Plant sterol metabolism. Δ<sup>7</sup>-Sterol-C<sub>5</sub>-Desaturase (STE1/DWARF7), Δ<sup>5,7</sup>-Sterol-Δ<sup>7</sup>-Reductase (DWARF5) and Δ<sup>24</sup>-Sterol-Δ<sup>24</sup>-Reductase (DIMINUTO/DWARF1) show multiple subcellular localizations in <em>Arabidopsis thaliana</em> (Heynh) L
Sterols are crucial lipid components that regulate membrane permeability and fluidity and are the precursors of bioactive steroids. The plant sterols exist as three major forms, free sterols, steryl glycosides and steryl esters. The storage of steryl esters in lipid droplets has been shown to contribute to cellular sterol homeostasis. To further document cellular aspects of sterol biosynthesis in plants, we addressed the question of the subcellular localization of the enzymes implicated in the final steps of the post-squalene biosynthetic pathway. In order to create a clear localization map of steroidogenic enzymes in cells, the coding regions of ¿(7)-sterol-C(5)-desaturase (STE1/DWARF7), ¿(24)-sterol-¿(24)-reductase (DIMINUTO/DWARF1) and ¿(5,7)-sterol-¿(7)-reductase (DWARF5) were fused to the yellow fluorescent protein (YFP) and transformed into Arabidopsis thaliana mutant lines deficient in the corresponding enzymes. All fusion proteins were found to localize in the endoplasmic reticulum in functionally complemented plants. The results show that both ¿(5,7)-sterol-¿(7)-reductase and ¿(24)-sterol-¿(24)-reductase are in addition localized to the plasma membrane, whereas ¿(7)-sterol-C(5)-desaturase was clearly detected in lipid particles. These findings raise new challenging questions about the spatial and dynamic cellular organization of sterol biosynthesis in plants.