of free sterols, but the total sterol content in the transgenic seeds increased by up to 60% compared to that in wild type. We conclude that AtSAT1 mediates phytosterol ester biosynthesis, alternative to the route previously described for phospholipid:sterol acyltransferase, and provides the molecular basis for modification of phytosterol ester level in seeds.

Acyl-CoA-Binding Protein ACBP1 Modulates Sterol Synthesis during Embryogenesis

“…In this study, FA profiles (particularly 16:0-and 18:0-FAs) were significantly altered in mutants and OEs of SMO1-1 and ACBP1 (Table I). As 16:0-and 18:0-FAs were the two major species elevated in Arabidopsis OEs of acyl-CoA:sterol acyltransferase with substrate specificity for 16:0-and 18:0-CoA esters (Chen et al, 2007), our results suggest a regulatory role for ACBP1 in sterol acylation via its association with these acyl-CoA species. Deviations of 16:1-and 16:3-FA levels in mutants and OEs of SMO1-1 and ACBP1 also revealed a compositional change in plastidial lipids (Table I).…”

Involvement of the Phospholipid Sterol Acyltransferase1 in Plant Sterol Homeostasis and Leaf Senescence

“…PSAT1 catalyzes a phospholipid-dependent (acyl-CoA-independent) formation of SE (Banas et al, 2005). Recently, Chen et al (2007) demonstrated the involvement of ASAT1, an acylCoA:sterol acyltransferase, in sterol esterification by expressing the corresponding gene both in yeast and in planta, where they showed that the SE content of Arabidopsis seeds expressing an ASAT1 cDNA driven by a NAPIN promoter was about 2-fold higher than that of wild-type seeds. We performed here a detailed analysis of the SE content of psat1-1, psat1-2, and asat1-1 deficient mutant lines.…”

“…This enzyme was shown to prefer saturated fatty acyl-CoAs as acyl donors and cycloartenol as the acyl acceptor. Overexpression of AtASAT1 in seeds of Arabidopsis resulted in a strong accumulation of cycloartenol fatty acyl esters accompanied by an increase of the whole SE content of these seeds and, in spite of a slight decrease of the free sterol pool, an increase of the total sterol content of the transgenic seeds by up to 60% compared with that of the wild type (Chen et al, 2007). We took advantage of the availability of Arabidopsis T-DNA insertion mutants of these two genes to investigate their respective physiological roles.…”

“…Sterol coincubation studies performed with this microsomal enzymatic assay showed that sterol precursors such as cycloartenol or obtusifoliol, which were poor substrates when incubated alone, were preferentially acylated in the presence of sitosterol, suggesting an activation of the enzyme by sitosterol (Banas et al, 2005). Another sterol acyltransferase gene, AtASAT1 (At3g51970), was identified in a survey of members of the Arabidopsis superfamily of membrane-bound O-acyltransferases with a yeast ACAT mutant functional complementation approach (Chen et al, 2007). AtASAT1 encodes a protein structurally related to the animal and yeast ACATs.…”

Biosynthesis and Biotechnology of Seed Lipids Including Sterols, Carotenoids and Tocochromanols

“…Fatty acid acylation of phytosterol can be catalyzed by CoA-dependent enzymes. Recently a plant gene, acyl-CoA:sterol acyltransferase, catalyzing the acyl-CoA-dependent acylation of phytosterol was identified (Chen et al, 2007). The enzyme has a preference for palmitoyl-CoA as acyl donor and cycloartenol as acyl acceptor, however, end product phytosterols, such as sitosterol, were also acylated.…”