De novo phytosterol synthesis in animals

Abstract

Sterols are lipids that regulate multiple processes in eukaryotic cells, and are essential 17 components of cellular membranes. Sterols are currently assumed to be kingdom 18 specific, with phytosterol synthesis restricted to plants while animals are only able to 19 synthesize cholesterol. Here, we challenge this assumption by demonstrating that the 20 marine annelids Olavius and Inanidrilus synthesize the phytosterol sitosterol de novo. 21 Using multi-omics, high-resolution metabolite imaging, heterologous gene expression 22 and enzyme assays, we show that sitosterol is the most abundant (60%) sterol in these 23 animals and characterize its biosynthetic pathway.